HP 431B power meter - University of California, Berkeley
Contents
1.2. ns I i 18 i r i i i Mie T 1 i El i 1 Riss 1 i i 113 i os i RI66 8171 2 52V RI77 l i i H H 38 05 41 46 2481 256 8 1 171 i i o 1L03 Me E FEEDBACK z E _ CURRENT NE i m 5 DC FEEDBACK des 854 0003 4 1 t 1 1 1 1 8 t V 1 25V UNREG NOTES 18V REG 1 DC VOLTAGE LIMITS GIVEN AT REFERENCE DESIGNATORS VARIOUS POINTS THRU CIRCUIT 1 THERMISTOR MOUNT 15 1 CONNECTED TO THE 4318 CIO 106 HO 125 FOR THESE MEASUREMENTS CRIOI 18 2 L CHASSIS GROUND 102 103 110 105 GROUND ON THE PRINTED Mo CIRCUIT BOARD DIFFERENTIAL a 4 ALL VALUES IN OHMS AND AMPLIFIER qol PICOFARADS UNLESS RIOI 103 105 11 H3 144 OTHERWISE INDICATED Qio4 Q105 150 155 160 181 241370 5 FRONT PANEL 5101 102 ENGRAVING gt 8 p 4 REAR PANEL wt t 7 J2 ENGRAVING Deo UNASSIGNED Ril2 6 ROTARY SWITCH OR POTENTIOMETER RI43 7 DC 990 OKC ERROR SIGNAL t1 5V 8 X VALUE SELECTED AT FACTORY UNREG AVERAGE VALUE SHOWN 9 0 VOLTAGE MEASURED WITH RESPECT TO 18 REG 10 TEST POINT COPYRIGHT 1962 HEWLETT PACKARD COMPANY 4318 PWR METER I PAGE 2 OF 2 i A Figure 5 3 Pomer Meter Assembly
3. 25 N m gt to a A ED EE o de ES NOTES D CONNECTION BETWEEN PIN 5 ON AND TERM 4 ON ASSY 101 IN STANDARD 4318 8 OPTION 10 OTRER OPTIONS CONNECTION PRESES 10 1821 112822 13823 318 11 320 10 323 40 329 8n fro on R103 202 5N 206 60 213 00 RIOS 190 2 192 720 197 7 20820 256 00 258 00 261 40 26820 NULL i THERMAL CONDUCTING ADJUST H BLOCK sema m um um ma ei ce mii an as es Se COAXIAL THERMISTOR MOUNT 2 478A PAGE J OF 2 P O _ ee a nl Figure 5 3 Power Meter Assembly 25 12 01370 2 431 ci20 47UF R152 3000 01370 2 10 OSCILLATOR AMPLIFIER FOLLOWER 2N383 2N388A AMPLIFIER IOKC AMPLIFIER AMPLIFIER AMPLIFIER 8 SHORT BASE TO EMITTER OF QIO FOR THIS MEASUREMENT IST IOKC AMPLIFIER 241370 POWER METER ASSEMBLY Section V FEEDBACK CURRENT GENERATOR 9107 1854 0003 15 UNREG ge Figure 5 3 P H RIGO RIGT i i 10 46 R173 i 1 1194 22 82 09K 57 46 1 i i o H ww or i 3 2130 e 1 XRI62 i mio o md
4. OPERATING AND SERVICE MANUAL MODEL 431B POWER METER SERIALS PREFIXED 301 233 amp 221 HEWLETT PACKARD COMPANY 1962 1501 MILL ROAD PALO ALTO CALIFORNIA U S A 01370 2 Printed MAR 1964 1 0 Section Figure 1 1 Figure 1 1 Mode 4315 Power Meter Model 4318 Model 4318 Section Paragraphs 1 1 to 1 4 SECTION 1 GENERAL INFORMATION 1 1 DESCRIPTION 1 2 The B Model 4318 Power Meter with jtemper ature compensated thermistor mounts measures rf power from 10 microwatts 20 dbm to 10 milliwatts 410 dbm in the 10 to 40 gc frequency range Direct reading accuracy of the instrument is 3 of full scale Instrument specifications are given in table 1 1 1 3 The design of the Model 431Band itsthermistor mount results in almost complete freedom from measurement error caused by ambient temperature changes The instrument incorporates two self balancing bridges with one arm of each bridge being a thermistor The two matched thermistors both located within the mount are thermally coupled but Table 1 1 Instrument Type Automatic self balancing for temperature com pensated mounts Power Ranges ranges with full scale readings of 10 30 100 and 300 uw 1 3 and 10 mw Also calibrated in from 20 to 10 External Bolometer Temperature compensated thermistor mounts required for operation dd 478A and 486A series Accuracy 3 of fu
5. thus the action which balances the rf bridge unbalances the metering bridge The metering bridge loop automatically re balances by substituting de power for 10 power Since the 10 kc power equaled the applied rf power the substituted de power is also equal to the applied rf power Instead of metering the feedback current directly which would require the use of a nonlinear meter scale an analog current is derived which is proportional to the square of the feedback Since power is square law function of current the analog current thus derived is pro portional to rf power making possible the use ofa linear scale on the meter 4 5 There is little drift of the power meter zero point when ambient temperature at the thermistor mount changes If for example ambient temperature at the mount increases a decrease in electrical power to the thermistors is required to hold their operating resistances constant The decrease for both therm istors is made automatically by the detection loop figure 4 1 which reduces 10 kc power The amount of dc power in the metering loop remains unchanged however and since this de power controls the meter action the ambient temperature changes do not affect the meter indication The compensation capa bility depends upon the match of thermistor tempera ture characteristics When thermistor mounts are built the thermistors are selected to insure optimum match of thermal characteristics 01370
