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model 128a lock-in amplifier operating and service manual

1. 4 In principle the instrument does not respond to even harmonics at all because the reference input being a square wave does not contain any even harmonics However very slight deviations from perfect symmetry in the applied reference square wave result in some even harmonic response the worst case being about 1 Note that the overall instrument response to harmonics is not necessarily as high as indicated in the preceding paragraphs Phase and pre mixer attenuation effects cannot be ignored With either the tuned amplifier or the hi or low pass filters in use input harmonics will be both attenuated and phase shifted Inasmuch as the reference input to the Mixer is a square wave the harmonics all odd are at the same phase as the fundamental As mentioned previously the even harmonic response is quite small and is seldom of any consequence particularly if the tuned amplifier is used There is one application where one has to be concerned with the subharmonic response of the Mixer and that is when measuring second harmonics using the 2f mode When the instrument is operating in this mode it has a funda mental response the fundamental can be thought of as a subharmonic of the signal of interest the second harmonic of about one or two percent Hence for accurate second harmonic measurements it is essential that the fundamental be attenuated ahea
2. 6 6C Final AC Amplifier VI 2 6 7 PACE Oy ole k d GL tye cb A ina dps athe S LAG ud VI 2 6 8 DC Amplifiers 4 9 5 X 7 ce 3 8 t tt ce UR t 5 dece VI 2 VII SCHEMATICS Table of s e toe t t s n ng ng VII 1 FIGURES Number Page 1 1 Model 128A Lock In Amplifier Dre du Msi SP ae IV 1 Ground Loop Suppression by Ten Ohm T Ground 2 dn d gota duni Ed ate rdum Oe IV 2 Differential Measurement of Single Ended Signal IV 1 V 3 Differential Measurement of Off Ground Signal t t ot 1 2 IV 4 Example of Everything Done Wrong eo t tot tom t t t t t t n IV 2 5 Errors Depicted Figure IV 4 Corrected t o t s t t ot n IV 6 Typical Model 128A Noise Figure Contours oe e e toto t t t t t t t n IV 3 IV 7 Pulse Train as Reference Drive 7 pk AVES IV 8 Net Phase Shift Between Signal and Referente Channels as a Eine E 0 Oph aC
3. eee eee s TIME sa m I CONSTANT 1 1 2225 1610203 204 1208 1224 J233J2 3 12391240 j222 23 J22i J210 OPTIONAL C212 a MIXER 100 v MONITOR 57 i i CIRCUIT C23 i C230 ISK 5 5 REF SQUARE OrP20 WAVE 422 R260 RN4004 8220 0201 196 15 5 R284 09 215 01 0 1 TP203 C232 V lt 555 AMPL Ue ZERO ADL 5 CR202 155v 0 1 SENSE ot SENSE VU FINAL ac anpe mixer N f SFT O C AMPL MIXER POWER SUPPLY BOARD SUE ASSY 1281 17 00095 PARTIAL 1 gt 15 J213 24 4 J220 2 R243 155 MODEL 128A 455 ZZ Jt A 2197 v 0 RECORDER OUTPUT REAR PANEL OUTPUT 8 OL SENSE NOTES ALL RESISTORS 5 Yo COMPOSITION UNLESS OTHERWISE NOTED 2 ALL 25 AND 1 RESISTORS I OW METAL FILM GROUND PLANE v SIGNAL GROUND 4 ALL 196 RESISTORS ARE I 2w METAL FILM COPYRIGHT 97 BY THE PRINCETON APPLIED RESEARCH CORPORATION THIS DRAWING IS INTENDED FOR THE OPERATION AND MAI
4. D Rist 56K 5 Rus cio 7 RII 25v 2 RIG UK 10 6 6958 1905 3 CIO6 Z OO EMR 105 1597 22 00 6106 2N4250 3 V 9 09 204250 cioe 2508 9107 71597 ar i amp Po RIS 15K 1 15 TO Cu SH 2 04 O L SENSE SENSE 774107 8 96 CRIS 4009 58 2 58 2 BLL RESISTORS yaw S 56 COMPC IGN UNLESS OTMER WISE NOTED ALL DIODES ARE 4525 UNLESS CTHERWISE NOTED ALLER RESISTORS ARE 4 ALL AR RES STORS ARE Yow METAL FILM 5 lt gt SIGNAL SROUND v GROUND PLANE COPYRIGHT 1272 BY THE PRINCETON APPLIED RESEARCH CORPORATION THIS DRAWING IS INTENDED FOR THE OPERATION AND MAINTENANCE OF PRINCETON APPLIED RESEARCH EQUIPMENT AND IS NOT TO BE USED OTHERWISE OR REPRODUCED WITHOUT WRITTEN CONSENT OF THE PRINCETON APPLIED RESEARCH CORP SHEET OF 2457 5150 D p TO MIXER BOARD MOTEL 15 PIN r ICH PASS SELECTIVE AMPLIFIER x REAR LE SOCKET 20 8 OP TIONAL Sic 6 8nF Bm END fc OP TIONAL E 15 5 415 5V ISN deci Y SENSE IN SENSE
5. 7487 MD A 15 8254 0 2229 P50 00 0227 CR205 TO SIG IN400S BD 5V Lu 12V OL SENSE 4237 16 0 7 4 N CR207 R263 1 0 390K CR208 2230 R262 C220 1 4228 TO SIG a Y _ OL SENSE 8259 Yo 15 5V R26 C215 x REFERENCE SQUARE WAVE ee MIXER POWER SUPPLY MODEL l2 SUB ASSY 1281 17 00095 PARTIAL AAA SR 15 5V R270 V gaia TIS93 a 2N4250 R283 18 41 2 CR212 i IN 4009 CR2I3 C223 500nF 272 62 r J226 O L LAMP 144009 5 15 R269 26 7K 1 5 5 OVERLOAD CIRCUIT i SEE SHEET 2 OF 3 COSPORATI N Ti 1971 BY THE PRINCETON APPLIED RESEARCH CORPORATION THIS DRAWING IS INTENDED FOR THE OPERATION AND MAINTENANCE OF PRINCETON APPLIED RESEARCH EQUIPMENT AND IS NOT TO BE USED OTHERWISE OR REPRODUCED WITHOUT WRITTEN CONSENT OF THE PRINCETON APPLIED RESEARCH CORP SHEET I OF 7444 521 OL HA A TO 516 80 1 J206 J205 5900 R206 909K 1 26 15 1 INTERFACE CONN XER MONITOR OPTIONAL r
6. RELATIVE VOLTAGE GAIN qw 01 02 05 1 2 5 1 NORMALIZED FREQUENCY gt NORMALIZED FREQUENCY f fo WHERE f APPLIED FREQUENC AND fo TUNED FREQUENCY 02 05 1 2 5 1 NORMALIZED FREQUENCY gt NOTCH PHASE Figure 1V 15 PHASE AMPLITUDE CHARACTERISTICS OF TUNED AMPLIFIER IV 14 removed is other than the one measured As result the signal frequency used for setting up the Tuned Amplifier should be the one to be notched out and not the one to be measured As can be seen from Figure IV 15 the Notch response is a sharp function of frequency Hence any slight frequency difference between the setup signal and the real signal to be notched out may result in the notch not being as deep as it could be Hence when the setup for notch operation is completed it is generally a good idea to connect the signal produced by the experiment to the input of the Model 128A and then while monitoring the signal at the SIG MON connector to adjust the Null Amplitude adjustment on the Tuned Amplifier as required to make the signal being nulled disappear into the noise It might be men tioned that the appearance of the signal may not be as expected For example in measuring harmonics it is generally desirable to notch out the fundamental Many observers are quite surprised at the appearance of a square wave for example that has had its fundamental frequency co
7. m SD 9 Amplitude and Phase Characteristics of Hi Pass Filter o o o n t t t t t n IV 7 IV 10 Amplitude and Phase Characteristics of Low Pass Filter o o o e t t t t n 7 IV 11 Examples of Output Filter Interactions Se yeh te ie vie cA et Gee um ua ase aed 8 1V 12 Mixer Output for In Phase and Quadrature Signals E cad Hoven he grecs 2 2 NES IV 13 Internal Oscillator Board Installed t oo moe eo tot ot t t t n IV 12 IV 14 Tuned Amplifier Installed P re tie Sk Oe a Be Bes m 1 15 Phase Amplitude Characteristics of mune S Ge oe aah 1 V 1 Model 128A Adjustments and Testpoints 2 o t tot tot t t on 2 TABLES Number Page IV 1 Interface Connector Pin Assignments eot motor tor rot 5 t t n IV 10 2 Frequency Range as a Function of Capacitors e s e t t t t t t t n IV 11 CONSIDERATIONS INTRODUCTION apparatus to which this instruction manual applies has been supplied in a safe condition This manual contains some information and warn ings that have to be followed by the user to ensure safe operation and to retain the apparatus in a safe condition The described apparatus has been designed for indoor use
8. ECG MODEL 128A LOCK IN AMPLIFIER SEE SAFETY NOTICE PRECEDNG SECTION I EFORE OPERATING INSTRUMENT BROOKDEAL ELECTRONICS PRINCETON APPLIED RESEARCH M128A 9 83 500 MIC MODEL 128A LOCK IN AMPLIFIER OPERATING AND SERVICE MANUAL Elsels BROOKDEAL ELECTRONICS lt PRINCETON APPLIED RESEARCH Copyright 1983 EG amp G PRINCETON APPLIED RESEARCH Printed in U S A SHOULD YOUR EQUIPMENT REQUIRE SERVICE A Contact the factory 609 452 2111 or your local factory representative to discuss the problem In many cases it will be possible to expedite servicing by localizing the problem to a particular plug in circuit board B If it is necessary to send any equipment back to the fac tory we need the following information 1 Model number and serial number 2 Your name instrument user 3 Your address 4 Address to which instrument should be returned 5 Your telephone number and extension 0 Symptoms in detail including control settings 7 Your purchase order number for repair charges does not apply to repairs in warranty 8 Shipping instructions if you wish to authorize ship ment by any method other than normal surface transportation C U S CUSTOMERS Ship the equipment being returned to EG amp G PRINCETON APPLIED RESEARCH 7 Roszel Road Off Alexander Road East of Route 1 Princeton New Jersey D CUSTOMERS OUTSIDE OF U S A To avoid del
9. n gt 80 1083 r Eley 80 3 03 J302 J301 6 2911 8125 5 S222 2 4623 p 1120 92 J310 J309 J311 301 J306 2120 w 6020 9928 26282 3221 202d 10 18 rn Se amp J202 J225 J238 O uJ N x 4241 0 C 6222 4224 4208 4253 4203 9204 8 8220 3tCt CR215 a J228 J227 CR202 69280 CR214 N 1 T 81818 BIS J240 R222 508 6120 9222 9858 W o m gt M 85 8 N OE ejeje ot 5 e 8 2 5 1 cizo goyr 1 3 orate Ley re 20582 w 2209 Se 4226 Llgu l20 3t eeu M R273 Goa gzzy 4214 9 ese 60280 9028 oczy GLZ 692 N gt gt 2022 WI u x 1053 514 4315 BAL 4307 9205 9234 J206 z mrt 822 ZERO OFFSET CAL SYMBOLIZATION MODEL 128A 129A MIXER POWER SUPPLY BOARD
10. 3 Set the Model 128A controls as follows Input Selector A Sensitivity 100 mV Filters HI PASS MIN LO PASS MAX Phase Quadrant switch 270 Phase dial 90 Mode f Zero Offset switch OFF center position dial 1 00 one turn from fully counterclockwise position Time Constant 3 SEC DC Prefilter OUT Reference Tracking Rate switches two internal switches FAST unless unit is equipped with Tuned Amplifier or Internal Oscillator set for frequency below 5 Hz in which case switches should be set to SLOW NOTE Instruments are normally shipped with these switches set to FAST Power ON 4 Applies only to units not equipped with the Internal Oscillator modification If the Model 128A has this modification go to step 5 Connect the output of the external oscillator set for 100 mV rms sinewave out at 1 kHz to both the A and Reference Inputs If the unit in question has the Tuned Amplifier modification set the oscillator to the tuned frequency specified when the unit was ordered 5 Applies only to units having the Internal Oscillator modification Connect a cable from the rear panel REF OSC OUT connector to front panel A Input There is no need to connect this signal to the Reference Input connector labeled MONITOR in units equipped with the Oscillator modification The connection to the Reference Channel of the Model 128A is made internally at the factory The rear panel REF
11. SYMBOLIZATION MODEL 1294 95 MODIFICATION INTERNAL OSCILLATOR BOARD FAB 6662 MD B asy REF BD 3419 15 iv RMS REOL R606 R63 b BUT 1 R602 15K 3 57K 15K 45V 64 R620 15 v 402 cw MAX OUT 0 neta 6 04K CR605 C60 IN4828 015 0 AN ya s 7 5 SELECT I SCW R629 8 AMPLITUDE cece l REF FREQ RANGE d osc TOL 5 Yo 15 OUT 6604 15 4 0601 9 e 245819 E cw R605 m 8 15 35 UF 25 po red 5 V J405 yei ON 15V J406 REF 5 80 J404 35yF 47 25v t 3V C606 3 R612 7 34 8K SelM NOTES 5 ALL RESISTORS 81 METAL FILM UNLESS OTHERWISE 15 5 2 FREQUENCY RANGE 31 3 C IO f UF 4 ALL 5 RESISTORS 4 CARBON COMPOSITION INTERNAL OSCILLATOR TAT FBS 7 DIFI CATION MODEL 128 OPERATION AND MAINTENANCE OF PRINCETON APPLIED RESEARCH EQUIPMENT AND IS NOT TO BE USED 128 99 OTHERWISE OR REPRODUCED WITHOUT WRITTEN THE PRINCETON APPLIED RESEARCH CORP 6645 C 50 IA VL MA cien GLIA NOTCH BANDPASS SWITCH TUNED FREQUENCY A
12. 5 14 2 7 004 ASQ 1 40 R492 a09 f Q 4 0 5 QNT R Oi SN7274 97 Sv 5 0424 5 11597 m 178K 8495 71 144009 4 57 E oj 5486 SNTAOIN 10K 8490 4085 5468 76K 5 1 f R asi 00 A t 660205 vit f C430 Rave 29uF 150K 10 SJE ASSEMBLY 1281 19 00075 PARTIA xw m 4 14009 a Sv Se REAR REFERENCE MONITOR 2409 OPTIONAL d REF OUTPUT 4132 ER BOARD 7 13 po 24 2 2444 o Jail E ed Jair TO veces SHEET I 15 509 SK 1 1 fer REF UNLOCK 15v NOTES 1 SEE SHEET 1 4430 ON MIZEL ONLY COPYRIGHT 1971 BY THE PRINCE TON APPLIED RESEARCH CORPORATION THIS DRAWING IS INTENDED FOR THE OPERATION AND MAINTENANCE OF PRINCETON APPLIED RESEARCH EQUIPMENT AND IS NOT TO BE USED OTHERWISE OR REPRODUCED WITHOUT WRITTEN CONSENT OF THE PRINCETON APPLIED RESEARCH CORP SHT2O0FZ 6622 o 501 VII 8 15 V ADJ 5 v 15 O a lE N TE EE 0 O coer f e 5 2 m 90 9 C C307 R R310 3 MK R39 d 2 OLED
13. A or B as appropriate the Sensitivity switch is set for as near full scale output as possible and the filters are used to narrow the bandwidth ahead of the Mixer In some applications it may be desirable to incorporate the optional tuned amplifier into the Signal Channel as well A further discussion of these topics follows Input Selector Switch With this switch set to the signal applied to the A Input is processed by the instrument Signal applied to the B Input is dead ended Similarly when the switch is set to B the situation is the same except that the roles of the A and B inputs are reversed Another difference is that the B Input is 180 out of phase with respect to the A Input In other words if a signal which yields positive output meter deflection when applied to the Input is applied to the Input an equal but negative reading will be obtained In the position the instrument operates differentially that is only the difference between the signals applied to the two inputs is processed and read out As discussed in Subsection 4 25 it is generally advantageous to operate differentially even when process ing signals from a single ended source Sensitivity Switch The Sensitivity switch should be set to provide as near full scale output as possible In very high noise situations it may be necessary to operate with less sensitiv
14. EN 7 MIXER POWER SUPPLY BD TP 202 R201 BAL C210 HF ZERO TP201 R234 AMP 1 ZERO TP205 R491 ADJ OUT RI43 GAIN C PREAMP OUT C106 HF CMR 6 O f R124 CMR R111 BIAS R242 A2 ZERO R247 METER CAL R243 ZERO SUPP CAL 204 TP203 C num It 438 O ADJ R437 90 ADJ R405 S T SYM ADJ R521 HF CAL REF BD 9 Increase the amplitude of the reference input to about 3 V pk pk 3 A3 Level Adjust R498 a b Being sure to use the 10 1 attenuator probe connect the oscilloscope to TP407 The oscillo scope should be dc coupled Adjust R498 A3 Level Adj for an indicated voltage of 4 V Then wait a minute If the voltage changes readjust R498 as required to obtain the desired 4 V reading 4 Adjust R491 a b c Transfer the oscilloscope to ac coupling and transfer it from TP407 to TP405 The suggested horizontal sensitivity is 1 ms cm Rotate 8491 Adj fully counterclockwise Increase the vertical sensitivity of the oscillo scope until small spikes are observed Then adjust R491 EQ Adj until the spikes disappear If the adjustment is turned too far the spikes will reappear but with reversed polarity Remove the oscilloscope 5 4C SIGNAL BOARD ADJUSTMENT 1 Preamp DC Bias Adjust R111 a b Connect the DVM to TP101 Adjust R111 Pre
15. t i 6BOnF 0 E 27 4115 Jno Jil4 JU 4412 6 3130 4129 120 419 48 JM 7 3428 3123 4122 9124 4125 4126 27 1 27 14 2 4 427 2743 NM aui ost oonr Sur Em 8157 9 GND 15V 15 RIIS 5 1 TO CAINS TC CR 116 DWG NO DWG NO 7457 0 50 7457 0 50 58 5 1 2 a 1015 NOTES We SEE SHEET 1 NAL AMPLIFIER AR COPYRIGHT 1972 BY THE PRINCETON APPLIED RESEARCH MODEL 28 CORPORATION THIS DRAWING INTENDED FOR THE 1281 18 5 PARTIAL OPERATION AND MAINTENANCE OF PRINCETON APPLIED GRAY Se an TEN CONSENT OF THE PRINCETON APPLIED RESEARCH CORP SHEET 2 OF 2174571 D SL VHA ot GIA REF O PHASE ADJ LOCK RATE SWITCH E ADJ LEVEL ADJ LOCK ON RATE SWITCH 428 8515 e R458 04128 _ 8455 24419 9 8 0412 O S o fs 10 5 s gt Tola a cT 0423 e T lt 402 R453 24315 O CR411 5401 ol l c Tey CR407 e S Z assa amp 20429 3 o n N X 1405 0406 R451 315 o
16. 2 positive full scale panel meter indication 13 Set the Zero Offset toggle switch to The panel meter indication should go to 0 5 of full scale 14 Begin rotating the Offset dial counterclockwise The panel meter indication should increase linearly track ing the dial setting When the dial is fully counter clockwise the panel meter should indicate full scale 5 Reset the Zero Offset toggle switch to the center OFF position This completes the initial checks If the indicated results were obtained one can be reasonably sure that the Model 128A is functioning normally SECTION IV OPERATING INSTRUCTIONS 4 1 INTRODUCTION Even though operation of the 128 5 straight forward there are a number of factors of which the operator should be aware to be assured of achieving optimum performance in all situations This section of the manual treats these considerations in some detail Topics covered include grounding noise performance harmonic sensitivity operation in conjunction with the plug in accessories and others For an overall quick look at how the instrument is operated the operator is referred to Section the condensed operating instructions 4 2 PRELIMINARY CONSIDERATIONS 4 2A POWER REQUIREMENTS The Model 128A requires 100 130 V ac or 200 260 V ac 50 60 Hz The power consumption is 15 watts The unit may also be powered by batteries by applying 24 V to the app
17. Power Supply Board 7444 D SD sheet 2 of 3 ixer Power Supply Board 7444 D SD sheet 3 of 3 Chassis Wiring Diagram 7589 E SD 4 internal Oscillator Board Parts Location E Internal Oscillator Modification 6645 C SD uned Amplifier Board Parts Location uned Amplifier Modification 6646 C SD Page VII 1 VII 2 VII 3 VII 4 VII 5 VII 6 VII 7 VII 8 VII 9 VII 10 VII 11 VII 12 VII 13 VII 14 VII 15 VII 16 VII 1 Rx3 should be high ohm resistor to minimize shunting effect on input impedance R x1 two possible positica according to whether SIGNAL AMP OUT detector bias is to be positive or negative Rx2 121 2 5 S101 8 5101 SIOI E 5101 0 5101 TR 4128 J123 R157 WEE kHz LO PASS 8142 rogos 125 158 Rxl 2 9 8160 CMR R21 w 5506 AM MUT J126 R161 R165 cn 50 Hz HI PASS xad RI62 T 22 J124 oj 0 ot In o Sos 5 Hz HI PASS 82 2155 zez ar Q c E is a 004 2 9 g 2 Oo 451 e x c 5 T wl K SK else 8199 3 0110 amp 55 c11 oh SL OG occu 3 122 EE Z ere QNI 5 S R148 w 115 333 7