6. Section IV Paragraphs 4 1 to 4 11 SECTION IV THEORY OF OPERATION 4 1 OVERALL DESCRIPTION 4 2 Figure 4 1 is a block diagram which shows the Model 431B Power Meter and its associated thermistor mount The thermistor mount contains twothermistor elements and Re Thermistor element absorbs the rf power applied to the mount thermistor element converts the applied rf power toa meter indication and provides compensation for ambient temperature changes at the thermistor mount 4 3 The power meter circuitry incorporates two bridges which are made seif balancing by means of Separate feedback loops Regenerative positive feed back is used in the detection loop degenerative feed back in the metering loop One thermistor element is used in one arm of each of the self balancing bridges In the detection loop the 10 ke oscillator amplifier supplies enough 10 kc power 110 tobias thermistor element Rg to the operating resistance which balances the rf bridge The same amount of 10 ke power is also supplied to thermistor element by the series con nected primaries of transformers T101 and T102 4 4 When rf power is applied to thermistor element Rg an amount of 10 kc power equal to the rf power is removed from thermistor element by the self balaneing action of the rf bridge Since the primaries of 101 and T102 are series connected the same amount of 10 kc power is also removed from thermistor element
7. and zero set the meter with ZERO and VERNIER controls Note Zero Set accuracy of 1 can be obtained by zero setting the meter on the most Sensitive range 01 MW only and assum ing the meter is properly zeroed on allless Sensitive ranges For maximum accuracy Zero set the meter onthe range to be used Apply rf power at the thermistor mount and read power on Model 4318 meter Power is indieated on the meter directly in mw or dbm Note This instrument is accurate to within 39 Aceuracy to 1 or better is possible using the de substitution technique de seribed in figure 3 3 Seealsoparagraphs 3 16 and 3 17 Figure 3 2 Turn On and Nulling Procedure 01370 2 Section Model 431B Figure 3 3 POWER DIGITAL VOLTMETER gt 9 0 20ma CEI wooe 431B 971A OWER METER IK 05 IW 11 41 10K 10 IOW 1 With power supply turned off connect the CAUTION Model 431B as shown above Never apply more than 20 ma dctothe DC CALIBRATION amp SUBSTITUTION termi 2 Set the Model 431B for normal operation on the appropriate range using the procedure given in nale or the Model 4918 figure 3 2 6 Readthe voltmeter which monitors the substitu 3 Apply rf power at the thermistor mount and tion current The voltmeter reading can be note and record the reading of the Model 431B interpreted as current in milliamperes because meter This is the reference for the substi the voltage is measured aeross 100
8. 4 Nulling Circuit 4 16 The reactive components of the metering bridge are balanced with variable capacitor C103 and inductor L102 Null adjust C103 is an operational adjustment and L102 is a maintenance adjustment Null adjust C103 is adjusted with the RANGE switch inthe NULL position simplified schematic diagram ofthe NULL circuit is shown in figure 4 4 The 10 kc signal is taken at the synchronous detector rectified by CR105 and read on the meter The rectified signal contains both reactive and resistive voltage components of the bridge unbalance 4 17 SYNCHRONOUS DETECTOR 4 18 The synchronous detector converts the 10 error signal from the metering bridge to avarying de signal A simplified schematic of the synchronous detector is shown in figure 4 5 The detector isa bridge rectifier which has a rectifier in series with a linearizing resistance in each of its arms Two 10 ke voltages designated E3 and Ed in figure 4 5 are applied to the bridge 1 voltage E3 induced in the secondary of transformer T103 is proportional tothe metering bridge error signal and is incoming from 10 ke tuned amplifier Q103 2 voltage E4 induced in the secondary of T104 is proportional to a voltage supplied by the 10 kc oscillator amplifier Voltage E4 is much larger than voltage E3and switches appro priate diodes in and out of the circuit to rectify voltage Section a of figure 4 5 shows the current path through diodes CR102 an
9. compensated thermistor mounts 7 1 2 ft 2 3 m power cable NEMA plug Accessories Available 431 95 Rechargeable Battery Pack for field installation Models 478A and 486A Thermistor Mounts Model 8402A Power Meter Calibrator 8 Model H01 8401A Leveler Amplifier Options 01 Rechargeable battery installed provides up to 24 hours continuous operation 02 Rear input connector wired in parallel with front panel input connector 10 With 20 foot cable for 100 Q or 200 Q mount 11 With 50 foot cable for 100 amp mount 12 With 100 foot cable for 100 2 mount 18 with 200 foot cable for 100 9 mount 21 With 50 foot cable for 200 mount 22 With 100 foot cable for 200 0 mount 23 With 200 foot cable for 200 2 mount Section I Paragraphs 1 5 to 1 11 Table 1 2 Model 4318 Thermistor Mounts Frequency Resistance Coaxial Waveguide Range in ohms X486A 8 2 to 12 4 ge 100 M486A 10 0 to 15 0 6 100 12 4 to 18 0 gel 100 K486A 18 0 to 26 5 K486AC R486A 96 5 to 40 0 2 R486AC With circular contact flange adapter 1 5 The Model 4318 has provisions for using the 40 substitution method of measurement and for check ing ealibration accuracy of the power meter The dc substitution method of measurement which requires other equipment provides greater power measurement accuracies than can be obtained by the power meter 1