18. particular ly in that this interference is coherent in phase and directly adds to the signal of interest It is not hard to envision situations where this interference signal could well be larger at the input of the lock in amplifier than the signal of interest itself SHIELD MODEL 128A INDUCED emf A C GROUND PATH Figure 4 EXAMPLE OF EVERYTHING DONE WRONG MODEL 128 T CONTACTS TO SHIELO AT SINGLE POINT GROUND PATH THIS PATH BROKEN IN IDEAL Figure 1 5 ERRORS DEPICTED IN FIGURE IV 4 CORRECTED Figure IV 5 shows the steps which can be taken to circumvent this problem All of the drive signal current is returned directly to the drive signal source except for the very small component reference input resistance of Model 128A is 10 which is applied to the Model 128A way of the Reference Input Second no current whether drive current reference current or signal current is allowed to flow through the experiment shield the shield contacts ground at one point only The only coherent signal which can flow through the parallel path of the signal cable braid is a small portion of that allowed by the ten megohm Reference Input resistance Furthermore the use of differ ential operation assures that even this small amount can have no effect By using the arrangements indicated one could operate with very large drive currents without concern that they might contaminate the signal
19. resistor value should be one megohm or smaller 2 DC Amp 1 Zero Adj R234 a b c d Set the Sensitivity switch to 2 5 mV Remove the jumper from J221 Adjust R234 DC Amp 1 Zero Adj for 0 on the DVM Set the Sensitivity switch to 100 mV 3 Meter Cal Adjust R247 a b c d e Set the Zero Offset Polarity switch to Adjust the Zero Offset dial clockwise about one full turn for a DVM indication of 1 00 V Adjust R247 Meter Cal Adj so that the panel meter reads exactly full scale to the right Set the Zero Offset Polarity switch to the center OFF position Remove the DVM 4 AC Bal Adj R201 a b c d Set the Sensitivity switch to 2 5 mV Connect the oscilloscope to the front panel OUT connector Decrease the reference frequency to 40 Hz Adjust R201 AC Bal Adj for minimum square wave signal observed at the oscilloscope NOTE This square wave will drift around because of short term temperature fluctuations caused by air currents on the components Remove the oscillo scope 5 HF Zero Adj C210 b c Increase the reference signal to 100 kHz Adjust trim capacitor C210 HF Zero Adj for 0 panel meter indication Reset the reference frequency to 400 Hz 5 4 OTHER ADJUSTMENTS 1 Reference 0 Phase Adj R438 a b c d e Set the Sensitivi
20. 2 3 636 OSC BD 15 27 5 1 GND 0v 4 8459 01 444 J405 40 2 196 th m AME AL 194009 aie Q i NOTES ALL RESISTORS i aw 5 9 COMPOSITION UNLESS 10 Baas wi SE NOTED Sir 9 6 5v 2 aLi RESISTORS I BW METAL FILH TSN 5 5 4 saos 3 CONTRO SETTINGS 2 ASE CONTROL 5 SHEET REF 1 RMS 740042 AMP 2 REF MODE A COPYRIGHT BY THE PRINCETON APPLIED RESEARCH REFERENCE BOARD SUE ASSEMBLY 1281 19 00075 PART IAL MODE 128 28A CORPORATION THIS DRAWING IS INTENDED FOR THE T dnd RATION AND MAINTENANCE OF PRINCE TON APPLIED RESEARCH EQUIPMENT AND IS NOT TO BE USED OTHERWISE OR REPRODUCED WITHOUT WRITTEN CONSENT OF THE PRINCETON APPLIED RESEARCH SHEET or 3 55 DHA RAS 12K 12 8 Ray 3v ERA 15 SHTI 5 57 7 REF 4 5v 7 ait E TS eer R 5 100 4828 40 48 iw4009 gt O 43642 m 22 a H Rae d 10K 4075 423 x x 5 ui noes 42 22pf we REFEREN QAR LIA 5425 565 43K 2n 4250 47 isv tec 35yf 25v
21. SIGNAL REQUIREMENT 100 mV pk pk any waveshape crossing its mean only twice each cycle Minimum time required on either side of the mean is 100 ns Amplitude excursions must be at least 50 mV on each side of the mean Maximum input signal is 5 V pk to mean Best phase accuracy is obtained with a 1 V rms sinewave LOCK ON TIME A function of internal switch setting as follows Selected Range Lock On Time SLOW 0 5 Hz to 100 kHz 20 sec per octave FAST 5 Hz to 100 kHz 2 sec per octave PHASE Calibrated Phase controls allow the phase of the reference drive to the Phase Sensitive Detector to be set at any angle relative to the input signal The controls consist of a Phase Dial with a range of 100 and a Phase Quadrant switch which provides incremental phase shifts of 90 The phase shift 11 2 7 accuracy of the dial is better than 0 2 over the entire frequency range The resolution of the dial is better than 0 1 The incremental phase shifts provided by the Quadrant switch are accurate to 0 2 The overall phase accuracy of the instrument including shifts in both the reference and signal channels is typically better than 5 DETECTOR BIAS Internal network allows dc bias current of either polarity to be provided at the REF IN connector to facilitate operation in situations where the reference signal is taken from diodes requiring biasing See page VII 6 and Parts Location
22. SWITCH Figure 14 TUNED AMPLIFIER INSTALLED IV 13 6 Set the Notch Bandpass slide switch to 7 Connect a 250 mV rms sinewave 0 7 V pk pk to 8 both the A Input and Ref Input of the Model 128A This signal should be at the intended operating frequency NOTE If the unit is also equipped with the internal oscillator modification use the internal oscillator as the signal source Monitor the rear panel SIG MON connector with the oscilloscope and alternately adjust the two Tuned Amplifier trim adjustments for a null in the observed signal The rear panel accessible adjustment sets the center frequency The one which is adjusted from the side sets the and amplitude response These adjust ments do interact so it will be necessary to go back and forth until no further improvement in the observed null can be obtained NOTE Any time the operating frequency is changed the Null Amplitude adjustment must also be reset If the Null Amplitude adjustment is properly set for one end of the frequency range and the tuned frequency is then shifted to the opposite end of the same range the amplitude response of the Tuned Amplifier will be in error by about 20 unless the Null Amplitude adjust ment is reset Bete NER LIZED FREQUENCY f fe ts APPLIED FREQUENCY TUNED FREQUENCY SE MN adc I 2 01 02 05 1 2 5 1 2 5 NORMALIZED FREQUENCY 9 RE
23. accurately measured even in situations where it is completely masked by noise Signals applied to the input single ended or differential routed through a series of amplifiers which allow dll scale sensitivity ranges down to one microvolt Switch selectable low pass and high pass filters allow considerable noise reduction ahead of the phase sensitive detector This pre detector noise reduction can be further enhanced by making use of the optional plug in internal selective amplifier At the phase sensitive detector the signal is compared with the reference signal derived from the experiment Only those signal components which are synchronous with the reference yield a net dc detector output Noise and other non synchronous signals do not contribute a net dc output but only ac fluctuations which can be reduced to any arbitrary value according to the amount of filtering selected with the Time Constant switch This switch allows time constants as large as 100 seconds to be selected with provision for achieving larger externally determined time constants if necessary Post detector dc amplifiers drive the panel meter and signal output connectors Other features include provision for calibrated zero suppression of up to 10 x full scale a two position dc _ prefilter and the capability of driving the reference input the detector at double the frequency of the signal applied to the Reference Input connector to facilitate second harmonic meas
24. correct one can reasonably assume that the AMP 1 circuit the AMP 2 circuit and the U401 switching circuits at the output of the Input Schmitt Trigger are all working normally Remove the DVM 6 Connect the oscilloscope to TP402 The observed 7 8 signal should negative sawtooth at 400 Hz The upper plateau should be at 10 V and the lower points should be at 4 5 V The plateau intervals should have the same duration as the sawteeth If this signal is as indicated one can reasonably assume that all of the circuits depicted on the page VII 6 schematic are functioning normally Connect the oscilloscope to TP404 The signal there should be a 1600 Hz square wave having its upper level at about 4 V and its lower at 0 V If this signal is as indicated one can reasonably assume that the 4f Oscillator and the oscillator control circuitry 0427 0428 and all components to the left on the schematic are working properly It might be instructive to check the voltage at TP407 The allowable voltage range at this point extends from OV to 8 However under the measurement conditions established this voltage is nominally 4 V The voltage at TP406 should be about 0 6 V relative to that at TP407 The correction signal can be observed at TP405 However when the loop is operating correctly there is little to observe One might see small amplitude fuzzy blips at 400 Hz When the loop i
25. matter if one is interested in the absolute phase of the input signal Another problem of low frequency operation is that of 1 f noise including both that which develops in the Model 128A and that which originates in the experiment itself to degrade the signal to noise ratio ahead of the lock in amplifier Increased response and settling time could be a IV 1 significant problem if one were operating in conjunction with the optional plug in tuned amplifier At high fre quencies radiation and associated pick up tend to be bothersome Another high frequency problem is that of signal attenuation as a result of the input cable capacitance This is especially a problem when working from a high source resistance Other frequencies to avoid are 60 Hz and its lower order harmonics By avoiding these frequencies the operator assures that he will be measuring the signal of interest only uninfluenced by power frequency pick up either internal or external 4 2bE GROUNDING In any system processing low level signals proper grounding to minimize the effects of ground loop currents usually at the power frequency is an important consideration In the case of the Model 128A special design techniques have been employed to give a high degree of ground loop signal rejection in single ended applications Even so it will often prove advisable to operate differentially even when ex amining a single ended signal source to achieve the greatest possib
26. mine the range of the frequency adjustment The trim adjustment accessible through an opening in the rear panel sets the center frequency of the Tuned Amplifier That which is accessible from the side of the Model 128A sets the Q and amplitude response These two adjustments interact to some degree The two switches determine the tuned amplifier function One of them allows the operator to select either Tuned Amplifier operation or Flat opera tion The other gives the choice of Bandpass or Notch operation These latter two functions have relevance only when the first switch is set to SELECTIVE If it is set to FLAT the tuned amplifier circuitry is bypassed and the 128A operates exactly as if the Tuned Amplifier had never been installed The frequency range of the rear panel adjustment as function of the value of the two replaceable capacitors is the same as for the Internal Reference Oscillator Also the restrictions as to the types of capacitors which can be used are the same as outlined in Subsection 4 7B There is however one difference namely that the capacitors used in the Tuned Amplifier must be matched to 176 whereas in the case of the Oscillator they need only be matched to 5 Capacitors purchased from P A R C for use in the Tuned Amplifier are matched to 196 even though they may be marked 5 This comes about because they are selected from a large stock of 5 capacitors Such 1 capacitors are marked by co
27. nevertheless important that the reference signal used be relatively noise free Any superimposed noise can cause small zero crossings to occur in the region of the main waveform zero crossings with the result that the Reference Channel momentarily sees a much higher reference frequency than what is really there When this happens the reference lock can be lost Frequently moderately noisy signals can be cleaned up sufficiently for satisfactory operation by interposing a single section low pass filter between the reference signal source and the Reference Input connector of the Model 128A On later instruments space is provided for mounting the filter components on the Reference printed circuit board Location of the mounting holes is indicated on page To install the filter transfer the wire which normally goes to quick disconnect J419 over to quick disconnect J429 Then install the filter components a resistor for RX4 and a capacitor for CX1 RX4 and CX1 are designations given on the schematic and parts location diagram for these com ponents In most instances optimum performance is obtained by setting the filter corner frequency f 1 4n RC to the intended reference frequency Switches Three switches are associated with operation of the Reference Channel One of them the f 2f switch is located at the front panel The other two are internal The front panel switch determines whether the mixer will be driven at the freq
28. octave high pass filter which can be set to 3 dB down frequencies of 50 Hz 5 Hz or MIN below 0 5 Hz 11 OVERLOAD DETECT Front panel indicator lights if applied signal plus noise is large enough to cause overload at any of several critical overload monitor points 12 GAIN STABILITY 0 1 C 13 GAIN LINEARITY 0 05 14 OVERALL GAIN ACCURACY 2 REFERENCE CHANNEL The Model 128 reference channel automatically locks onto and tracks an applied reference signal over the entire operating frequency range of the instrument As a result the instrument is immune to frequency and phase shifts as long as the reference and signal to be recovered change together 1 TRACKING RANGE 2 3 4 5 6 5 Hz to 100 kHz FAST or 0 5 Hz to 100 kHz SLOW as determined by the setting of two internal switches Faster lock on time and slewing rate obtained with switches set to FAST make this range preferable except when operating below 5 Hz MODES Either of two modes f and 2f can be selected by means of a front panel switch In the f position the phase sensitive detector is driven at the same frequency as the applied reference signal In the 2f position the phase sensitive detector is driven at twice the frequency of the applied reference signal to facilitate second harmonic measurements INPUT IMPEDANCE 10 shunted by more than 20 pF MINIMUM REFERENCE
29. turns ratio The source resistance is increased by the square of the turns ratio For example if one had a ten ohm source one could use a 1 100 step up transformer in which case the amplifier would see a source resistance of 100 At 100 the amplifier adds little additional noise Even though the thermal noise of the transformer adds to that of the source avery considerable improvement is usually achieved P A R C manufactures a line of suitable signal transformers each designed for optimum operation over a given frequency range Perfor mance information can be obtained from the factory or one of its representatives In using an external transformer in conjunction with the Model 128A single ended operation of the lock in amplifier is advised The extremely high inherent common mode rejection of the transformer makes differential operation of the lock in amplifier unnecessary When working from a high source resistance one could in principle use a transformer in the same manner to improve noise performance Unfortunately practical transformer design considerations usually prevent one from doing so As a result the options available to an experimenter working with a high source resistance device such as a photo multiplier are limited Practically speaking the best one can do is to make the load resistor as large as possible The larger the source resistor the less the shunting effect it will have and the better the s
30. varies with the square root of the time constant As a result the measurement times rapidly become lengthy as the time constant is increased to obtain better signal to noise ratios As a practical guide the correct filtering time constant is the one which reduces the noise to an acceptable level Two separate dc filters are provided The first called the DC PREFILTER provides filtering time constants of 100 ms 15 or OUT in which the filtering time constant is negligibly small The second or main TIME CONSTANT filter allows filtering time constants from 1 ms to 100 s to be selected in addition to MIN time constant about 7 ms and EXT time constant determined by external capacitors connected to rear panel octal sockets Both filters are single section filters having a 6 dB octave rolloff The filters have an accumulative effect as shown in Figure IV 11 FREQUENCY 6 AMPLITUDE AMPLITUDE TIME CONSTANT FILTER 8 gt 3 AMPLITUDE AMPL TUDE AMPLITUDE 01 016 05 1 FREQUENCY Figure 11 EXAMPLES OF OUTPUT FILTER INTERACTIONS The usual procedure for setting these filters is to leave the Prefilter set to OUT and to adjust the main Time Constant filter as required to reduce the noise to an acceptable level However if the noise level is sufficiently high to cause dc amplifier overloading it will be necessary to set the Prefilter 10 100 ms or pe