10. operation When the battery is fully charged the Model 431B can be continuousiy operated for 24 hours with 48 hours of charge time However it is recom mended that battery operated instruments be operated for eight hour periods with 16 hour recharge time This makes the Model 431B available for portable use daily yet maintains the battery at full charge 3 13 BATTERY CHARGE CHECK 3 14 Under normal conditions a fully charged battery will start at approximately 27 volts and drop to about 22 volts after 24 hours of continuous use at room temperature a Connect the Model 431B to primary power Set POWER to AC and perform the turn onand nulling 01370 2 procedure given in figure 3 2 This will check for normal operation from ac primary power If perform ance is normal proceed to step b b Set POWER to BATTERY CHARGE the AC CHARGE lamp will glow Allow Model 431B to charge the battery for 48 hours This wil allow the battery to obtain a full charge After the recharge interval set POWER to BATTERY ON Since battery is now fuily charged you should be able to zero set and null the meter figure 3 2 If not the battery or battery charging circuit is at fault Refer to Battery and Charging Checks paragraph 5 40 3 15 MAJOR SOURCES OF ERROR MICROWAVE POWER MEASUREMENTS 3 16 In microwave power measurements the follow ing are the major sources of error 1 mismatch error or tuner loss when a tuner is use
11. the same degree dc power Sub stitution error and mount efficiency are often combined forsimplicity of measurement into what 18 termed calibration factor Typically the calibration factor of the Model X486A waveguide mount is 97 to 98 8 1 Section Figure 3 1 4318 PONER METER THERMISTOR MOUNT i POWER The POWER switch sets up connections 3 2 THERMISTOR MOUNT to the selected power sources or to the battery charging circuit When the power switch is in the AC position externally supplied 115 or 230 volts is applied to the instrument Ifthe instru ment contains a battery a trickle charge is applied to maintain the battery at full charge With POWER at BATTERY ON 24 vde battery within the instrument supplies primary power to the instrument With POWER at CHARGE 115 and 230 volt power is used to charge the battery 16 to 24 hours is required to obtain full battery charge The instrument is in operative in this position Note Batteries are installed at the factory for option 01 instru ments only RANGE The RANGE switch can be set for full scale power readings from 01 to 10 milliwatts in seven steps It also includes a NULL posi tion which in conjunetion with the adjacent null serewdriver adjust insures that meter ing bridge is reactively balanced The THERMISTOR MOUNT connector is a female receptacle that accepts a specially made cable which is supplied with the instrume
12. thermistor returns to operating resistance and the detection loop regains equilibrium 4 11 The MOUNT RES switch 101 changes the resistance arm of the rf bridge so thatthe bridge will function with either a 100 or 200 ohmthermistor mount BIAS METERING BRIDGE RF SHIELD 1056 OSCILLATOR AMPLIFIER 3 no ERROR SIGNAL Figure 4 2 RF Circuit Section IV Paragraphs 4 12 to 4 19 4 12 METERING BRIDGE CIRCUIT 4 13 A simplified schematic diagram ofthe metering bridge circuit is shown in figure 4 3 Operationof the metering bridge circuit is similartothe rfbridge cir cuit It uses the same principle of self balancing through a closed loop metering loop The major dif ference is that dc rather than 10 ke power is used to rebalance the loop The resistive balance point is adjusted by the ZERO and VERNIER controls which constitute one arm of the bridge The MOUNT RES switch not shown in figure 4 3 whichis mechanically linked to both the rf bridge and metering bridge changes metering bridge reference resistance from 100 to 200 ohms When the MOUNT RES switch is in the 200 ohm position some of the feedback current is shunted to ground through R101 This maintains the I R function constant when mount resistance is changed from 100 or 200 ohms The switch also adds the necessary reactance for each position 4 14 The same 10 kc power change produced in the rf bridge by rf power
13. 0 ohms tution measurement This current is Iac Note 7 Calculate power in mw from the expression second digital voltmeter in parallel with 1000 ohm 4109 1 watt resistor 12 R connected in series with the RECORDER Power MW de d output of the Model 431B will increase 4 x 103 accuracy of reference duplication where operating resistance of the term 4 Turn off or disconnect the rf source istor 100 or 200 ohms 5 Turn power suppiy on adjust the output voltage and I de 7 substitution current in milliamps of the power supply until the reference of step 3 from step 6 is duplicated A potentiometer arrangement may be substituted for the adjustable power 9 To minimize error due to drift in either the supply However at least 10 000 ohms must reference or substituted power level steps 1 remain in series with the supply through 6 should be repeated 3 4 Figure 3 3 DC Substitution Technique 01370 2 Model 431B Instrument Error This is the inability of the power meter to accurately measure interpret the information available at the thermistor elementi In Specifying the accuracy power meter instrument erroris the figure usually given For the Model 431B instrument error is 3 of full scale 20 to 35 This error can be reduced by special techniques such as the dc substitution method discussed in para 3 17 d Error Due to the Unilateral Properties of a Thermi
14. 2 4 6 CIRCUIT DESCRIPTION 4 1 BRIDGE CIRCUIT 4 8 simplified schematic diagram of the rf bridge circuit is shown in figure 4 2 The rf bridge circuit consists of the rf bridge and 10 kc oscillator amplifier The rf bridge includes thermistor the secondary winding of T101 resistors R102 and R103 the MOUNT RES switch 5101 and eapacitance represented by and Cy The rf bridge and 10 kc oscillator amplifier are connected in a closed loop the detection loop which provides regenerative feedback for the oscillator amplifier This feedback causes the 10 kc oscillator amplifier to oscillate 4 9 When the power meter is off thermistor is at ambient temperature and its resistance is about 1500 ohms the rf bridge is unbalanced When the power meter is turned on this unbalance of the rf bridge causes a large error signal to be applied to the 10 oscillator amplifier Consequently maximum 10 bias voltage is applied to the rf bridge As this 10 ke voltage biases to its operating resistance 100 or 200 ohms the rf bridge approaches a state of balance and regenerative feedback diminishes untilthere is just sufficient 10 kc bias power to hold at operating re sistance This condition is equilibrium for the detection loop 4 10 With application of rf power thermistor resistance decreases causing the regenerative signal from the rf bridge to decrease Accordingly 10 power diminishes the
15. 2 Model 431B alone In additiona jackin series with the panel meter permits digital or chart recording of measurements operation of alarm or control systems and use ina closed loop leveling system 1 6 ACCESSORIES 1 7 Two accessories are supplied with the Model 4318 Power a 7 1 2 foot detachable power cable and 5 foot cable that connects the thermistor mount to the Model 4318 Thermistor mounts are available see table 1 2 but not supplied with the instrument A rechargeable battery with installation kit is also available A list of supplied and available accessories is given in table 1 1 Specifications 1 8 INSTRUMENTS WITH OPTIONS 1 9 The options available with the Model 4318 Power Meter are given in table 1 1 The thermistor mount cable options require modification and recali bration of the Model 431B Power Meter The recali bration procedures for the cables are given in section V Maintenance under Oscillator Frequency Adjust ment paragraph 5 58 and Coarse Null Adjustment paragraph 5 63 1 10 INSTRUMENT IDENTIFICATION 1 11 Hewlett Packard uses a two section eight digit serial number 000 00000 If the first three digits of the serial number on your instrument do not agree with those on the title page of this manual change Sheets supplied withthe manual will define differences between your instrument andthe Model 431B described in this manual 01370 2 Model 4318 Section Paragr
16. 431B D MODEL 405 DIGITAL VOLTRE TER 80 5 253 Figure 8 5 Obtaining Increased Resolution 3 27 INCREASED RESOLUTION Digitai readout of power to three decimal places can be obtained with the arrangement shown in figure 3 5 The value of R1 18 316 2 ohms 1 and R4 is 1000 ohms 4 196 Correct placement of the decimal in the readout is determined by the setting of the power meter RANGE switch On the divider switch arrangement at the voltmeter input may be replaced by a single 1000 ohm 1 resistor With this arrangement on the 01 1 and 10 MW ranges power is read in the same way as when the arrangement shown in figure 3 5 is used decimai placement being determined by the setting of RANGE On the 03 3 and 3 MW ranges however to obtain the power readings the volimeter indication must be multiplied by the factor given in table 3 1 Table 3 1 Voltmeter Readout to Power Multipliers 0 0316 0 316 3 16 3 28 LEVELER Figure 3 6 is a block diagram of a closed loop control circuit for maintaining output power at a constant level It is recommended for use in leveling the output of various types of microwave equipment such as bwo sweep oscillators twt micro wave amplifiers and rf generators In additionto the Model 431B and its thermistor mount such a leveling system requires the H01 8401A Leveler Amplifier and a directional coupler with good directivity suchas one of the 752 series of waveguide coupl
17. also affects the metering bridge through the series connection of T101 and T102 pri maries Although this change of 10 kc power has equal effect on both the rf and metering bridges it is initiated by the rf bridge circuit alone The metering bridge cannot control 10 ke bias power but the 10 ke bias power does affect the metering circuit Once a change in the 10 kc bias power has affected unbalanced the metering bridge a separate closed dc feedback loop metering loop re establishes equilibrium in the metering circuit 4 15 Variations in 10 ke bias level initiated in the rf bridge circuit cause proportional unbalance of the metering bridge and there is a change in the 10 ke error signal 810 ke applied to the 10 ke tuned ampli fiers in the metering loop These error signal vari ations are amplified by three 10 kc amplifiers and rectified by the synchronous detector From the synchronous detector the de equivalent Ige of the 10 ke signal is returned to the metering bridge and is monitored by the metering circuit to be indicated by the meter This dc feedback to the metering bridge acts to return bridge to its normal near balance condition FROM IO KC METERING RF BRIDGE BIAS BRIDGE lioke ph ey L 104 105 1107 0 5 178 Figure 4 3 Metering Bridge Circuit 4 2 Model 431B FROM BRIDGE BIAS AND 10 ERROR SIGNAL COMPENSATION THERMISTOR Figure 4