31. which acts as a heat sink 6 5 REFERENCE CHECKS schematics on pages VII 6 and VII 7 1 Set the controls as follows Input Selector Sensitivity 100 m V Phase switch 0 dial 0 Reference mode f Zero Otfset switch OFF center position dial fully counterclockwise Time Constant 3 SEC DC Prefilter OUT Power ON Reference Tracking Rate switches internal FAST 2 Set the oscillator controls to provide a 280 mV pk pk 100 mV rms sinewave at 400 Hz 3 Connect the output of the oscillator to the Reference Input of the Model 128A 4 Connect the oscilloscope use 10 1 probe to TP401 The observed s gnal should be a 400 Hz square wave having its lower level at about 0 6 V and its upper level at 2 5 V this signal is as indicated one can assume that the Input Schmitt Trigger circuit is working correctly NOTE Here and throughout the remainder of the troubleshooting procedure the opera tor should concern himself primarily with gross discrepancies Generally when a circuit malfunctions the error in the output signal of that circuit is so great as to leave no doubt of a malfunction Much time may be saved by going through the checks fairly rapidly without wasting undue time and effort trying to verify that each signal of voltage conforms to the value indicated down to the last decimal point 5 Connect the DVM to TP403 The voltage should be 4 5 V 0 1 V If this voltage is
32. with 10 1 probe 6 3 INITIAL STEPS 1 Remove the top cover which is secured by four screws two on each side 2 If the unit contains a tuned amplifier or an internal oscillator take the necessary steps to render these accessories inactive that is the Model 128A should be operated in the External Reference mode and the signal channel response should be flat 3 Plug in the Model 128A turn on the power and allow a fifteen minute warmup 6 4 POWER SUPPLY CHECKS schematic on page VII 11 1 On the Power Supply board check for a 15 5 V 01 at the positive end of capacitor C309 b 15 5 V 0 2 V at the negative end of capacitor C310 and c 5 V 0 2 V at the positive end of capacitor C312 If these voltages are correct go to Subsection 6 5 If any of these voltages are incorrect or missing proper power supply operation must be established before any further checks can be made Note from the schematic on page VII 11 that the 15 V regulator VI 1 supplies the reference voltage for both the 5 V and 15 V regulators Thus any trouble with the 15 V regulator would cause loss of regulation in the 15 V and 5 V circuits as well 2 the unregulated supply levels are incorrect nominal ly 24 V check the unregulated supply components line fuse transformer rectifiers and filter capaci tors Note that the pass transistor for each of the three regulator circuits is bolted directly to the rear chassis
33. 0 AMPL 2 CONTROL PHASE 1 15v REF OUT 0 448 0 Y i l 4409 413 0 REAR REF MON 14 7 OPTIONAL REF OSC eme e Ut 7 OPTIONAL CONNECT TO 2 J419 WHEN wn REAR SIGNAL BOARD OPTIONAL REF REF OSC REAR 89 8 09 49 OPTIONAL 3407 54 Jiz2 J03 N t 5 6 2 5 2284227 1406 205 J225 4241 0 7202 MIXER 1 MONITOR 7T ist 7 AMPL 7 MIXER POWER SUPPLY 100nF I BOARO lone 1281 1700 Ec cp 60 0 AC AMPL L EI 11 111 Jj 4 1 Jg04 5298 4233 4240 239 926 G Lal a nta Wied 1 6 SIG AMP i Pep HASSIS WIRI IAGRAM SECTION VII x SCHEMATICS Table of Signal Amplifier Board Parts Location Signal Amplifier Board 7457 D SD sheet 1 of 2 Signal Amplifier Board 7457 D SD sheet 2 of 2 Reference Board Parts Location Reference Board 6622 D SD sheet 1 of 2 Reference Board 6622 D SD sheet 2 of 2 Mixer Power Supply Board Parts Location j Mixer Power Supply Board 7444 C SD sheet 1 of 3 ixer
34. 0 7 ms and EXT which allows time constants longer than 100 s to be achieved by means of external capacitors separate front panel toggle switch allows another 6 dB octave filter to be inserted if desired This filter has a time constant of either 100 ms or 1 s whichever is selected 5 ZERO OFFSET A calibrated ten turn Zero Offset dial with up to ten times full scale capability is provided 6 FULL SCALE OUTPUT 1 V 7 OUTPUTS a Panel meter full scale b Front panel BNC connector One volt out corresponds to full scale panel meter deflection Output resistance is 600 ohms c Rear panel Recorder Out binding posts spaced to accept standard double banana connector Output resistance is 600 ohms GENERAL 1 AMBIENT OPERATING TEMPERATURE RANGE 15 C to 45 C 2 AUXILIARY POWER OUTPUT 15 5 V regulated dc at 20 mA is provided at rear panel connector 3 POWER REQUIREMENTS 100 130 V ac or 200 260 V ac 50 60 Hz May also be powered from 24 V dc source such as batteries Power consumption 15 watts 4 SIZE 17 3 A W x 3 1 2 H x 14 D 45 W x 9 H x 36 cm D II 3 5 WEIGHT 14 15 6 4 kg a b c 6 MODIFICATIONS Model 128 97 Monitor Modification Three rear panel BNC connectors are installed which permit monitoring of 1 Output of Signal Channel before demodulation please note that the Signal Channel monitor is provided as part of the Tun
35. 5 V 0 2 V Remove the DVM 3 5 4B REFERENCE BOARD ADJUSTMENTS 1 Set the controls as follows Sensitivity 100 mV Input Selector switch A Lo Pass switch MAX Hi Pass switch MIN Phase switch 270 Phase dial 90 Reference Mode switch f Time Constant MIN Zero Offset switch OFF center position dial 0 00 fully counterclockwise Fast Slow switches located on Ref board FAST DC Prefilter switch OUT 2 Schmitt Trigger Symmetry Adjust 8405 a Connect the oscillator to the Ref In connector Set the amplitude to 3 V pk pk at 400 Hz b Monitor the signal at TP401 with the oscillo scope The observed signal should be a 400 Hz square wave with a pk pk amplitude of about 3 V c Gradually reduce the amplitude of the applied signal until rounding of the square wave is observed As the amplitude is further reduced the symmetry of the observed waveform may become degraded Adjust R405 Schmitt Trigger Symmetry Adj as required so that ideal sym metry is maintained Keep reducing the ampli tude of the input signal to the point where R405 is adjusted for best symmetry with the lowest possible reference input which gives proper oper ation 15 R498 A3 LEVEL N 15V 5 V 4128 OUT SIG FINAL AMP OUT 2 4232 FINAL AMP TRACKING RATE OUT 1 J 231 w ww UL 7 4 99 oid gh t a
36. 9 BIAS RI52 w m 5 65 tel B R164 yy R153 5 10 LOP 107 J104 Iz Sole R163 yy 6120 Ae ok of uj RI34 9107 o o cit 5 J127 9130 129 9L Jue 114 133 sno E 103 102 T GAIN pan ap O o R24 on C109 Om NSN 9119 18 732 J117 J134 113 ZZ ADJ o 8135 Cx LFCMR 2 J101 SYMBOLIZATION MODEL 128A 129 SIGNAL AMP 80 FAB Z 7484 x 1 64028 QIO3A midi RIG 105 15K 4 99 2 499k x 1 2 284250 CRIO 10 3 TO 5 SELECTED 0 ies 400 gt PARC SPEC 4 Rios CRIO3 12020 PARC MATCHED SPEC 730215 PER PAR 700528 RUO 8104 T 1 6816 on PARC 10999 SPEC 0215 ed Riit E 01038 PARC SPEC CE 720212 s MATCHED PER R C SPEC CR 105 8 2 RIO 4 SPEC win spec MIN 5996 ed 7302i5 8 SPEC 730215 8122 2 1 Yaw CRIN RII 4 CIA SIGNAL AMPLIFIER 12 amp 18 OOOBS PART FAL B BOARD
37. AM PLITUDE adjustment should be set so that the amplitude of the signal at the REF OSC OUT connector is 100 mV rms An accurate ac voltmeter may be useful for setting this level units leave the factory set for anominal 100 mV rms out 6 Applies only to units having the Tuned Amplifier modification In the case of units not having this modification go directly to step 7 a Connect the oscilloscope to the rear panel SIG MON connector b Vary the frequency of the signal applied to the Input use the rear panel REF OSC FREQ ADJ control in units equipped with an Internal Oscillator for peak signal amplitude as observed with the oscilloscope 7 Set the Phase Quadrant switch to 0 Then adjust the Phase dial for 0 panel meter indication 8 Set the Phase Quadrant switch back to 270 The panel meter should indicate full scale to the right a few percent of full scale The accuracy of this reading will depend the amplitude accuracy of the signal applied to the A Input 7 9 Adjust the amplitude of the signal applied to the Input as required to obtain exactly full scale panel meter indication 10 Set the Phase Quadrant switch to 180 The panel meter should indicate 0 5 of full scale 11 Set the Phase Quadrant switch to 90 The panel meter should indicate negative full scale 5 of full scale 12 Set the Phase Quadrant switch to 270 to restore the
38. B INSPECTION Newly received apparatus should be inspected for shipping damage any is noted immediately notify EG amp G PARC and file a claim with the car rier The shipping container should be saved for possible inspection by the carrier WARNING THE PROTECTIVE GROUNDING COULD BE RENDERED INEFFECTIVE IN DAMAGED PARATUS DAMAGED APPARATUS SHOULD NOT BE OPERATED UNTIL ITS SAFETY HAS BEEN VERIFIED BY QUALI FIED SERVICE PERSONNEL DAMAGED PARATUS WAITING FOR SAFETY VERI FICATION SHOULD BE TAGGED TO INDI CATED TO A POTENTIAL USER THAT IT MAY BE UNSAFE AND THAT IT SHOULD NOT BE OPERATED C SAFETY MECHANISM As defined in IEC Publication 348 Safety Require ments for Electronic Measuring Apparatus the 128A is Class apparatus that is apparatus that depends on connection to a protective con ductor to earth ground for equipment and operator safety Before any other connection is made to the apparatus the protective earth terminal shall be connected to a protective conductor The protec tive connection is made via the earth ground prong of the M128A s power cord plug This plug shall only be inserted into a socket outlet provid ed with the required earth ground contact The protective action must not be negated by the use of an extension cord without a protective conduc tor or by use of an adapter that doesn t main tain earth ground continuity or by any other means The power c
39. DJUST R708 8713 R719 712 8711 O 7276 W R705 RT 02 o 101 R702 R706 Be 9 B 709 E 5 5 eros RC 6704 _ 0 x NULL AMPLITUDE ADJUST SYMBOLIZATION MODEL 129A 84 TUNED AMPLIFIER MODIFICATION BOARD 6973 0 SELECTIVE FLAT SWITCH R702 10K C703 RT27A SBT 59 SELECT FOR OPERATING FREQ RANGE MATCH C704 0 300pF 55 45 P R701 59 15702 8719 NOTES u s a 6 4 TN 9 ALL 1 RESISTORS SELECTIVE Es E METAL FILM Spr O 6 lt SIQIA 15 5 oe C707 30 4725 J R726 N W VIO W GRN ER a 9130 4129 4 O5 7 6 RED BLK YEL TO SIG BD _ _____ COPYRIGHT 1971 BY THE PRINCETON APPLIED RESEARCH TUNED AMPLIFIER MODIFICATION MODEL 12 CORPORATION THIS DRAWING 5 INTENDED FOR THE OPERATION AND MAINTENANCE OF PRINCETON APPLIED lt 128 98 RESEARCH EQUIPMENT AND IS NOT TO USED OTHERWISE OR REPRODUCED WITHOUT WRITTEN CONSENT OF THE PRINCETON APPLIED RESEARCH CORP E 6646 C 50
40. Diagram on page 5 PHASE SENSITIVE DETECTOR DC AMPLIFIER 1 OUTPUT DRIFT 0 1 C 2 OVERLOAD CAPABILITY 3 NON COHERENT REJECTION 1000 times full scale up to a maximum at the input of 650 mV rms Overload capability is defined as the ratio at the input of the Model 128A of the maximum pk pk non coherent signal which can be applied without overloading the Model 128A to the pk pk coherent signal required to yield full scale Model 128A output Note that expressed as the ratio of the pk pk non coherent signal to the rms value of the coherent signal required for full scale output this number can be as great as 2800 Maximum acceptable signal is a 650 mV rms sinewave 50 ppm maximum Non coherent rejection is defined as that offset which results from applying a non coherent signal having a pk pk amplitude 1000 times the pk pk amplitude of the coherent signal required to obtain full scale output Example With a non coherent signal applied having a pk pk amplitude 1000 times the pk pk coherent signal required to obtain full scale output there will occur an offset at the output caused by the non coherent input signal The amplitude of this offset will be no greater than 50 x 1076 x 1000 50 x 10 of f s output 50 mV f s 1 V 4 TIME CONSTANT Front panel switch allows selection of 6 dB octave filter time constants of 1 ms 10 ms and 1 s through 100 in 1 3 10 sequence Also MIN time constant
41. E ASSUMING THE CAPACITORS ARE DIS CHARGED SECTION VI TROUBLESHOOTING READ SAFETY NOTICE ON FACING PAGE BEFORE PROCEEDING 6 1 INTRODUCTION This section consists of a series of procedures to be followed in troubleshooting the Model 128 The purpose of the procedure is to narrow the trouble down to one of the three plug in circuit boards by making voltage and waveform checks at critical points Once the faulty board has been identified the operator can contact the factory or the authorized representative in his area for advice on how to get the instrument back into operation in the shortest possible time In the case of units still in Warranty it is particularly important that the factory or one of its representatives be contacted before doing any work on the board itself because any damage that occurs as a result of unauthorized work could invalidate the Warranty Although past experience indicates that some instrument failures turn out to be the fault of a specific component failure on one of the boards it is of course perfectly possible that some component other than one located on a circuit board could go bad Where this is the case the person troubleshooting will have to appropriately adapt the procedure to isolate the faulty component 6 2 EQUIPMENT REQUIRED 1 Digital Voltmeter such as the Fairchild Model 7000 2 Oscillator sinewave to provide a 100 mV rms signal at 400 Hz 3 General purpose oscilloscope
42. ILM 3 ALL 196 RESISTORS METAL FILM 7 GROUND PLANE COPYRIGHT 1971 BY THE PRINCETON APPLIED RESEARCH CORPORATION THIS DRAWING IS INTENDED FOR THE OPERATION AND MAINTENANCE OF PRINCE TON APPLIED RESEARCH EQUIPMENT AND 15 NOT TO BE USED OTHERWISE OR D WITHOUT WRITTEN CONSENT OF THE PRINCETON APPLIED RESEARCH CORP SHT 3or 3 0150 J3 3 OP WDNAL 4222 3 UXER BOAR 1 1 OF FLAT RIBBON CABLE AC SWITCH LAMP 59 052 JWER Power 28v 40mA prr ON whee 436 5100 30 35302 1 tisv 154 157 REGULATOR Sv REGULATOR 9 45V 3209 0207424 213 215 9211 J219 J218 7 4 37 REAR IGHT 197 BY THE PRINCE COP TRI CORPORATION THIS DRAWING OPERATION AND MAINTE RESEARCH EQUIPME NY AND IS MODEL 128A OR REPRODUCED WITHOUT CONSENT Or THE PRINCETON APPL M j jjpowa p os san Rem F h 000011101066 S AREE Eu QUU tete 9 na ae IDEM L E E er sannana ELHA nieces SC TO REF OSC OUT CONNECTOR BLACK WHITE GREEN OSCILLATOR FREQUENCY ADJUST AMPLITUDE ADJUST 5
43. J405 15 V R522 J406 Cx SYMBOLIZATION R x1 two possible positions according to whether MODE 128A 129A detector bias is to be positive or negative REFERENCE BOARD Rxe FAB 6618 MD K Rx3 should high ohms resistor to minimize BIAS NETWORK shunting effect on Reference Input impedance OPTIONAL INPUT FILTER 15 Cx 3 1429 TU C 40 oy sais M um cree S 8429 8 nase 4 1 499 5 NOTE 3 gt ix PT SNT40IN 5 Q4064 1 956 0465 n 6 N ZU 0410 7 ioopr 2N5952 0433 7 0 2 as 11597 40 CRaO 2 2 K syent TAY MI28 390pF CAL 522 R523 7 aM CR4O2 CR4O5 MiZ8A 2409F gt 9 0 DELAY wk SS 144009 n 4009 uk 8 6 5 v 5 v 5v 041111597 DELAY GENERATOR v Oot 2405 lt 100A OSC 80 3427 nF 100 2 e SCHMITT TRIGGER 1 15 15 IN caog 35 25v 26 87 5 1 28 6 90 wi28A 5 62K 045 ER NAA NE 4 5v 15 gt 15 5 Ras Raa BET SEE NOTE 3 IK 8 i 5402 o i Raa 1 i FAST D MAN LS NA Rare REF MODE yaaa 9 v
44. LATIVE VOLTAGE GAIN mw 1 11 5 BANDPASS AMP PHASE OF Egy RELATIVE TO 5 1 o NORMALIZED FREQUENCY f fe WHERE APPLIED FREQUENCY AND fo TUNED FREQUENCY o 6 02 65 2 NORMALIZED FREQUENCY gt BANDPASS PHASE 9 Set the Notch Bandpass switch to BANDPASS Then using an accurate ac voltmeter establish the rms amplitude of the nput signal at 250 mV 1 10 Adjust the Phase dial and Phase switch if need be for peak panel meter indication alternatively one could use a digital voltmeter connected to the front panel OUT connector 11 Adjust the Model 128A GAIN CAL trim poten tiometer R143 for exactly full scale panel meter indication 1 000 V on a DVM 8 143 is mounted on the Signal Amplifier board The Model 128A is now tuned for bandpass operation at the intended operating frequency and is normalized for operation with the tuned amplifier Figure IV 15 shows the phase amplitude response of the Selective Amplifier in both Notch and Bandpass operation For bandpass operation the Notch Bandpass switch should be set to BANDPASS For Notch operation it should be set to NOTCH Neither position of the Notch Bandpass switch has any relevance unless the Selective Flat switch is set to SELECTIVE Note that this procedure must be modified somewhat for notch operation where the frequency component to be o
45. NSENT OF THE PRINCETON APPLIED RESEARCH CORP 6649 so 128A 90A MODIFICATION ADDED APPENDIX MODIFICATION 1281 70 INSTRUCTIONS FOR OPERATING THE MODEL 128 OR MODEL 129A WITH THE MODEL 189 INCORPORATED AS A TUNED AMPLIFIER NOTE Before proceeding first verify that the lock in amplifier is in good working order One way of doing this is to run through the initial checks procedure provided in the lock in amplifier instruction manual Since the underlying assump tion is that the lock in amplifier has been specifi cally modified for operation in conjunction with a Model 189 be sure the rear panel switch is in the FLAT position in performing the Initial Checks TUNING PROCEDURE The procedure that follows is written in terms of tuning the system to 450 Hz This same procedure could be used to tune the system to other oper ating frequencies The only difference would be that the oscillator and the Model 189 would be set to the new frequency instead of to 450 Hz 1 Set the lock in amplifier controls as follows a CONTROL SETTINGS FOR M128A ONLY Input switch A Sensitivity 100 mV Filters MIN and MAX Phase switch 270 Phase dial 90 9 turns from fully ccw position Reference Mode FUND f Zero Offset dial 0 00 fully ccw Zero Offset switch OFF Time Constant 0 3 SEC dc Prefilter OUT b CONTROL SETTINGS FOR M129A ONLY Input switch A Sensitivity 100 mV Filters MIN and MAX Ph