18. aphs 3 1 to 3 16 SECTION OPERATION 3 1 INTRODUCTION 3 2 The Model 4318 Power Meter measures rf power ranging from 01 to 10 millivatts with power meter accuracy of 3 Since the zero carries over within 1 accuracies of at least 4 can be obtained on any range by a single zeroing on only the lowest range 3 3 MECHANICAL ADJUSTMENT OF METER ZERO 3 4 The procedure for performing the mechanical adjustment of the meter zero is given in section V paragraph 5 54 3 5 CONTROLS AND INDICATORS 3 6 The front and rear panel controls and connectors are explained in figure 3 1 The explanations are keyed to corresponding controls and indicator on the drawing of the front and rear panels ofthe instrument provided with the figure 3 7 OPERATING INSTRUCTIONS 3 8 Figure 3 2 Turn On and Nulling Procedure and figure 3 3 DC Substitution Technique give step by step instructions for operating the Model 431B In figure 3 2 each step is numbered to correspond with numbers on the accompanying drawing of the power meter 3 9 BATTERY OPERATION 3 10 The following applies to power meters having a factory or a field installed rechargeable nickel cad mium battery See figure 3 1 Turn On and Nulling Procedure for step by step instructions for operating the Model 431B from a battery 3 11 BATTERY CHARGING TIMES 3 12 The battery used in the Model 4318 requires two hours of charge time for one hour of battery
19. ch to change metering loop gain so that the meter will read full Scale for each power range Diode CR107 provides additional temperature compensation for 9107 4 25 METER CIRCUIT 4 26 The meter circuit is shown in figure 4 8 It includes feedback current squared generator 9106 squaring circuit the meter and RECORDER jack J102 The purpose of the meter circuit is to convert linear voltage function proportional to applied power to a squared function so that power may be indicated on a linear meter scale The linear voltage function is applied to the base of Q106 and is converted to square law function by the squaring circuit in Series with Q106 emitter 4 21 SQUARING CIRCUIT The squaring circuit in cludes diodes CR109 113 and resistors Ri67 177 Temperature compensation for the squaring circuit is provided by CR108 4 28 The design of the squaring circuit is such that individual diodes conduct at discrete values of emitter voltage so that emitter conductance approximates a Square law function Thus the collector current of Q106 is made to approximate a square law function and the meter indicates power on a linear scale 4 29 RECORDER OUTPUT The current which drives the meter can be monitored at the RECORDER output a telephone type two wire jack A RESISTOR OF 1000 OHMS MUST REMAIN IN SERIES WITH THE METER FOR ALL APPLICATIONS USING THE METER DRIVING CURRENT 4 30 ZEROING Perfect balance of the metering bri
20. d CR104 for a positive going signal section b shows the current path through diodes CR101 and CR103 for a negative going signal The rectified output is taken at the center taps of trans formers T103 and T104 4 19 Operation of the circuit is as follows When the left side of T104 is positive with respect to the right side as in figure 4 5a diodes CR102 and CR104 on duct while diodes CR101 and CR103 are biased off With the polarities reversed as in figure 4 55 the 01370 2 Model 431 SYNCHRONOUS DETECTOR FROM iOKC TUNED AMPLIFIER RECTIFIED JOR gt OUTPUT FR t OSCILLATOR AMPLIFIER 1 on cde c lt gt a Section Paragraphs 4 20 to 4 24 SYNCHRONOUS DETECTOR FROM IOKC TUNED AMPLIFIER 9105 loz RECTIFIED IOKC OUTPUT 4 ue b Figure 4 5 Synchronous Detector diodes CR102 and CR104 are biased off The resultant output is a pulsating dc signalequivalenttothe applied 10 kc error signal This puisating de signalis filtered and applied to differential amplifier Q104 Q105 4 20 Proper synchronous detector output requires an in phase relationship between E3 and E4 and for ampli tude of E4 to be larger than that of 4 21 DIFFERENTIAL AMPLIFIER 0104 0105 4 22 simplified schematic diagram of the amplifier is shown in figure 4 6 The pulsating dc from the synchrono
21. d to tune out mismatch error 2 bolometer mount efficiency 3 substitution error 4 instrument error and 5 error due to the unilateral properties ofa thermistor Thus five errors must be known if accurate power measurements are to be obtained Expressed mathematically Total measurement error mismatch or tuner loss calibration factor instrument error error due to the unilateral properties of a thermistor a Mismatch Loss Unless the mount and rfsource are perfectly matched to the transmission system a fraction of incident power is reflected and does not reach the thermistor Since there generally is more than one source of mismatch in a microwave meas urement system and the resulting error signals inter act loss cannot be calculated from the swr figure it can only be expressed as lying between two limits Limits of mismatch loss generally are determined by means of a chart such as the Mismatch Loss Limits chart included in each of the thermistor mount Operating Notes A tuner such as the Model 872A or 870A can be used to minimize loss although the tuner itself will introduce some 1088 b Bolometer Mount Efficiency and Substitution Error Not al the rf power applied to the mount is used to heat the rf thermistor Some of it is absorbed by the other elements in the mount such as the walls of the rf chamber the heat sinks theleads etc Sub Stitution error results because rf power does not affect the thermistor to