46. NTENANCE OF PRINCETON APPLIED RESEARCH EQUIPMENT AND IS NOT TO BE USED OTHERWISE OR REPRODUCED WITHOUT WRITTEN CONSENT OF THE PRINCETON APPLIED RESEARCH CORP SHEET 20F 317444 0 SD LEHA SUB ASSY TO INTERFACE COLLECTOR CONNECTOR OF A 30 exe y EXT 34505 JP 294 yaw 24 2 CECE TSS 2 csos 10 yF 25 CR30 14955 R307 6 8K 5 6 R308 2 8K 196 1 0 w TOINTERFACE CONNECTOR EXT J306 24 r lt IN 1 R3 20 C30z ZIOC VF 7 1 Y2w 40v C306 Hoye 25v R306 10K 96 yaw Q304 7 245122 0305 2 4122 eZ T gt 0306 2 R308 CR303 Yew SIM fisv POWER SUPPLY 1281 17 0009 9v 1 SERVICE JUMPER O 15 25 m 153 our Z 3303 15 5 uere 2 gt 5 5 6 9 14 51 C309 25v I 309 16 9K to 20K i 96 2w 1 j SERVICE Ls JUMPER Q 30 COL gt MODEL 128 LECTOR Sv tisy E 4302 Sw LAMP NEUE o 3301 J MT sv Ba yew JH 7 8 amp gt 5 9308 CR304 11598 SIM NOTES ALL RESISTORS i 4W 5 COMPCCITION UNLESS OTHER w SE NOTED 2 ALL 196 RESISTORS I 2W METAL F
47. NUTES BEFORE ASSUMING THE CAPACITORS ARE DIS CHARGED IV 16 SECTION V ALIGNMENT READ SAFETY NOTICE ON FACING PAGE BEFORE PROCEEDING 5 1 INTRODUCTION The Model 128A Lock In Amplifier is a reliable conserva tively designed instrument High quality stable components have been used throughout in its construction and one can reasonably expect a long period of troublefree operation without any need for realignment However to be assured of continued high confidence in the experimental data obtained with the Model 128A it may be advisable to run through the following alignment at one year intervals and after doing a repair on the instrument Due to possible interactions between some of the adjustments it is neces sary that they be carried out in the indicated sequence Any decision to make a partial alignment should be reserved to someone having sufficient knowledge of the Model 128A to fully understand all possible interactions Figure V 1 identifies the adjustments and testpoints Note that this alignment is not intended to be used in troubleshooting If the instrument is suspected of malfunc tioning go directly to Section VI which deals with troubleshooting The instrument must be working properly before it can be aligned 5 2 REQUIRED EQUIPMENT 1 General purpose oscilloscope having a sensitivity of at least 1 mV cm with a 10 1 attenuator probe 2 Sinewave oscillator providing both an adjustable output and a fi
48. OCK light to go out before proceeding Usually this will be but a few seconds However at low frequencies a longer time is required STEP THREE SIGNAL CHANNEL Set the Input Selector to A single ended differential or single ended whichever is appropraite Set the Sensitivity switch to 250 If signal channel filtering 15 desired set the HI PASS and LO PASS Filter switches as required Connect the signal source to the Model 128A Input 1 1 gone For this reason it is advisable to read Section IV the complete Operating Instructions to be assured of achieving optimum performance NOTE As written these condensed instructions do not apply to units having either the Internal Oscillator or Tuned Amplifier modifications If the unit in question has either or both of these modifications the operator is referred to Subsections 4 7 and 4 8 STEP FOUR OUTPUT CONTROLS Set the Zero Offset toggle switch to the center OFF position Then set the DC Prefilter to 100 ms and the Time Constant switch to 0 3 SEC STEP FIVE FINAL ADJUSTMENTS Rotate the Sensitivity switch counterclockwise until the panel meter begins to deflect Adjust the Phase Quadrant switch and Phase dial for maximum meter indication When the Sensitivity and Phase adjustments are optimally set the meter should indicate the maximum possible on scale reading If the Overload light comes on inc
49. ODIFICATION INTERNAL OSCILLATOR BOARD FAB 6662 MD B A 1 FREQ ADJUST 702 10K 1 727 SBT 5 o 15 i STOIA Q2 30 WGRN SOS 4129 4 1 TO 6 TO 16 80 TUNED AMPLIFIER MODIFICATION 128798 128A 90A MODIFICATION ADDED 15V 15V R NOTES l ALL 1 RESISTORS m METAL FILM 2 16 4131 132 KE COPYRIGHT MO S ca AND MAINTENANCE OSE ARCH EQUIPMENT AND 15 LUE OTHERWISE OR REPRODUCED WITHOUT WRI CONSENT OF THE PRINCETON APPLIED RESEARCH CORP 6646 501 15 060 2N35819 TO REF 8D J419 1 v 157 605 144828 6 3 ek R629 cw 06 00A AMPLITUOE 1 R 5 2w oe Ap TO 05 C604 2 UE Sv of amp MAX OUT IOV yet N 15 3406 5 4 55 NOTES 7 ALL RESISTORS Vg W 176 METAL FILM UNLESS OTHE MORSE 2 FREQUENCY RANGE 3 1 5 CXIO f UF 4 ALL 5 RESISTORS 4 W CARBON COMPOSITION COPYRIGHT 1971 BY THE PRINCETON APPL CORPORATION THIS ORAWING IS D INTERNAL OSCILLATOR MODIFICATION MODEL 128 OPERATION MAINTENANCE OF PRI RESEARCH EQUIPMENT ANO 15 WRITTEN 128 99 CO
50. Oe e R517 297 9 o 5 7 5 403 R450 R50 S o E yene o E Y R449 6 Tx lt 43 D q 406 a 1409 J oc 406 206 51 5 8 8435 9999 R434 amp R457 0412 9445 W W DF OF S gt R439 Q411 0408 407 o c 5 S R444 E g T SCHMITT M R432 90 T Q406B wWe PHASE ADJ MES GRA DE P 3 SYMMETRY ADJ 433 W C406 kazo lt P ANA N N nt R431 R430 T a 3999 02329 415 5 9 S mn 405 QW 5 6 5 0405 6410 9r 3 0404 5 R428 T P 402 L o 5 Rag 9404 w R436 1 22 T T S gt Mes cR405 8405 segs si lt 4 gt a 2 a udo lt gt I u4o8 U403 9407 c d it y C408 DK beg 9 T me 11 e feed dle bY S 3 2 m 5557 i E Q a I v J417 J416J415 9414 J413 J412 J411 9410 408 409 J407 J427 J403 N R523 J402 a i
51. amp DC Bias Adj for 3 V Remove the DVM 54D MIXER ADJUSTMENTS 1 DC Amp 2 Zero Adj R242 a b c Connect a jumper from J221 to J223 ground Connect the DVM to the front panel OUT connector Adjust R242 DC Amp 2 Zero Adj for 0 on the DVM NOTE If the instrument has been repaired and 0206 A B replaced the following procedure should be used to select R273 select by test resistor l After performing steps a and b connect a jumper from TP204 to TP205 2 Connect another jumper from TP203 to J223 or any ground 3 Adjust R242 DC Amp 2 Zero Adj for 0 panel meter indication NOTE This is a sensitive high gain adjustment and setting a true zero will probably prove impossible 3 Initially the meter will probably be against one stop or the other As the adjustment is turned a point will be reached where the meter indication snaps to the other ex treme If the adjustment is then turned in the opposite direction about a half turn will typically be required to make the indication snap back The correct setting is midway through the dead zone In other words one must adjust the pot until the indication snaps to the other extreme then stop and go back half way to the setting required to make the meter snap back 4 Remove the two jumpers connected in steps 1 and 2 5 Select for R273 that resistor which yields a DVM indication of 0 40 2 mV The
52. and to Princeton Applied Research Corporation but before using the instrument for experimental measurements Should any difficulty be encountered in carrying out these checks contact the factory or one of its representatives It might be noted that it is not the purpose of these checks to demonstrate that the instrument meets all specifications but rather simply to show that it is functioning normally If normal indications e obtained for the functions checked one may reasonably assunie that those functions which are not checked are working properly as well 3 2 EQUIPMENT NEEDED 1 Sinewave Oscillator to provide a 100 mV rms sinewave at 1 kHz NOTE If the instrument to be checked is equipped with the Tuned Amplifier modification then the oscillator will have to provide a 100 mV rms sinewave at the tuned frequency If the instrument in question is equipped with the Internal Oscillator modification no external oscillator will be required This applies whether or not the unit is equipped with the Tuned Amplifier modification 2 General purpose oscilloscope This item is required only in the case of units having the Tuned Amplifier modification 3 Suitable cables for interconnecting the above instru ments 3 3 PROCEDURE 1 Check the position of the rear panel 115 230 switch Be sure the number showing in the window corre sponds to the line voltage to be used 2 With the Power switch set to OFF plug in the line cord
53. as been found to this point the instrument is either functioning normally or the problem is beyond the scope of this procedure For additional aid or advice the operator is advised to contact the factory or the authorized representative in his area APPENDIX MODEL 128A 90A MODIFICATION The Model 128A 90A is a Model 128A modified to operate at two different combinations of frequen cies Both a Tuned Amplifier and an Oscillator are incorporated into the Model 128A 90A However instead of setting the tuned frequency of these ac cessories by means of capacitors mounted on the accessory boards themselves as explained in the instruction manual the capacitors are mounted on rear panel switches One switch allows the Tuned Amplifier to be tuned to either 2 kHz or 17 kHz Another allows the oscillator frequency to be set to either 1 kHz or 17 kHz The two are operated together so that when the oscillator frequency is 1 kHz the tuned amplifier is tuned to 2 kHz and when the oscillator frequency is 17 kHz the tuned amplifier frequency is also 17 kHz In addition to the changes required to achieve two frequency operation changes have also been made in the time constant circuitry The range of available time constants is changed and the Exter nal Time Constant capability is eliminated A plate TUNED FREQUENCY ADJUST with the new Time Constant switch symbolization is added to the panel so that the Time Constant sw
54. ase switch 270 Phase dial 90 9 turns from fully ccw position Reference Mode FUND f Vector switch 2 PHASE Zero Offset dial 0 00 Zero Offset switch OFF Time Constant 0 3 SEC Output Expand x 1 dc Prefilter OUT both channels 2 Set the SELECTIVE FLAT switch on the rear panel of the modified Model 128A or Model 129A to the SELECTIVE position B 1 3 Connect a cable from the Model 189 INPUT connector to the M189 IN connector on the rear of the lock in amplifier 4 Set the controls of the external oscillator as required to provide a 100 mV rms sine wave output at 450 Hz Then connect this signal to both the A and Reference Inputs of the lock in amplifier 5 Set the Model 189 controls as follows Pushbuttons all to OUT position Frequency control 4 50 Frequency range x 100 6 Turn the power on at both the Model 189 and lock in amplifier 7 Monitor the Bandpass output of the Model 189 with an oscilloscope The observed signal should be a 450 Hz sine wave Carefully adjust the inner dial of the M189 dual concentric Fre quency control for maximum observed signal The amplitude of the observed signal should be about 125 mV pk pk 8 Remove the oscilloscope and connect a cable from the BANDPASS output of the Model 189 to the M189 OUT jack on the rear panel of the modified Model 128A or Model 129A 9 The panel meter on the Model 128A IN PHASE meter in the ca
55. ay in customs clearance of equipment being returned please contact the factory or the nearest factory distributor for complete shipping information E Address correspondence to EG amp G PRINCETON APPLIED RESEARCH P O Box 2565 Princeton 08540 Phone 609 452 2111 TELEX 84 3409 WARRANTY EG amp G PRINCETON APPLIED RESEARCH warrants each in strument of its manufacture to be free from defects in material and workmanship Obligations under this Warranty shall be limited to replacing repairing or giving credit for the purchase price at our option of any instrument returned freight prepaid to our factory within ONE year of delivery to the original purchaser provided prior authorization for such return has been given by our authorized representative This Warranty shall not apply to any instrument which our in spection shall disclose to our satisfaction has become defec tive or unworkable due to abuse mishandling misuse acci dent alteration negligence improper installation or other causes beyond our control Instruments manufactured by others and included in or supplied with our equipment are not covered by this Warranty but carry the original manufacturer s warranty which is extended to our customers and may be more restrictive Certain subassemblies accessories or com ponents may be specifically excluded from this Warranty in which case such exclusions are listed in the Instruction Manual supplied with each inst
56. btained at 90 and 270 It might be mentioned that the waveforms illustrated in Figure IV 12 apply only at frequencies below 50 kHz and with a noise free input signal At higher frequencies switching spikes become visible and some Mixer filtering effects become evident Even relatively small amounts of noise accompanying the signal could completely obscure it at the Mixer output especially if one were operating without the tuned amplifier 4 4 MIXER FUNCTION AND HARMONIC SENSITIVITY The purpose of the Mixer Phase Sensitive Detector is to convolute the input signal in such away that the output of the detector is the sum and difference frequencies of the signal and the reference If the input and reference signals are at the same frequency and they must be for normal lock in amplifier operation one of the output frequencies of the detector will be zero that is dc Noise or other interference is not normally at exactly the same frequency as the input signal and so does not produce zero frequency at the Mixer output The dc output is proportional to the amplitude of the in phase component of the input signal This dc is passed by the low pass filter s which follow the Mixer while the ac components representing the input noise and interference are shunted to ground Thus the signal to noise ratio at the output connectors and front panel meter is much improved over what it is at the input of the instrument The reference inp
57. ce Mixer Monitor Output An optional Mixer Monitor output is provided at the rear panel The signal applied to this output is taken directly from the output of the Mixer and before any filtering Figure IV 12 illustrates the Mixer output corresponding to in phase and quadrature signals respectively If the signal and reference inputs to the Mixer are either in phase or 90 out of phase the signal at the output of the Mixer will be as shown For signals 180 out of phase the Mixer output will be the inverse of the in phase output and for signals 270 out of phase the output will be the inverse of the 90 output Taking the maximum possible area which can be enclosed by one cycle one polarity as a unit output the output averaged over a cycle for any Mixer input phase relationship will be the unit output times the cosine of the angle between the input and reference signals A SIGNAL REF IN PHASE APPLICABLE ONLY BELOW 50 FOR NOISELESS x x SIGNALS lt 1 B SIGNAL AND REF 90 OUT OF PHASE Figure IV 12 MIXER OUTPUT FOR IN PHASE AND QUADRATURE SIGNALS IV 9 The cosine response depends on the sinusoidal nature of the input signal If the signal were a square wave and the tuned amplifier were not used the Mixer output would vary linearly with the angle between signal and reference inputs Nevertheless maximum output would still be at 0 and 180 and zero output would be o
58. ctor and chassis ground After the connector is securely mounted twist the two wires together moderately tight over their entire length 2 3 Note the pink wire which extends from the Ref In connector to J419 near the front of the Ref Bd Remove this wire from J419 and allow it to hang free Current production units use a different kind of quick disconnect pin than was used previously a current production oscillator board is to be installed in an older unit the quick disconnect terminals at the end of the involved leads will have to be cut off and new ones installed The new terminals are supplied with the oscillator board 4 7 9 REF AMP ADJ r Figure IV 13 INTERNAL OSCILLATOR BOARD INSTALLED Remove the Reference INPUT connector from the front panel Then locate the Monitor tag over the panel opening and remount the connector so that the tag is secured in place by the connector mounting flange Position the oscillator board as shown in Figure IV 13 but do not snap it into place yet Dress the red yellow and black wires out to the right and the longer white orange wire towards the front of the instrument Connect the ied yellow and black wires as follows FOC tees 4405 yellow AE E J406 black oo a a a O a Ser RA J404 Connect the pink wire from the Ref In connector to J418 Locate th
59. ctor to pin J128 rear edge of Signal Amp Bd and the black wire to J121 Current production units use a different kind of quick disconnect pin than was used previously If a current production tuned amplifier board is to be installed in an older Model 128A the quick disconnect terminals at the end of the involved leads will have to be cut off and replaced by the new type terminals supplied with the Tuned Amplifier board IV 15 3 Position the Tuned Amplifier as shown in Figure IV 14 but do not snap it into place yet It is easier to make the wire connections first 4 the Model 128A remove the jumper white orange which interconnects J129 and J130 Also remove the jumper white violet which interconnects J131 and J132 It may be a good idea to tape these jumpers somewhere to a chassis surface inside the Model 128A to prevent their becoming lost 5 Make the wire connections from the Selective Ampli fier to the following listed pins Wire Color Connect To black eere CAE I Ene 4119 aka atid det NULL EL 4120 MIDI uswa J118 white violet J130 white green J129 white orange J131 white gray J132 Press the Tuned Amplifier circuit board down so that it snaps into place 6 This completes the installation The Tuned Amplifie
60. d first set the polarity switch to assume initial meter indication were to the right followed adjusting the dial for null The dial setting required would be 0 70 and the meter sensitiv ity would 100 uV with respect to the 70 uV ambient level A recorder connected to the output would allow the amplitude variations as a function of some experimental parameter to be recorded Because of the Offset dial range 10 times full scale the sensitivity of the measurement could be greatly expanded In the example at hand the Sensitivity switch could be set to 10 uV The signal amplitude 70 uV would be less than ten times full scale 100 uV and so would fall within range of the Offset dial If the dial were adjusted for null setting 7 00 the meter would read 10 uV full scale with respect to the 70 uV ambient signal level Outputs The output of the instrument is provided at both the front and rear panels These two outputs are in parallel The output resistance is 600 ohms and full scale output is 1 V which corresponds to full scale deflection of the panel meter At the front panel the output is provided at a BNC connector At the rear panel it is applied to one of a pair of binding posts The second black binding post is ground These binding posts are spaced to accept a standard double banana connector is frequently more convenient to use the rear panel output when using a strip chart recorder as the readout devi
61. d of the Mixer One convenient way of doing this is to use the optional tuned amplifier By operating in the Notch mode with the Notch tuned to the frequency of the fundamental the fundamental is reduced to a negligible level with only 196 loss in the amplitude of the second harmonic It follows that the Tuned Amplifier can also be used to good advantage when measuring harmonics higher than the second 4 5 INTERFACE CONNECTOR An 11 pin Interface connector is provided at the rear panel Table IV 1 indicates the function of each pin Observing the connector from outside the instrument the pins are counted counterclockwise from the key Pin Function Chassis Ground PIONEER IMP MN EE 15 5 V OUT load limit 20 mA E tens oe 15 5 V OUT load limit 20 Ayka Rec Ku IE No connection e upian 24 V IN battery operation 300 mA req d 0 ts taa ks No connection 24 V IN battery operation 300 mA req d O SNN External Time Constant Capacitor External Time Constant Capacitor LO dcs cp No connection Essa pha ispa NTC UN No connection Table IV 1 INTERFACE CONNECTOR PIN ASSIGNMENTS 4 6 BATTERY OPERATION Battery operation of the Model 128A Lock In Amplifier may be necessary where no ac power is available or as a last resort where power line interference is a problem Battery operation of the Model 128A is stra
62. derations Should he desire to operate without the oscillator he has only to transfer a few wires as described in the preceding paragraph IV 11 If the Internal Oscillator is ordered separately the installa tion is generally made by the customer Three items are supplied the oscillator board itself a BNC connector with two wires attached and a metal tag bearing the word MONITOR The oscillator circuit board is secured to the Model 128A by standoffs which plug into openings in the Model 128A Reference board as shown in Figure IV 13 This figure also shows the location of the various inter connecting wires all of which are terminated in quick disconnect contacts so that the installation can be com pleted in a matter of minutes with no special tools or soldering required The oscillator adjustments are ordi narily preset at the factory for 100 mV rms out at the frequency specified by the customer The appropriate frequency range capacitors are factory inserted The follow ing procedure can be used to make the installation 1 Remove the top and bottom covers of the Model 128A The top cover is secured by four screws two on each side The bottom is secured by ten screws Mount the BNC connector in the REF OSC OUT opening in the rear panel it will be necessary to first push out the plug which can then be discarded Be sure to use the insulating bushings supplied so that there is no contact between the shell of the conne
63. e amount of noise contributed by the amplifier is 1 4 times the source thermal noise At 1 4 times the thermal noise the amplifier noise just begins to be noticeable At lower noise figures the amplifier for all practical purposes may be regarded as noiseless Generally speaking if one can operate anywhere inside the 3 dB contour amplifier noise considerations may be neglected As critical as amplifier noise is in certain applications it is nevertheless possible to overemphasize its general impor tance For example if the signal amplitude 1 significantly higher than the amplifier noise the subject becomes purely academic Similarly if preamplification is provided ahead of the Model 128A with the result that the amplified source noise at the input to the Model 128A is far greater than the amplifier noise there is little point in striving to operate inside the 3 dB contour of the Model 128A However where a preamplifier is used it is important that these same considerations be carefully evaluated for the preamplifier In other words when using a preamplifier try to operate inside the 3 dB contour of the preamplifier A quick check of Figure 6 followed by a computation of the total noise Equation IV 3 should give one a realistic idea of the importance of amplifier noise considerations to the measurement at hand Where amplifier noise is a consideration one should try to operate inside the 3 dB contour by appropriately adju
64. e white orange wire from the oscillator and connect it to J419 Dress this wire along the Ref Bd Press the Oscillator board down until it snaps into place REF ised m IV 12 OSC FREQ ADJ BLACK J404 RED J405 YELLOW J406 FREQUENCY RANGE CAPACITORS MATCHED TO 5 BLACK REAR PANEL WHITE GREENJ CONNECTOR WHITE ORANGE J419 10 Connect the black and white green twisted leads fr the rear panel connector to the Oscillator bo quick disconnect contacts as shown in Figure IV This completes the installation The top and bottom co may now be reinstalled To check the oscillator sim turn on the power and monitor the oscillator output wit an oscilloscope 4 8 OPERATION WITH THE INTERNAL TUNED AMPLIFIER 4 8A INTRODUCTION In some applications it is desirable to narrow the n bandwidth ahead of the mixer or to notch out a partic frequency component of the input signal These operatio are made possible if the Model 128A 15 operated conjunction with the Model 128A 98 Accessory Tu Amplifier With this accessory installed bandpass or not operation at a of 5 is possible from 1 Hz to 100 kHz Note from Figure 1V 14 a photograph of the Model 128 with the Tuned Amplifier installed that there are trim adjustments and two switches on the Tuned Ampli circuit board In addition there are the two capacitors mounted on special spring loaded terminals which deter
65. ed Amplifier modification as well 2 Squarewave output of Reference Channel and 3 Full wave demodulated Mixer output before Time Constant filter Model 128A 98 Tuned Amplifier Modification Internal plug in board is available which provides a tuned bandpass or notch characteristic at a Q of 5 The frequency can be adjusted over a 3 1 range by means of a rear panel adjustment and can be set to any frequency from 1 Hz to 100 kHz by changing capacitors mounted on component clips located on the plug in circuit board Model 128A 99 Internal Oscillator Modification An internal low distortion oscillator is available which provides a sinewave output adjustable from O to 10V pk pk at 600 ohms The frequency is adjustable over about a 3 1 range by means of arear panel adjustment with the actual frequency range spanned by the adjustment being determined by a pair of internal capacitors mounted the oscillator circuit board Operation from about 1 Hz to 100 kHz is possible This option in conjunction with the 2f mode of operation is particularly useful for harmonic detection where the modulation frequency must be at one half the detected frequency SECTION INITIAL CHECKS 3 1 INTRODUCTION The following procedure is provided to facilitate initial performance checking of the Model 128A In general this procedure should be performed after inspecting the instrument for shipping damage any noted to be reported to the carrier
66. ence between the center conductor of the cable and the braid In the second it sees the potential difference between the Input and B Input The ground loop signal current flowing in the signal cable braid is of no consequence The very high common mode rejection of the amplifier assures that common mode power frequency pickup will not be a problem either However when operating differentially it is important to take a little trouble to assure that common mode inter ference arising in ground loops is just that that is without a significant differential component This should not prove a problem as long as both signal cables follow the same path Figure IV 3 shows the Model 128A operated differentially to measure an off ground signal The most important consideration in an application of this type is to be sure that the common mode signal component is not so large as to exceed the common mode input limit of the Model 128A See Specs 2 MOEL 128A RESISTANCE VARIES AS FUNCTION OF EXPERIMENTAL Figure IV 3 DIFFERENTIAL MEASUREMENT OF OFF GROUND SIGNAL The reduction of power frequency interference is not the only benefit to be derived from proper grounding and differential operation A much more serious source of interference is coherent interference at the signal frequency which results when drive signal current is allowed to flow through the braid of the signal cable Figure
67. erator according to the impedance and voltage requirements of his detector The parts location diagram on page shows where to install the resistors Phase Controls A high resolution potentiometer covering a range of 0 100 degrees works in conjunction with a Quadrant switch to determine the phase of the synchronous detection process with respect to the phase of the applied reference signal The Phase Sensitive Detector provides a dc output propor tional to the amplitude of the input signal This output varies with the cosine of the angle between the reference and input signals When the Phase controls are adjusted for maximum output the signal amplitude can be read from the panel meter and the phase of the signal can be read from the Phase dial It may happen that meter fluctuations due to noise will make it difficult to find the setting which gives maximum output Where this is the case it will usually prove more accurate and expedient to adjust for the null obtained when the reference phase is set at 90 relative to the signal phase Once the null is achieved the Phase Quadrant switch can then be rotated the one position necessary to achieve maximum output so that the amplitude can be read from the meter The absolute accuracy and resolution of the Phase controls are stated in the specifications Figure IV 8 shows the typical net phase shift through the unit Additional phase shifts are introduced in the Signal Channel at
68. ffect in smoothing a recording can be significant With the Time Constant switch set to EXT intermediate time constant values or time constants longer than 100 seconds can be obtained by connecting an external capaci tor between pins eight and nine of the 11 pin socket at the rear panel The formula relating the capacitor value and time constant is C TC 100 uF Any low leakage film capacitors rated at 50 V or higher can be used Do not use electrolytic or tantalum capacitors Offset The ten turn dial and its associated polarity switch allow calibrated offsets of up to ten times full scale to be applied Two applications for this feature are that it allows small amplitude variations in a signal to be expanded and examined in detail and that it allows a signal amplitude to be read with greater resolution than is possible with the panel meter alone For example suppose one had a meter indication to the right To read the amplitude with the greatest possible resolution the polarity switch would be set to and the dial adjusted for null at which time the signal amplitude could be read directly from the dial The following example illustrates how the Zero Suppress feature can be used to read signal amplitude variations Suppose one had a 70 signal Assuming this signal were measured on the 100 sensitivity range the resulting meter indication would be 70 of full scale To examine small variations in this signal one woul
69. he result will be signifi cantly increased noise and at high frequencies increased phase shift OPTIONAL 2 53 3 TUNED AMP BOARD Ve d E 1 E aj EMOVE WHE 1 WHEN 9 1 TUNED AMP sa 15 d poy ut Y JR ae 115 veo J 412 3117 3114 JHE 30 129 0 E IVS cte nct Ny Susu ira a Sy Sasu Qasi SIGNAL AMPL BOARD ull 206 1 1 T vt 1281 1800 15 45 m 15 INPUT 4103 PL E 316 PREAMP 1 l 45 ME 4 02 1 INPUT B 42 N a N E 7 z 1 S10 TOW 7 e lax We Gamme eoe 1 4 J107 227 J228 4 boue q 9 MIXER BOARD peak 9 IOK N IQ MIXER BOARD 3206 J205J221 222 3 9423 44224424 gt gt gt 94426 425 3408 0407 U asas SCHMITT 1 yaar TRIGGER d HIS 6 999 1 JK FLIP FLOP 4 ae 6 5 3418 WHEN OSC 15 f w 5 8 lt s REFERENCE BOARD i 1280 190
70. ied b c d e Set the Zero Offset Polarity switch to Then rotate the Zero Offset dial to the fully clockwise position Adjust R243 Zero Offset Cal for 0 00 V on the DVM The panel meter will also indicate 0 Set the Zero Offset Polarity switch to OFF and the Sensitivity switch to 100 mV The DVM should indicate 1 V 5 mV If it does go on to the following step If it does not it is because of interaction between R143 Gain Adj and R243 Zero Offset Cal If necessary repeat steps 3 and 4 as required to achieve the desired adjustment objectives Reset the Sensitivity to 100 mV Also set the Offset Polarity switch to the center OFF position and rotate the Offset dial fully counter clockwise Remove the DVM 5 Low Frequency CMR Adj R124 a b c d e Remove the shorting plug from the 8 Input Then connect the 100 mV rms signal from the generator simultaneously to both inputs Set the Input Selector switch to Increase the level of the input signal to 1 V rms 2 8 V pk pk The Sensitivity should remain set to 100 mV Set the Sensitivity switch to 100 uV Then adjust R124 for O panel meter indication Increase the Sensitivity to 10 Again adjust 8124 for 0 panel meter indication 6 High Frequency CMR Adj C106 a b c d e f a Set the Sensitivity switch to 250 mV Increase the signal frequenc
71. ightforward because the points to which one must gain access are provided at the rear panel 11 pin socket Two batteries are required one to supply 24 V 300 mA and the other to supply 24 V 300 mA The 24 V source should be connected to pin 7 The 24 V source should be connected to pin 5 Ground for both is at pin 1 11 is generally a good idea to fuse the battery lines external to the instrument and to provide an ON OFF switch as well The front panel ON OFF switch does not control the instrument s power when it is operated from batteries Nevertheless the battery drain is minimized if the front panel power switch is kept in the OFF position so that no current flows through the switch s built in light Keep the line disconnected during battery operation 4 7 OPERATION WITH THE INTERNAL REFERENCE OSCILLATOR 4 7 INTRODUCTION In some applications the experimental apparatus does not generate a suitable reference output but is itself capable of being driven by an external signal source To facilitate use of the Model 128A in such applications an internal reference oscillator modification is provided With this modification installed a signal of variable frequency anc amplitude is generated by a sinewave oscillator inside the Model 128A The output of this oscillator in addition tc being provided at a rear panel connector for easy routing tc the experiment can be applied internally to the Reference Channel so that it is directl
72. ignal to noise ratio at the input of the amplifier will be That this is so becomes clear when one recalls that the signal amplitude varies directly with the load resistance while the thermal noise varies with the square root of the resistance Note that the entire preceding discussion of noise is based on comparing the noise generated by the amplifier with the source thermal noise In many situations other types of noise of interference may accompany the signal as well and could even dominate it Where this is the case the amplifier can only perform better than the noise figure contours indicate because the noise figures are based on a compari son of amplifier noise with the minimum possible noise which can accompany any signal namely the source thermal noise 4 3 OPERATING THE MODEL 128A 4 3A INTRODUCTION Operationiof the Model 1284 is straightforward In most instances the operator simply connects the reference signal waits for the REF UNLOCK light to go out and then connects the signal to be measured The Sensitivity and Phase controls are then adjusted for maximum output without overload Should overload occur the dc filtering Prefilter and Time Constant is increased and or the sensitivity is reduced as required to eliminate the overload The reading can then be taken n many situations achieving a successful measurement will depend not so much on critically adjusting the Model 128A controls as on taking the pro