22. dge would mean that no 10 kc error signal would be applied to the 10 kc amplifiers there would be no de feedback from Q107 and the metering loop would be open With an open metering loop zero reference could not be accurately established In the Model 4 4 Model 4318 TO METERING RF IOKC 8145 BRIDGE BRIDGE TIOI DETECTION foc caLiBRaT ION L 8 SUBSTITUTION 102 I 0 KC ERROR SIGNAL SD 176 Figure 4 9 DC Calibration and Substitution 431B this occurrence is prevented by insuring a closed metering loop even when the ZERO control causes the meter pointer to deflect downseale from zero By the combined actions of R141 and R179 the zero setting of the meter pointer does not coincide with absolute balance of the metering bridge A slight unbalance of the bridge is maintained by R 4i while R179 provides counter action in the feedback current squared generator Q106 so that the meter can indicate zero even though the metering bridge is not perfectly balanced Resistor R179 aiso sets the full scale accuracy of the meter 4 31 DC CALIBRATION AND SUBSTITUTION 4 32 simplified schematic diagram of the de cali bration and substitution circuit is shown in figure 4 9 Highly accurate rf power measurements can be made using the dc substitution technique given in figure 3 3 In the dc substitution method is used to duplicate the rf power reading An accurate known current is s
23. e at that frequency RECORDER 4102 TEMPERATURE 8401A COMPENSATED bir THERMISTOR AMPLIFIER MOUNT C 418h 486A AMPLITUDE MODULATION INPUT DIRECTIONAL POWER COUPLER SOURCE 4 CONTROL VOLTAGE NEGATIVE FEEDBACK B0 254 Figure 3 6 Leveler Setup 3 6 01370 2 Section Model 4318 Fx uu E DE Figures 3 7 and 3 8 x Jette TEMPERATURE Soda ALARM COMPENSATED 141 06 T CONTROL THERMISTOR H AMPLIFIER PRTECTIvE OR DEVICES B0 8 255 RF SOURCE Figure 3 7 Monitoring Control Systems TEMPERATURE COMPENSATEO THERMISTOR M 2 4 TI MICROWAVE DEVICE 80 5 256 Figure 3 8 Determining Insertion Loss Gain 01370 2 Section IV Figure 4 1 06 CALIBRATION N i DETECTION THERMISTOR MICROWAVE POWER CURRENT CONTROL amp VERNIER 115 230 50 1000 5 OR 24V DC OPTIONAL BATTERY tork AMPLIFIER 0101 0103 I 5102 ES 7771 FEEDBACK CURRENT GENERATOR 0107 se ge 1 5V0C OWE 18V DC REG SUPPLY 25V0C REG REGULATED POWER Model 4318 Ue OX OSCILLATOR AMPLIFIER 0108 0111 LEGEND v 7 lioke Toe SYNCHRONOUS DETECTOR 8101 08104 DIFFERENTIAL AMPLIFIER 0104 0105 FEEDBACK CURRENT SQUARED GENERATOR 0106 SO L 3t B Figure 4 1 Block Diagram 01370 2 Model 431
24. ers or 760 series of coaxial couplers The output of the power source is sampled by the coupler and applied to the Model 431B A de signal proportional to the power sample is fed from the Model 431B RECORDER jack to the Leveler Amplifier In the H01 8401A the signal from the Model 431B is compared to an internal ref erence voltage and the difference is amplified and fed back as a control voltage to hold output power constant 3 29 MONITOR CONTROL SYSTEMS By adding a de amplifier and relay circuit tothe rf monitoring arm of a system the de signal provided by the Model 431B can be used to actuate alarm or control circuits Ar rangement of equipment to provide an alarm or control system is shown in block diagram form in figure 3 7 3 30 DETERMINING INSERTION LOSS OR GAIN AS A FUNCTION OF FREQUENCY Arrangement of a system to obtain information on insertion loss or gain as a function of frequency is indicated in figure 3 8 Initially the device under test is not connected into the system connect the thermistor mount dir ctly to the sweep oscillator Set the sweep oscillator for the band of interest and record variations in amplitude as frequency is swept this curve is the reference Next insert the device under test between the sweep oscillator and the thermistor mount and again record frequency response The difference between the second reading and the reference at any one frequency is the insertion loss or gain of the devic
25. instrument operation is entirely dependent on the battery In the CHARGE position 25 volts is connected to the battery for recharging the Model 431B cannot be operated during this time Approximately 37 ma dc is applied to the battery during charge time JI 1 on Inc EF POWER TRANSFORMER 25 VOLT RECTIFIER 40 VOLT R2 RECTIFIER Section IV Paragraphs 4 36 to 4 37 40V FROM CRI CR4 R4 25 VDC REG 25V FROM CR2 CR3 18 VOC REG REGULATOR 1 5 VDC UNREG CENTER TAP OF TI 50 5 177 CR5 CR6 EE SV eds i j i I R3 I8V RE 1 2 REGULATOR wi L OPTIONAL BATTERY 1 54 UNREG CRS LO M 888 Figure 4 11 Power Switch Arrangement 01370 2 Section V 431B Figure 5 3 WAVEGUIDE RF SHIELD THERMAL CONDUCTING CABLE TO i IOKC BIAS SIGNAL P O _ c 2 3 DC BIAS 4 5 DETECTION COMPENSATION i THERMISTOR THERMISTOR i R FERME IN ETE j WAVEGUIDE THERMISTOR MOUNT Q 4 d 486A 3 103 THERMI STOR NT EMEN TA RF SHIELD LIOI 1 9 20UK 1 COMPENSATION THERMISTORS Wr Cl GU PEIS 27 METERING BRIDGE 81058 clos 200 THERMISTOR MOUNT CABLE 7 144 N