73. itch pointer symbolization indicates correctly The new range extends from 300 ys to 30 s in 1 3 10 sequence Several component value changes have been made in implementing the new time constants Resistors R239 and R235 are changed in value to 3 MQ and 301 respectively Capacitors C1 through C9 are all decreased in value by a factor of ten and a new capacitor one microfarad has been added to achieve the 30 SEC time constant EXT position in standard in struments The diagrams below illustrate the wiring of the rear panel switches to the spring loaded contacts of the individual accessory boards Schematics of the affected circuits are also provided Because the time constant changes only involve compo nent value changes as described above no separate schematics are furnished for the Time Constant circuits NOTCH BANDPASS SWITCH SELECTIVE FLAT SWITCH SYMBOLIZATION MODEL 129A 84 TUNED AMPLIFIER MODIFICATION BOARD FAB Z 6973 MD A 8 87278 RTOS M 6714 o2 siot A gt BRN WHT t lt REF OSC GRN WHT SWITCH pO NULL AMPLITUDE ADJUST GRY WHT BRN WHT TUNED AMP BLU WHT SWITCH TO REF OSC OUT CONNECTOR A BLACK WHITE GREEN OSCILLATOR FREQUENCY T WANS AMPLITUDE Q ADJUST SYMBOLIZATION MODEL 1294 95 M
74. itor signal is a constant 1 V rms and its source resistance is 10 The operator is not advised to use this monitor signal as the reference drive for his experiment The only other operating consideration is that of transfer ring the Model 128A from external reference operation to internal reference operation or vice versa Internal wires fitted with quick disconnect contacts determine whether the Reference channel is driven by the internal oscillator accessory or by an externally derived reference signal applied to the front panel jack To gain access to these wires it is necessary to first remove the instrument s cover which is secured by four screws When the cover is removed locate the three adjacent board contacts J418 J419 and J420 For operation in the Internal reference mode the pink wire from the front panel Ref In connector is connected 10 4418 The white orange wire from the Internal Oscillator is connected to J419 J420 is not used For operation in the external mode the pink wire from the front panel connector is connected to 419 The white orange wire is connected to J420 and J418 is not used Also the two frequency determining capacitors should be removed when operating with an external reference source 4 7C INSTALLATION When a Model 128A is ordered with the Internal Oscillator modification the instrument is shipped with the oscillator installed and the operator need only concern himself with operating consi
75. itude of 130 mV If this signal is as observed one can go on to 6 6C If the signal is not as indicated the following checks can be made to determine in which of the two Intermediate Ampli fiers the trouble is located First connect 1 resistor in series with the oscilloscope probe Then monitor the signal at the negative end of capacitor C135 The signal observed should be a sinewave with a pk pk amplitude of 1 4 V If this signal is as indicated one can conclude that the first Intermediate Amplifier is working correctly Connect the oscilloscope to quick disconnect pin J131 The signal there should be a sinewave with a pk pk amplitude of 1 4 V If this signal is as indicated the second Intermediate Amplifier is also functioning normally 6 6C FINAL AC AMPLIFIER schematic on page VII 10 Connect the oscilloscope to each of the two outputs of the Final AC Amplifier The signal observed at both points should be a 7 V pk pk sinewave at 400 Hz If this signal is as indicated one may conclude that all of the Signal Channel ac amplifiers including the two final amplifiers are functioning normally 6 7 MIXER schematic on page VII 10 1 Set the Phase switch to 270 and the Phase dial to 90 2 Verify that the 280 mV pk pk signal from the oscillator is still applied to both the Signal and 3 Reference inputs of the 128 Connect the oscilloscope to TP201 The observed signal should be a positi
76. ity than would be employed if processing a noise free signal of the same amplitude If overload proves a problem at the sensitivity which yields maximum on scale output meter deflection there are a couple of things the operator can try before resorting to lowered sensitivity operation First he can increase the output filtering using the Prefilter and Time Constant switches With a very low time constant output amplifier overload can occur when processing noisy signals This type of overload problem can generally be resolved by operating with a time constant setting of 3 SEC or higher and with the Prefilter set to 100 MSEC 1 SEC as required Additionally one can narrow the noise bandwidth ahead of the Mixer by means of the high pass and low pass filters If neither increased time constant nor the filters brings the overload under control there still remains the additional step of using the internal tuned amplifier in the case of a unit equipped with this option Assuming none of these steps helps the overload problem such as would happen if noise of sufficient amplitude to cause overload is at the frequency of the signal being measured there is no alternative but to reduce the sensitivity Hi Pass and Low Pass Filters The function of these filters is to eliminate as much interference and noise as possible while having minimal effect on the signal of interest Under most conditions by setting these filters so that they bracket the
77. le rejection Figures IV 1 and IV 2 illustrate this point Note from Figure that the signal source is located inside a grounded enclosure shield to which signal source common is attached at one point The braid of the hA SIGNAL GROUND PATH SHIELD RESISTANCE TYPICALLY 20 MODEL 128A INDUCED B DISTRIBUTED AROUND LOOP RESISTANCE OF AC GROUND PATH Figure IV 1 GROUND LOOP SUPPRESSION BY TEN OHM INPUT GROUND SHIELD MODEL 128A INDUCED emf DISTRIBUTED AROUND LOOP RESISTANCE OF AC GROUND PATH Figure 1 2 DIFFERENTIAL MEASUREMENT OF SINGLE ENDED SIGNAL signal cable is grounded directly to signal source common as well thereby assuring that no signal currents or ground loop currents will flow through the shield a desirable condition for the best possible shielding The Model 128A is operated single ended using the A input Note that the low side of the amplifier input is not grounded to the chassis directly but by way of a ten ohm resistor Further note that the braid of the signal cable is returned to this resistor and not to the chassis A ground loop generator is indicated as being connected between the chassis of the Model 128A and signal source common This path would ordinarily consist of the ac ground third wire paralleled by the braids of other cables connecting the system components The ground loop generator will cause currents at the power frequency to flo
78. lored tape to prevent them from being confused with any other capacitors with which they might be stored BLACK J119 YELLOW J118 RED J120 WHITE VIOLET J130 WHITE GREY J132 WHITE GREEN J 12 9 WHITE ORANGE 1131 NULL 4 88 OPERATION The first step is to set the Tuned Amplifier to the intended operating frequency An appropriate procedure follows NOTE Each Tuned Amplifier is preset at the factory to the frequency specified by the customer 1 Remove the top cover of the Model 128A This cover is secured four screws two on each side 2 Set the Model 128A controls as follows Sensitivity 250 mV Input A Low Pass MAX Hi Pass MIN Reference Mode f Phase switch 270 dial 90 0 Time Constant 3 SEC Zero Offset OFF dial setting irrelevant DC Prefilter OUT 3 Select and install the two capacitors which set the frequency range Be sure to use the capacitors matched to 1 NOTE One set of capacitors having the value appropriate to the frequency specified by the customer is supplied with each Tuned Amplifier 4 Turn on the Model 128A power 5 Set the Flat Selective slide switch on the Tuned Amplifier to SELECTIVE TUNED AMP FREQ ADJ ORANGE WIRE FROM CENTER PIN OF SIG MON BNC CONNECT TO J128 BLACK WIRE FROM GND LUG OF SIG MON BNC CONNECT TO FREQ RANGE CAPACITORS MATCH TO 1 NOTCH BANDPASS SWITCH SELECTIVE FLAT
79. move the cable connected to the B Input d Set the Phase switch to 0 and the Phase dial to 90 0 e Adjust R521 High Freq Phase Adj for 0 on the panel meter f Set the Sensitivity to 10 mV and adjust R521 again this time for a 1090 of full scale meter deflection This is a meter indication to the left of 0 1 on the upper meter scale This completes the alignment The test equipment can now be removed and the cover secured in place 5 SECTION VI WARNING POTENTIALLY LETHAL VOLTAGES ARE PRESENT INSIDE THIS APPARA TUS THESE SERVICE INSTRUCTIONS ARE FOR USE BY QUALIFIED PER SONNEL ONLY TO AVOID ELECTRIC SHOCK DO NOT PERFORM ANY SERVICING UNLESS YOU ARE QUALIFIED TO DO SO ANY ADJUSTMENT MAINTENANCE AND REPAIR OF THE OPENED APPARATUS UNDER VOLTAGE SHALL BE AVOIDED AS FAR AS POSSIBLE AND IF UNAVOID ABLE SHALL BE CARRIED OUT ONLY BY A SKILLED PERSON WHO IS EXPERIENCED IN WORKING ON ELECTRONIC APPARATUS AND WHOIS AWARE OF THE HAZARD INVOLVED WHEN THE INSTRUMENT IS NECTED TO A POWER SOURCE TERMINALS MAY BE LIVE AND THE OPENING OF COVERS OR REMOVAL OF PARTS IS LIKELY TO EXPOSE LIVE PARTS THE APPARATUS SHALL BE DISCONNECTED FROM ALL VOLTAGE SOURCES BEFORE IT IS OPENED FOR ANY ADJUSTMENT RE PLACEMENT MAINTENANCE OR REPAIR CAPACITORS INSIDE THE UNIT MAY STILL BE CHARGED EVEN IF THE UNIT HAS BEEN NECTED FROM ALL VOLTAGE SOURCES USERS ARE ADVISED TO WAIT SEVERAL MINUTES BEFOR
80. mponent removed 4 86 INSTALLATION When a Model 128A is ordered with the Tuned Amplifier modification the instrument is shipped with the amplifier already installed and the operator need only concern himself with operating considerations he should wish to operate without the Tuned Amplifier there are no changes to make other than to set the Selective Flat switch to FLAT although it may be desirable to touch up the setting of R143 using a test signal of accurately known amplitude In the case of a Tuned Amplifier which is ordered separately the installation is generally made by the customer Two items are supplied the first being the Tuned Amplifier itself and the second a BNC connector with two attached wires that terminate in quick disconnect contacts of the interconnections between the Tuned Amplifier and the Model 128A are by means of wires attached to the Tuned Amplifier These wires also terminate in quick disconnect contacts The following procedure can be used to make the installation 1 Remove the top cover from the Model 128A This cover is secured by four screws two on each side 2 Mount the BNC connector in the SIG MON opening in the rear panel It will be necessary to first push out the plug which can then be discarded Be sure to use the insulated bushings supplied so that there is no contact between the shell of the connector and chassis ground Then connect the orange wire attached to the conne
81. of interest If electrostatic coupling of the drive signal to the detector is a problem mounting a conducting material around the signal source detector should prove helpful The electro static shield should be connected to the system at but one point signal source common 4 2F NOISE Any electronic signal processing system adds noise to that already accompanying the signal to be measured and a Lock In Amplifier is no exception Even though the method of signal processing used in a Lock In Amplifier allows very large improvements in signal to noise ratio to be achieved the amount of noise contributed by the Lock In Amplifier itself affects its performance and limits the achievable improvement One convenient way of specifying the noise performance of an amplifier is to speak of its noise figure which indicates the amount of noise the amplifier adds to the source thermal noise Source thermal noise is used as the basis for comparison because it is completely predictable always present and is the least amount of noise which can possibly accompany any signal Its value in volts rms is given by the following formula En V4kTBR IV 1 where En rms noise voltage within the bandwidth of the measurement IV 3 k Boltzmann s constant 1 38 x 10 23 joules kelvin T absolute temperature in kelvins Resistance in ohms of the resistive component of the impedance across which the voltage 15 mea sured B Bandwidth
82. onse characteristics should not be changed The other two can be The Parts Location Diagram on page VII 2 can be used to identify these capacitors Schematically they are shown on page VII 4 In the case of the Low Pass filter the relationship between Capacitance and 3 dB down frequency is given by 66 10 1 4 RESPONSE IN UNDEFINED EXCEPT AS GIVEN IN SPECIFICATIONS RELATIVE AMPLITUDE 20 59 FREQUENCY IN DEGREES OF LAG PHASE IN MAK POSITION 15 UNDEFINED y E 20 FREQUENCY 48 ru Figure IV 10 AMPLITUDE AND PHASE CHARACTERISTICS OF LOW PASS FILTER For the Hi Pass filter is 25 10 f IV 5 where for both formulas C the capacitance in Farads and f the desired 3 dB down frequency in Hz 4 3D OUTPUT CHANNEL CONTROLS Filters The primary function of the Output Channel is to act as a low pass filter and eliminate any ac components at the output of the Mixer Inasmuch as only dc at the Mixer output represents the in phase component of the signal of interest the ac results from noise an improvement in signal to noise ratio is obtained In principle the signal to noise ratio can be improved to any arbitrary degree simply by making the filter time constant long enough Practical considerations however generally set the limit to what can be achieved The improvement in signal to noise ratio
83. ord plug provided is of the type illus trated in Figure 1 If this plug is not compatible with the available power sockets the plug or power cord should be replaced with an approved type of compatible design WARNING IF IT IS NECESSARY TO REPLACE THE POWER CORD OR THE POWER CORD PLUG THE REPLACEMENT CORD OR PLUG MUST HAVE THE SAME POLARITY AS THE ORIGINAL OTHERWISE A SAFETY HAZARD FROM ELECTRICAL SHOCK WHICH COULD RESULT IN PERSONAL INJURY OR DEATH MIGHT RESULT LINE OR ACTIVE CONDUCTOR ALSO CALLED LIVE OR HOT NEUTRAL OR IDENTIFIED CONDUCTOR EARTH OR SAFETY GROUND Figure 1 POWER CORD PLUG WITH POLARITY INDICATIONS D POWER VOLTAGE SELECTION AND LINE FUSES Before plugging in the power cord make sure that the equipment is set to the voltage of the ac power supply CAUTION THE APPARATUS DESCRIBED IN THIS MANUAL MAY BE DAMAGED IF IT IS SET FOR OPERATION FROM 110 V AC AND TURNED ON WITH 220 V AC APPLIED TO THE POWER INPUT CONNECTOR A detailed discussion of how to check and if nec essary change the power voltage setting follows The line voltage is selected by means of a rear panel switch FOR SAFETY UNPLUG THE POWER CORD WHEN CHECKING THE LINE VOLTAGE SETTING OR WHEN CHECKING THE FUSES FUSES SHOULD ONLY BE CHANGED BY QUALIFIED SERVICE PERSONNEL WHO ARE AWARE OF THE HAZARDS INVOLVED Depend ing on the switch position either 115 or 230 both are printed on the switch
84. over which the measurement is made Mathematically expressed noise figure can be stated as Noise Voltage at Output of Amplifier 2010910 That Portion of Numerator Attributable to Source Thermal Noise NF dB 2 Noise figure is not constant but varies as a functipn of the source resistance frequency and temperature When the loci of all points having the same noise figure are plotted as a function of frequency and source resistance temperature fixed the result is a noise figure contour A full set of contours completely specifies the noise characteristics cf amiptifier over its working range Figure IV 6 contains a full set of contours for a typical Model 128A The utility of these contours are first of all that they clearly indicate the best noise performance region in terms of operating frequency and source resistance and secondly that they allow one to directly compute the total noise accompany ing the signal amplifier noise and source thermal noise considered other noise sources neglected relating formula is 4 x 10 20 where is the total noise referred to the input in volts rms and all other terms are as defined previously Note that IV 3 x N lt eo z x o z lt n amp 5 a Figure IV 6 TYPICAL MODEL 128A NOISE FIGURE CONTOURS r t with noise figure of 3 dB th
85. per steps indicated by the preliminary considerations discussed in Subsection 4 2 Factors such as proper grounding and operating inside the 3 dB contour are most important in making low level measurements of noisy signals 4 38 REFERENCE CHANNEL see Subsection 4 9 for additional information Referende Signal Requirements An outstanding feature of the Model 128A is its unique reference channel circuitry which allows it to lock onto and track a wide range of possible reference input waveforms Once locked on the reference remains locked on even if the reference input signal changes in frequency There are no Reference Channel controls of any kind which must be adjusted for proper reference channel operation Once the light goes out all that remains is to adjust the Phase controls so that the Reference signal applied to the mixer is t the proper phase relative to the signal to be measured As stated in the specifications the only requirements on the reference signal are that it swing at least plus and minus 50 mV with respect to its mean that it cross its mean twice each cycle and that it remain on each side of the mean for at least 100 ns Sinewaves square waves triangle waves and many others are all suitable However for best phase accuracy rms sinewave is recommended One waveshape which at first glance may seem to suffice but which does not is the very narrow low duty factor pulse For example suppose one intended to u