26. ll scale from 20 C to 35 C 5 of full scale from 0 to 55 C Zero Carry Over Less than 1 of full scale when zeroed on most sensitive range Recorder Voltmeter Output Phone jack on rear with 1 maximum into 1000 ohms 10 one side grounded Calibration Input Binding posts on rear for calibration of bridge with d 8402A Power Meter Calibrator or precise de standards Power Supply 115 or 230 volts 10 50 to 1000 cps 2 1 2 watts Dimensions 6 17 32 in 166 mm high 7 25 32 in 198 mm wide 12 1 2 in 318 mm deep 01370 2 electrically isolated One thermistor is used to absorb rf power the other is used to provide temper ature compensation Thus thethermaldriftproblems normally associated with the thermistor power meter arrangement have been greatly reduced single setting of the ZERO control on the most sensitive power range is maintained within 195 for all higher power ranges 1 4 The temperature compensated thermistor mounts used with the instrument are specifically de signed for Model 431 Power Meters Coaxial and waveguide thermistor mounts cover the 10 mc to 40 gc frequency range Table 1 2 gives thermistor mount operating frequency mount configuration and operating resistance Specifications Weight Net 8 lb 3 63 kg with cover and cables 11 1 2 10 5 44 kg including battery shipping approx 131b 5 9 kg Accessories Furnished 5 ft 1 5 m cable for temperature
27. nt The cable connects the mount thermistors into their respective bridges within the power meter Model 431B THERMISTOR MOUNT OC CALIBRATION SUBSTITUTION 4 MOUNT RES This two position slide switch sets the power meter to accommodate therm istor mounts of 100 or 200 ohm nominal resistance ZERO and VERNIER The ZERO control coarsely sets the meter pointer near zero the VERNIER control is a more exact adjustment which sets the meter pointer on zero In Option 02 instruments only mount connector wired in parallel with front panel connector Two mounts cannotbe connected simultaneously RECORDER The RECORDER input is a grounded telephone jack for monitoring the current which operates the Model 431B meter DC CALIBRATION amp SUBSTITUTION This terminal permits application of known direct current to the rf bridge The power reading obtained with the accurately known de power applied is then compared with the reading ob tained when rf power was applied The de sub stitution technique is used both to calibrate the 431 and to increase the accuracy of power measurement LINE VOLTAGE The LINE VOLTAGE switch S is a two position slide switch that selects the mode of ac operation The line voltage for which the instrument is set to operate appears on the slider of the switch A 15 100 slow blow fuse is used for both 115 and 230 volt operation Figure 3 1 Front and Rear Panel Controls and Indicator
28. s 01870 2 Model 431B OWER OFF BATTEAY OM CHARGE 1 Connect thermistor mount and cable to the THERMISTOR MOUNT thermistor mounts and their frequency ranges are given in table 1 2 Model 431B Thermistor Mounts Note When possible the Model 431B should be zeroed and nulled with the power source to be measured connected to the thermistor mount Ifthisis notpossible anda coaxial thermistor mount is used terminate the rf input into 50 ohm load Power source Should be off while zero and null setting the Model 4318 Power Meter Set MOUNT RES to match thermistor mount resistance 100 or 200 ohms Set RANGE to 01 MW Set POWER to AC amp CHARGE lamp wili glow If instrument is battery operated rotate POWER to BATTERY ON Adjust ZERO control for 25 to 75 of full scale on meter Rotate RANGE to NULL and adjust null screw driver adjust adjacent to NULL on RANGE Switch for a minimum reading Section Figure 3 2 4318 POWER METER RANGE ww 01 03 4 8 0 3 10 20 15 0 5 0 THERMISTOR Oy HOUR MAL NAZ CS N won Ron MOUNT RES t6 u 54 L0 M 604 7 Repeat steps 5 and 6 untll NULL reading is within NULL region on the meter Note Jf instrument is battery operated and you are not able to zero the meter orif meter pointer fluctuates rapidly battery needs recharging Refer to paragraph 3 11 Set RANGE switch to the power range to pe used
29. stor The thermistor used in conjunction with the Model 431A B exhibits unilateralproperties which when the source of power is a dc current causes a slightly different indication of power than is obtained by the calculation of I 2R Th s the power required to produce a reading on the Model 431A B Power Meter is not the same as the rf power required to produce the same reading on the Model 431A B Power Meter The maximum error produced from this source of error ts 0 3 typical error is 10 1 u watt Since the order o magnitude of this error is small 0 3 u watt it need be minimized only on the two most sensitive ranges of the Model431A B Power Meter Refer to the Model 8402A Power Meter Calibrator manual for procedure used to minimize this error 3 17 POWER METER ACCURACY OF 1 OR GREATER USING THE DC SUBSTITUTION METHOD 3 18 Highly accurate instruments are available for measuring direct current Thus where optimum accuracy is required there is considerable advantage in using a technique where the rf measurement is used only as a reference and the determination of rf power is based on precise de measurements In general the technique involves Applying rf power to the Model 431B inthe usual manner and noting the resulting meter indication for use as a reference b Removing the rf power and applying sufficient de at the DC CALIBRATION amp SUBSTITUTION terminals to exactly duplicate
30. the dc substitution measurements 3 23 Although the most convenient and accurate means of applying the de substitution technique is by using Model 8402A Power Meter Calibrator it is also possible to accurately measure power using the dc substitution technique with the arrangement shown in figure 3 3 The digital voltmeter is used to monitor the substitution current The power supply output and voltmeter input are ungrounded to eliminate ground currents 3 24 ADDITIONAL APPLICATIONS 3 25 At the RECORDER output the Model 431B fur nishes current 0 to 1 dc which is proportional to the power measured This feature makes possible measurement system with more capability than simply the indication ofpowerona meter Some of the more sophisticated measurement systems are shown in block diagram form in figures 3 4 through 3 8 3 26 PERMANENT RECORD Use of a recorder in the measurement system is indicated in figure 3 4 Resistance across the Model 431B RECORDER output should be 1000 ohms 10 for optimum measurement accuracy Any type of recorder may be used with the Modei 431B if input resistance exceeds 1000 ohms use a shunt across the recorder input J102 RECORDER MODEL 4318 METER 80 S 252 Figure 3 4 Making a Permanent Record 3 5 01370 2 Section Paragraphs 3 27 to 3 30 TEMPERATURE RF SOURCE THERMISTOR MOUNT 478A 486A 4p MODEL 438 COMPENSATED POWER Model
31. the meter indication produced by the rf power c Using the value of de which duplicated the ref erence in calculating rf power 3 19 Although the dc substitution technique is the most accurate method of measuring rf power there are sources of error that must be considered The accuracy of the dc Substitution technique depends largely upon a how precisely the reference is duplicated TEMPERATURE RF SOURCE COMPENSATED THERMISTOR Section Paragraphe 3 17 to 3 26 b how accurately the value of the substituted de is known c the actual operating resistance of the thermistor and 4 the actual ratio of current division in the rf bridge 3 20 With precision components in the substitution setup and careful procedure error produced by the Model 4318 Power Meter can be reduced to 1 or less This is assuming nominal thermistor mount resistance 100 or 200 ohms and that half the applied de flows through the rf thermistor The de substitution tech nique using the Model 431B is shown in figure 3 3 3 21 EQUIPMENT USED FOR DC SUBSTITUTION T 3 22 The Model 84024 Power Meter Calibrator was specifically designed to be used for calibration and de substitution measuremerits of rf power In addition the instrument will accurately measure the operating resistance of the thermistor mount being used Use the procedures giveninthe manual provided with the Model 8402A Power Meter Calibrator to perform
32. upplied externally atthe DC CALIBRATION and SUB STITUTION terminals Calculation of the substituted de power gives an accurate measure of the rf power Effectively dc power is substituted for rf power 4 33 REGULATED POWER SUPPLY 4 34 simplified schematic diagram of the power supply is shown in figure 4 10 The power supply operates from either a 115 or 230 volt 50 to 1000 source or from an optional 24 volt 30 ma rechargeable battery Three voltages and two current outputs are provided by the power supply Regulated voltages of 18 and 25 and unregulated 1 5 vde operate the power meter circuits The current outputs are used for maintaining battery charge trickle charge for recharging the battery 4 35 The 18 vdc is regulated by a conventional series regulator Q1 through Q5 The 25 vdc is developed across CR9 a 6 8 volt zener diode refer enced at 18 The unregulated 1 5 vdc is taken 01370 1 Model 431B across the s ries diodes 5 and CR6 The 18 supply is adjusted by R13 4 36 POWER SWITCH 4 37 A simplified schematic diagram of the power switching arrangement is shown in figure 4 11 The power switch S2 has four positions OFF AC BATTERY ON and BATTERY CHARGE In the AC position the instrument operates from the conven tional line voltage if a battery has been installed in the instrument a trickle charge is supplied to the battery In the BATTERY ON position
33. us detector is filtered by C117 C118 C119 OUTPUT 1 0106 2 0107 IBV Rt4t 8142 RECTIFIED OKC FROM SYNCHRONOUS DETECTOR 5 Figure 4 6 Differential Amplifier 01370 2 TO FEEDBACK CURRENT SQUARED GENERATOR Qo DC BIAS METERING BRIDGE FEEDBACK CURRENT GENERATOR FROM Q107 COLLECTOR OF 0104 i8VOC 50 174 Figure 4 7 Feedback Current Generator and R140 amplified by Q104 and fed to both the feed back current squared generator Q106 figure 4 7 and feedback current generator Q107 Temperature com pensation and low emitter circuit resistance for Q107 are provided Q105 Diode CR106 protects Q106 and Q107 from excessive reverse bias when Q104 is cut off 4 23 FEEDBACK CURRENT GENERATOR 9107 4 24 A simplified schematic diagram of the feedbaek current generator is shown in figure 4 7 Thede signal from the differential amplifier is applied to feedback current generator 0107 Q107 has two functions 1 it 4 3 Section IV Paragraphs 4 25 to 4 35 METER RECORDER 8178 4 1 5v FEEDBACK SQUAR GENERATOR RIT9 8 80 9106 FROM DIFFERENTIAL AMPLIFIER QI04 105 Su RIG 0 175 Figure 4 8 Meter Circuit completes the metering loop to the metering bridge and 2 it operates in conjunction with the first 10 kc amplifier Q101 and the RANGE swit
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