86. pplication where phase accuracy is just barely large enough to provide proper reference important use of a 1 V rms sinewave reference signal is channel operation the error will much to 90 advised Other waveforms can of course be used With a than to O Any instability in the amplitude of the square wave this problem is minimized With a triangular reference signal will only compound the problem Conse wave it is even more severe than with a sinewave SECTION V WARNING POTENTIALLY LETHAL VOLTAGES ARE PRESENT INSIDE THIS APPARA TUS THESE SERVICE INSTRUCTIONS ARE FOR USE BY QUALIFIED PER SONNEL ONLY TO AVOID ELECTRIC SHOCK DO NOT PERFORM ANY SERVICING UNLESS YOU ARE QUALIFIED TO DO SO ANY ADJUSTMENT MAINTENANCE AND REPAIR OF THE OPENED APPARATUS UNDER VOLTAGE SHALL BE AVOIDED AS FAR AS POSSIBLE AND IF UNAVOID ABLE SHALL BE CARRIED OUT ONLY BY A SKILLED PERSON WHO IS EXPERIENCED IN WORKING ON ELECTRONIC APPARATUS AND WHOIS AWARE OF THE HAZARD INVOLVED WHEN THE INSTRUMENT IS CON NECTED TO A POWER SOURCE TERMINALS MAY BE LIVE AND THE OPENING OF COVERS OR REMOVAL OF PARTS IS LIKELY TO EXPOSE LIVE PARTS THE APPARATUS SHALL BE DISCONNECTED FROM ALL VOLTAGE SOURCES BEFORE IT IS OPENED FOR ANY ADJUSTMENT RE PLACEMENT MAINTENANCE OR REPAIR CAPACITORS INSIDE THE UNIT MAY STILL BE CHARGED EVEN IF THE UNIT HAS BEEN DISCON NECTED FROM ALL VOLTAGE SOURCES USERS ARE ADVISED TO WAIT SEVERAL MI
87. r can now be operated as described in Subsection 4 8B 49 MORE REFERENCE CHANNEL OPERATING HINTS 4 9A REFERENCE CHANNEL SLEWING RATE When the input frequency to the Reference channel changes the internal Reference circuitry automatically tracks so that detection is always with respect to the applied frequency However the tracking is not instan taneous with the result that there is some phase difference between the applied signal and the reference drive to the detector while the frequency is changing The maximum rate at which the Reference Input frequency can change depends on how much phase shift one is willing to tolerate The relationship linking these factors is df dt where df dt is the slewing rate in Hz s is a constant 103 and 0 is the phase lag Example If the operating frequency were nominally 1 kHz and the maximum 0 one could accept were 1 then the maximum allowable df dt would be 3 Hz s One could as well use this equation to solve for the angle 0 given some value of df dt 4 9B PHASE ERRORS WITH SMALL REFERENCE SIGNALS With reference signals on the order of 1 V rms the firing point is very near O with respect to the zero crossover of the reference input sinewave However as smaller and smaller reference signals are applied speaking of sine waves the firing point becomes further and further from Egas the crossover point introducing phase error For sinewaves quently in any a
88. radation of the signal to noise ratio ahead of the lock in amplifier The series resistor adds its thermal noise to that already accompanying the signal Although the amplifier shows a better noise figure than before it is only because the amplifier noise is now less relative to the thermal noise of the combined resistances source plus 4 series resistor The signal is larger in fact it may be attenuated the noise is greater and the improved noise performance is illusory Recall that noise figure only relates amplifier noise to thermal noise and does not denote the absolute value of amplifier noise Connecting a parallel resistor to lower a high source resistance has a similar effect Even though the thermal noise does go down the signal amplitude goes down even more For example if a source of resistance R were paralleled by another resistor of the same value the signal amplitude would go down by a factor of two However the thermal noise would only be reduced to 707 of its initial value thermal noise varies directly with the square root of the resistance with a net degradation in signal to noise ratio In operating from low source resistances however one can usually improve the situation dramatically by using a transformer to raise the source resistance seen by the amplifier The improvement one obtains with a transformer is real because the amplitude of both the signal and the noise is increased by the
89. rease the prefilter and time constant settings Also try bracketing the signal frequency with the HI PASS and LO PASS filters If these techniques don t help rotate the Sensitivity switch as far clockwise as is required to eliminate the overload Set the time constant as required to reduce the output noise to an acceptable level STEP SIX READING The input signal level is read from the panei meter f the input signal is a sinewave the meter directly indicates the rms amplitude of the input signal However if the signal frequency is near enough to a selected HI or LO PASS filter frequency to be influenced the filter the filter attenuation effects must be taken into account If the input signal is not a sinewave then the harmonic response of the instrument must be taken into account as well NI 5201 V8ZL 13GON L I 221814 espe 1 2 SECTION Il CHARACTERISTICS 2 1 INTRODUCTION The Model 128A Lock In Amplifier enables the accurate measurement of signals contaminated by broad band noise power line pickup frequency drift or other sources of interference It does this by means of an extremely narrow band detector which has the center of its passband locked to the frequency of the signal to be measured Because of the frequency lock and narrow bandwidth large improvements in signal to noise ratio can be achieved allowing the signal of interest to be
90. rhaps even to 1 5 to stop the overload In many instances use of the Prefilter will prove to be unnecessary It might be noted that the prefilter is particularly useful when a recorder is being used to monitor the output of the instrument in that recorder jitter is significantly reduced Adjusting the Model 128A s controls is generally easier with the Prefilter OUT The equivalent noise bandwidth of a single section 6 dB octave filter is 1 4TC Its rise time from 10 to 90 of full amplitude is 2 2 TC 0 to 95 is 3 TC If both the Time Constant filter and the Prefilter are set the same the effect is the same as if one had a single two section filter with a 12 dB octave rolloff The equivalent noise band width of this filter would be 1 8TC and the 10 to 90 rise time would be 3 3 TC 0 to 95 4 8 TC When both filters are used but with different settings the relationship defining the equivalent noise bandwidth and rise time as a function of time constant is more complex For all practical purposes if the time constant of one is a factor of three or more longer than the other the one with the longer time constant dominates and the single section expressions using the longer time constant characterize the rise time and equivalent noise bandwidth to a good approximation Nevertheless even though the prefilter may do relatively little to further reduce the equivalent noise bandwidth if the main Time Constant setting is longer its e
91. ropriate pins of the rear panel octal connector see BATTERY OPERATION page IV 10 A rear panel slide switch determines whether the ac power circuits are connected for operation from 100 130 from 200 260 V For operation from 100 130 V 115 should show the window For operation from 200 260 V 230 should show 4 2B FUSING The Model 128A is protected by a single fuse mounted on its rear panel A slow blow 1 4 A fuse is used for operation from 115 V A slow blow 1 10 A fuse is used for operation from 230 V It occasionally happens that a slow blow fuse fails in shipment as a result of shock and vibration Hence if the fuse is found to be bad when the instrument is operated for the first time it is advisable to try and change the fuse If normal operation follows chances are there are no other problems However if the replacement fuse fails there is something wrong which will have to be corrected before proceeding 4 20 WARM UP PERIOD For most applications five minutes Where it is desired to achieve the best possible gain and output stability allow an hour 4 2D OPERATING FREQUENCY Although one can in principle make equally accurate measurements at any frequency within the operating range of the instrument operation is simplest and least subject to error over a range having as its lower limit perhaps a few hundred Hz and as its upper perhaps 10 kHz At very low frequencies phase offsets occur which could
92. rument We reserve the right to make changes in design at any time without incurring any obligation to install same on units previously purchased THERE ARE NO WARRANTIES WHICH EXTEND BEYOND THE DESCRIPTION HEREIN THIS WARRANTY IS IN LIEU OF AND EXCLUDES ANY AND ALL OTHER WARRANTIES OR REPRE SENTATIONS EXPRESSED IMPLIED OR STATUTORY IN CLUDING MERCHANTABILITY AND FITNESS AS WELL AS ANY AND ALL OTHER OBLIGATIONS OR LIABILITIES OF EG amp G PRINCETON APPLIED RESEARCH INCLUDING BUT NOT LIMITED TO SPECIAL OR CONSEQUENTIAL DAMAGES NO PERSON FIRM OR CORPORATION IS AUTHORIZED TO ASSUME FOR EG amp G PRINCETON APPLIED RESEARCH ANY ADDITIONAL OBLIGATION OR LIABILITY NOT EXPRESSLY PROVIDED FOR HEREIN EXCEPT IN WRITING DULY EXE CUTED BY AN OFFICER OF EG amp G PRINCETON APPLIED RESEARCH Section TABLE OF CONTENTS CONDENSED OPERATING INSTRUCTIONS CHARACTERISTICS 2 1 Introduction 2 2 Specifications INITIAL CHECKS 3 1 Introduction 3 2 Equipment Needed 3 3 Procedure OPERATING INSTRUCTIONS 4 1 Introduction j 4 2 Preliminary Considerations 4 2A Power Requirements 4 2B Fusing 4 2C Warm Up Period 4 2D Operating Frequency 4 2E Grounding 4 2F Noise 4 3 Operating the Model 128A 4 3A Introduction 4 3B Reference Channel 43C Signal Channel 430 Output Channel Controls 4 4 Mixer Function and Harmonic Sensitivity 4 5 Interface Connector 4 6 Battery Operation 4 7 Operation with the internal Referen
93. s oscillating and the operator will have to wait sevi seconds for normal operation to be restored Note that amplitude adjustments affect only the amplitude of IV 10 Approx Freq Range Capacitor Value 0 53 Hz to 1 58 Hz 10 uF 1 06 Hz to 3 2 Hz 5 uF 2 7 Hz to 7 9 Hz 2 5 3 Hz to 15 8 Hz 1 uF 10 6 Hz to 32 Hz 500 nF 27 Hz to 79 Hz 200 nF 53 Hz to 158 Hz 100 nF 106 Hz to 320 Hz 50nF 270 Hz to 790 Hz 20 530 Hz to 1 5 kHz 10 nF 1 kHz to 3 2 kHz 5 nF 2 7 kHz to 7 9 kHz 2 nF 5 3 kHz to 15 kHz 1nF 10 kHz to 32 kHz 500 pF 27 kHz to 79 kHz 200 pF 44 5 kHz to 130 kHz 120 pF P A R C EDP 5 for Osc 5 match EDP 5 for Tuned Amp 196 match 1521 0193 1560 0008 1521 0105 1560 0009 1521 0207 1560 0010 1521 0066 1560 0011 1521 0061 1560 0012 1521 0208 1560 0013 1521 0186 1560 0014 1521 0184 1560 0015 1521 0209 1560 0016 1521 0182 1560 0017 1521 0023 1560 0018 1521 0211 1560 0019 1501 0032 1560 0020 1501 0004 1560 0021 1501 0068 1560 0022 1501 0034 1560 0023 Table IV 2 FREQUENCY RANGE AS A FUNCTION OF CAPACITORS signal provided at the rear panel Ref Osc Out connector impedance 600 ohms The amplitude can be adjusted from OV to 10V pk pk The amplitude of the signal supplied to the Model 128A Reference channel does not change Neither does that supplied to the Ref In connec tor which acts as a monitor point when the unit is operated in conjunction with the internal oscillator The mon
94. s IV 4 and IV 5 are provided to illustrate this problem and the steps which can be taken to prevent it To begin with Figure IV 4 shows the experiment with just about everything possible done wrong The lock in amplifier is operated single ended The ground connections at the experiment are made to the enclosure allowing currents to flow through it and in particular the drive signal currents have the opportunity to flow through the braid of the signal cable The drive signal in addition to providing the reference input signal to the Model 128A can be presumed to be driving other components of the system as well Depending on the nature of the experiment these currents could range from very small to quite large perhaps even amperes if the experiment involves driving a low impedance coil Note that the various loads for the drive are represented by a single resistor returned to ground somewhere on the enclosure Most of this drive signal current can be presumed to flow through the shield back to the drive signal source However a small but significant part of it will flow through the parallel path consisting of the braid of the signal cable the ten ohm resistor and the braid of the reference signal cable The voltage drop of this current across the resistance of the signal cable braid even though attenuated by the ratio of the ten ohm resistor to the braid resistance can constitute a serious source of interference at low signal levels
95. s perturbed by changing the frequency of the applied reference signal these blips transform into definite spikes which can be either positive or negative according to the sense of the correction to be made Monitor J413 with the oscilloscope One should observe a 400 Hz square wave having its upper level at 4 V and its lower level at 0 V If this waveform is as indicated one can conclude that the logic circuits which provide the Ref Mon output are working normally If all of these voltages and signals are as indicated one can reasonably assume that all of the tracking circuits are functioning normally The only remaining reference circuit is that which controls the Reference Unlock lamp 6 6 SIGNAL CHANNEL AMPLIFIERS 6 6A PREAMPLIFIER schematic on page VII 3 1 Connect the oscillator output still set to 280 mV 2 pk pk at 400 Hz to the A input of the 128A This signal should still be applied to the Reference Input as well Connect the oscilloscope to the negative end of capacitor 111 NOTE This capacitor is shown schematically on page VII 4 The observed signal should be a sinewave with a pk pk amplitude of 2 8 V indicating that the preamplifier has the expected gain of ten VI 2 6 6B INTERMEDIATE AC AMPLIFIERS schematic on page VII 4 1 Connect the oscilloscope to disconnect pin J128 The 2 3 observed signal should be a 400 Hz sinewave with a pk pk ampl
96. se as a reference signal the pulse train depicted in Figure IV 7 that is pulses having an amplitude of 1 V a duration of 1 us and a period of 1 ms The mean of this signal would be about 1 mV and the excursions relative to the mean would be 999 mV and 1 mV Thus the individual pulses even though they exceed the minimum excursion requirement on one side by some 950 mV lack meeting the excursion requirement on the other side of the mean by a full 49 mV 50 mV required with the result that the Reference Channel will not function properly Thus when using pulses as the reference input take care that the duration of the pulses relative to the pulse period is great enough to give a mean of at least 50 mV In the example just cited if the pulse duration were increased to 100 5 as shown in Figure 1 5 AMPLITUDE PERIOD ms DURATION 1 p MEAN EXCURSIONS 27221 1 A PULSE TRAIN WHICH WILL NOT TRIGGER BECAUSE EXCURSION CRITERIA NOT MET AMPLITUDE PERIOD 1ms DURATION 100 MEAN 100 EXCURSIONS 930 100 mV B PULSE TRAIN WHICH MEETS ALL CRITERIA INCLUDING EXCURSION REQUIREMENTS Figure IV 7 PULSE TRAIN AS REFERENCE DRIVE IV 7B the mean would increase to 100 mV and the excursions 900 mV one side 100 mV the other would be more than adequate for proper triggering Even though the Model 128A can accept and track a wide range of possible reference signals it is
97. se of a M129A should indicate near full scale to the right 10 Adjust the lock in amplifier Phase dial for peak meter indication The lock in amplifier M189 system is now tuned to 450 Hz As explained previously this same proce dure could be used to tune to other frequencies as well the lock in amplifier is to be operated as a flat frequency response instrument simply set the switch at the rear panel of the lock in amplifier to FLAT It is not necessary to disconnect the Model 189 although it can be disconnected if desired With the switch in the FLAT position the lock in amplifier functions as described in the instruction manual AC LINE FREQUENCY OPERATION It is never a good idea to operate a lock in ampli fier at the power line frequency or one of its har monics This is particularly true if a tuned amplifier such as the Model 189 is incorporated into the system Significant pickup and measure ment error will occur if operation at the power fre quency or its harmonics is attempted EXTRA GAIN OF TEN OPERATION The Model 189 has a gain of one when both of the Model 189 GAIN pushbuttons are in the out posi tion The maximum sensitivity that can be select ed at the front panel of the lock in amplifier is one microvolt By operating with the Model 189 PREAMP GAIN pushbutton depressed this sensi tivity can be increased to 100 nV Do not operate the system with the Model 189 BANDPASS GAIN pushbutton depressed T
98. signal fre RESPONSE MIN POSITION UNDEFINED EXCEPT AS GIVEN IN SPECIFICATIONS RELATIVE AMPLITUDE DEGREES OF LEAD 15 UNDEFINED Figure 1V 9 AMPLITUDE AND PHASE CHARACTERISTICS OF HI PASS FILTER IV 7 quency as closely as possible the noise tolerance of the instrument will be increased and the noise fluctuations at the output of the Model 128A will be reduced However in using the filters it is important to take their effect on the signal of interest into account Figures IV 9 and IV 10 show the amplitude and phase characteristics of these filters as a function of switch setting and frequency To find the net effect in a region where both filters affect the signal multiply the amplitude transfer fractions and add the phase shifts Use of the MIN and MAX positions simultaneously provides a flat response curve over the full operating range of the Model 128A see specifications A flat response is useful for operation in situations where the signal fre quency changes by large factors during the course of the measurement In certain applications the operator might like to have 3 dB down frequencies other than those provided This can be done by changing the value of some internal capacitors Each of the two filters has a separate capacitor for each of the three switch positions The capacitors which determine the MIN and MAX resp
99. sting the operating frequency or source resistance The choice of operating frequency is usually determined by the type of sensor used and by the capabilities of the chopper where one is used Often the experimenter has some control over his choice of frequency and so can adjust things so that he is operating at the low noise end of the frequency range available to him The situation with regard to source resistance may be less flexible because the source resistance is usually determined solely by the type of sensor used Once the commitment to a particular sensor has been made it can be difficult to adapt the system to another one Hence in applications where noise is a potential problem the choice of sensor should be made carefully and with full regard for the noise characteristics of the amplifier Two situations deserve special attention the first being operation from a source resistance very much lower than optimum and the second being operation from a source resistance very much higher than optimum In either case there may be a temptation to improve the source resistance situation by the use of resistors In the case of the low source resistance one may be tempted to connect a resistor in series with the source In the case of the high source resistance one may be tempted to connect a resistor in parallel with the source Unfortunately neither approach to the problem does any good and in fact both will result in further deg
100. te Oscillatar 47 Introduction 47B Operation 4 7C Installation 4 8 Operation with the Internal Tuned Amplifier 4 8A Introduction 48 Operation 4 8C Installation 4 9 More Reference Channel Operating 4 9A Reference Channel Slewing Rate 49B Phase Errors with Small Reference Signals ALIGNMENT 5 1 Introduction TE 5 2 Required Equipment 5 3 Preliminary Steps 5 4 Procedure 54A 15V Adjust R310 15 Check and 5 Check 5 4B Reference Board Adjustments 5 4C Signal Board Adjustment 540 Mixer Adjustments 5 4E Other Adjustments Page 1 1 M 1 11 1 111 1 111 1 IV 1 IV 1 IV 1 IV 1 IV 1 IV 1 IV 1 IV 1 IV 3 IV 5 1 5 IV 5 IV 6 8 IV 9 IV 10 IV 10 IV 10 IV 10 IV 10 1 11 IV 12 IV 12 AV 13 IV 15 1 15 15 IV 15 TROUBLESHOOTING 2 6 464 eat eode de EES 6 1 6 2 Equipment Required 4 VI 1 6 3 Initial Steps ae Aces A cedo sa esie 1 6 4 Power Supply Checks qa oh Son WED s sa ake We te ie 17 6 5 Reference Checka a Mad 6 6 Signal Channel Amplifiers 4 2 2 a oot eot m e t s t t t t n VI 2 6 6A Preamplifier gt 6 68 Intermediate 79069
101. the frequency extremes Even at middle frequencies the HI PASS and LO PASS filters can have an effect on the signal phase A phase calibration can easily be made at any frequency by connecting the input signal noiseless to both the Signal and Reference Inputs followed by adjusting the Phase controls for maximum output The Phase controls then indicate the net phase shift in both the Signal and Reference Channels This shift can then be subtracted from any phase readings taken while operating at the calibration frequency IV 6 AST REF MODE d PERIERE REF SIG IN rms SINE WAVE SIGNAL IN 100 mV rms SINE WAVE z c e 5 w lt FREQUENCY gt Figure NET PHASE SHIFT BETWEEN SIGNAL AND REFERENCE CHANNELS AS A FUNCTION OF FREQUENCY Reference Monitor Connector An optional Reference Monitor output can be provided at a rear panel connector The Reference Channel output is available at this connector This signal is a square wave taken from ahead of the Mixer but after the Phase Control circuitry and so is at the frequency of the applied reference signal twice the frequency in the case of 2f operation and at the phase set with the Phase controls Standard TTL logic levels are employed Logic 0 0 2 V 0 2 V and Logic 1 3 5 1 V 4 3C SIGNAL CHANNEL x Introduction Operation of the Signal Channel controls is straightforward The Input Selector is set to
102. ty switch to 100 mV Remove the shorting plug from the A Input Then connect the signal generator output 400 Hz 280 mV pk pk to the A Input The Reference Input should still be driven from the same signal generator Set the Input switch to Note the panel meter indication should be near full scale to the right Set the Phase switch to 90 and the Phase dial to 0 fully counterclockwise The meter indication should go to very near 0 Adjust 8438 0 Adj for exactly O0 on the panel meter 2 Reference 90 Phase Adj R437 a b Set the Phase switch to O and the Phase dial to 90 0 nine turns clockwise the fully counterclockwise position Again the panel meter indication should be approximately 0 Adjust R437 90 Adj for exactly O panel meter indication 3 Gain Adj R143 a b c d e Connect the DVM to the OUT connector Adjust the level of the input signal to exactly 100 mV rms If necessary use a calibrated ac volt meter to be assured that the input signal level is accurate to at least 1 Set the Time Constant switch to 3 SEC Set the Phase switch to 270 and the Phase dial to 90 Then adjust the dial for peak output as indicated by the DVM Adjust R143 Gain Adj for 1 000 V at the DVM 4 Zero Offset Cal R243 a Set the Sensitivity switch to 10 mV The 100 mV rms signal should still be appl
103. uency of the applied reference signal or at twice the frequency of the applied reference signal The 2f position is used for second harmonic studies For normal operation the switch is set to f To examine harmonics higher than the second the operator would have to supply a reference signal at the frequency of the harmonic to be measured One should check the position of this switch before connecting the Reference signal when the switch position is changed during operation the reference channel will unlock and time will be lost in waiting for it to lock on again In most situations the lost time would be but a few seconds and of little consequence However if one were operating at a low frequency it could be lengthy The internal switches determine the lock on range either 5 Hz 100 kHz or 5 Hz 100 kHz With these switches set to either FAST or SLOW the unit will lock onto reference signals in the frequency range of 5 Hz to 100 kHz without difficulty However only when the switches are set to SLOW will it lock onto reference signals which are below 5 Hz The switches should be set to SLOW only for operation below 5 Hz because the time required to achieve frequency lock is longer with the switch set to this position than when itis set to FAST Detector Biasing There is no provision for detector biasing at the Reference Input by means of an internal network The network is not provided but must be furnished by the op
104. urements An optional plug in oscillator internal is available for use in applications where the experiment does not produce a reference signal itself but is capable of being driven by a signal furnished by the Lock In Amplifier With its wide range of capabilities and ease of operation the Model 128A Lock In Amplifier should find extensive application in situations where the accurate measurement of signals is complicated by the presence of noise and interference 2 2 SPECIFICATIONS SIGNAL CHANNEL 1 INPUT TYPE Single ended or differential as selected by front panel switch 2 INPUT IMPEDANCE 100 shunted by no more than 20 pF 3 SENSITIVITY 12 full scale ranges in 1 2 5 10 sequence from 1 uV to 250 mV 4 FREQUENCY RANGE 0 5 Hz to 100 kHz 5 COMMON MODE REJECTION At least 100 dB at 1 kHz 6 MAXIMUM COMMON MODE VOLTAGE 3 V pk pk to 20 kHz then 6 dB octave above 20 kHz 7 DETECTOR BIAS Internal network allows dc bias current of either Polarity to be provided at the A Input to facilitate operation with diode detectors which require biasing See page 3 and Parts Location Diagram on page VI 2 8 NOISE At 1 kHz the signal channel noise will not exceed 10 2 9 LOW PASS FILTER Switch selectable 6 dB octave low pass filter which can be set to 3 dB down frequencies of 100 Hz 10 kHz or MAX greater than 100 kHz 10 HIGH PASS FILTER Switch selectable 6 dB
105. ut to the detector is a square wave having a fundamental of some fixed amplitude and odd harrnonics of lesser amplitude From Fourier analysis the amplitude of the third harmonic is 1 3 the fundamental amplitude the amplitude of the fifth is 1 5 the seventh is 1 7 etc Because the detector demodulates with respect to all components of the reference input any odd harmonic components of the input signal contribute to the output as well The response of the detector to odd harmonics is in the same proportion as the amplitude of the harmonic in the applied reference In other words the Mixer s third harmonic sensitivity is 1 3 its fundamental sensitivity its fifth harmonic sensitivity is 1 5 its seventh is 1 7 etc In the case of a square wave input if the unit does not have a tuned amplifier and if the Signal Channel filters are set to a frequency far from the principal harmonics of the input signal then 180 mV square wave will yield full scale output on the 100 mV sensitivity range If the unit has a tuned amplifier set to the fundamental frequency so that only the fundamental reaches the Mixer the input square wave must have an amplitude of 220 mV pk pk to yield full scale output on the 100 mV range the rms value of the fundamental frequency component of a 220 mV pk pk square wave is 100 mV 5 _ _
106. ve full wave rectified sinewave with a peak amplitude of 0 35 V A slight adjustment of the Phase dial may be required to obtain this waveform If this signal is as indicated one may conclude that the Mixer circuit 0201 through 0204 is functioning normally If this signal is incorrect or missing check the Mixer Reference drive signal which can be monitored at TP202 One should observe a 400 Hz square wave having its upper level at 6 V and its lower level at 12 V If this signal is as indicated one may conclude that the Reference Schmitt Trigger which directly supplies the reference signal to the Mixer circuit is functioning normally 6 8 DC AMPLIFIERS schematic on page VII 10 1 Connect the DVM to quick disconnect pin J222 With 0 the Phase dial adjusted for maximum DVM indication the voltage at J222 should be 5 V 2 V If this voltage is as indicated the First DC Amplifier is functioning normally 2 The output of the final amplifier can be measured at either the Recorder Output or at the front panel Output connector At either place the voltage should be 1 V 50 mV assuming the input signal is the specified amplitude Also the panel meter should indicate full scale If readings other than those indicated are obtained the trouble is associated with the final dc amplifier This completes the troubleshooting procedure If no indica tion of trouble h
107. w through the braid of the signal cable through the ten ohm resistor and back through the ac ground path to complete the loop Because of the ten ohm ground employed in the Model 128A these currents are attenuated over what they would be if the Model 128A input were returned directly to the chassis More importantly most of the ground loop signal is dropped across the ten ohm resistor and little across the braid of the signal cable the ratio being the ten ohms of the resistor to the 10 to 20 milliohms typical of the braid resistance As far as the input of the Model 128A 15 concerned the ground loop signal is reduced by this ratio and the ground loop interference is thus perhaps a factor of five hundred or one thousand less than would be the case without the ten ohm ground However in some applications this would not be enough Figure 1 2 shows how this same signal could be measured operating the Model 128A differentially In this instance the Model 128A Input Selector is set to and two input cables are used one connected to the signal source and the other to signal source common At the source end the braid of both cables is returned to signal source common At the lock in amplifier end the ten ohm ground serves to attenuate the ground loop currents and maintain a small ground loop signal drop across the braids the same as in Figure IV 1 However in the first instance the amplifier looked at the potential differ
108. will be visible to the viewer For operation from a line voltage from 100 V ac to 130 V ac 50 60 Hz 115 should show SECTION 1 CONDENSED OPERATING INSTRUCTIONS The following condensed operating instructions are pro vided as an assistance in placing the Model 128A Lock In Amplifier into operation as quickly as possible Generally speaking these condensed instructions will allow good re sults to be obtained in most instances However because of the brevity of these instructions many considerations hav ing importance in particular applications have been fore STEP ONE PRELIMINARY STEPS Check that the rear panel 115 230 V switch is in the proper position Remove the top cover and verify that the two Reference Lock On Speed switches are properly set For operation below 5 Hz they should be set to SLOW For operations above 5 Hz they should be set to FAST Units are shipped with these switches set to FAST Replace the cover and plug in the line cord Turn the power STEP TWO REFERENCE CHANNEL Check that the Reference Mode switch is set to the proper position With the switch set f the detector is driven at the frequency of the applied reference signal Set to 2f the detector is driven at twice the frequency of the applied reference to facilitate second harmonic measurements Next connect the reference signal 100 mV pk pk or greater to the Reference Input connector and wait for the REF UNL
109. xed amplitude output with the two to be in phase The fixed output is used as the reference drive to the lock in amplifier and need not be a sinewave One suitable oscillator would be the Krohn hite Model 4200 One could also use a single output oscillator followed by a 10 1 attenuator 3 Digital Voltmeter such as the Fairchild Model 7000 4 Two shorting plugs CW 159 U Amphenol or equiva lent 5 Cables for interconnecting the above items 6 Three small jumper cables 5 3 PRELIMINARY STEPS 1 Remove the top cover which is secured by four SCrews two on each side 2 If the unit contains a tuned amplifier or an internal oscillator take the necessary steps to render these accessories inactive that is the Model 128A should be operated in the external reference mode and the signal channel response should be flat 3 Check and if necessary adjust the mechanical zero of the panel meter Then plug in the Model 128A turn on the power and allow a fifteen minute warmup 4 Connect BNC shorting plugs to both inputs 5 4 PROCEDURE 5 4A 15 V ADJUST R310 15 V CHECK and 5 V CHECK 1 Connect the DVM to the positive end of capacitor C309 Then adjust R310 415 V ADJ for a DVM reading of 15 50 V 2 Remove the DVM from C309 and transfer it to the n gative end of C310 The voltage should be 15 5 V 0 2 V Remove the DVM from C310 and transfer it to the positive end of C312 The voltage should be
110. y driven by the same signal a drives the experiment is also provided at the front pane Reference INPUT connector at an impedance of 10 fo monitoring purposes only The Phase controls remain full functional allowing the internal reference signal and th information signal from the experiment to be brdught int phase at the mixer 4 78 OPERATION Operation of the internal oscillator is straightforward Tw openings in the rear panel of the Model 128A give access the adjustments which set the amplitude and frequenc The frequency adjustment allows the frequency to varied over about 3 1 range with the actual ran spanned depending on the value of two capacitors mounte on the oscillator board Table 2 indicates the frequen range of the adjustment as a function of the value of the capacitors The capacitors can be easily changed they a held by spring loaded special clips which release t component lead when pressed downwards The capacitc should be low leakage types matched to within 5 Myl polystyrene polycarbonate teflon and other film capa tors rated at 50 V or better are all suitable Do not 1 electrolytic or tantalum capacitors Because the oscilla board plugs into the main board one must remove the t cover of the Model 128A to change the capacitors In setting the oscillator frequency adjustment it is imp tant that the control setting not be changed too rapidly the adjustment is turned quickly the oscillator will
111. y to 100 kHz and set the amplitude of the input signal to 250 mV rms 0 7 V pk pk Set the Sensitivity switch to 25 UV Connect the oscilloscope with probe to J128 quick disconnect at rear of Signal board Adjust trim capacitor C106 for minimum ob served signal should be possible to adjust the Observed signal to less than 60 mV pk pk Set the Sensitivity switch to 100 mV 7 High Frequency Phase Adj R521 Make provision for supplying in phase signals to the Model 128A The signal to be applied to the Reference Input should have an amplitude of b nominally 1 V rms while that applied to the Signal Input should be 100 mV rms is absolutely essential that the two signals be in phase This is assured by using a single source and a simple series 10 1 divider A 910 ohm com position resistor in series with a 100 ohm composition resistor makes a suitable divider No great divider accuracy or precision is required Ordinary 5 resistors are quite adequate A signal having a nominal one volt rms amplitude is applied to the divider and to the Reference Input of the lock in amplifier while the signal applied to the Signal Input connector is taken from the junction of the two divider resistors Set the signal frequency to 100 kHz and adjust the signal source output amplitude so that the signal applied to the Model 128A Input is 100 mV rms Set the Input Coupling switch to A Then re

model 128a lock-in amplifier operating and service manual

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