HP 55330M User's Manual
Contents
1. George D Pontis rm Born in Los Angeles Calfomia George t s Ponts attended the University of Catlomia at Santa Barbara receiving his BSEE degree in 1975 He joined HP in 1978 and contributed to the design of the 8903A Audio Analyzer Formerly a contributing editor of Audie Magazine he s a member of the Audio Engineer ing Society He lives in Palo Alto California and enjoys outdoor sports especially tennis sking and swimming h 0 High ov Oren Low Vea ae brest a K zoon 150088 om R2 3000 in eA eo Re 1091 f 2400 10v amp ie pe AA R3 RS RS Bee 24001 RE vi 1091n gt A71 IRS 4545 v2 tb ian Fig 3 a To demonstrate that the output of the crout is floating the input is grounded and a volage source is aped fo Doth outputs b Reduced circuit for Mis test any halt ofthe circuit i shown Current trough AE cancels the current Trough Ad and AB Currant rough VT and 24s supplied by the other hall of the circuit not shown Thus no current is drawn from tne test source ground and measures the potentals at several circuit nodes The measured values can be compared to the calculated values pub lished in a table in the 89034 manual A second set of measurements can be made il necessary with the high side grounded By observ ing the deviations batween the measured and calculated values it is easy to locate the faulty component networks for low distortion2. 20 No 4 197E pp 465 472 measurement time it was necessary to develop an accurate rapid fine tune mechanism with quick recovery from over loads and mistuning To achieve good Q over the frequency range an active RC filter is necessary To tune this device a Fig 2 Block diagram of the 8903 s wterna state varabve asciilatar variable resistance or conductance device is needed since achieving the tuning range with variable capacitors or In ductors is impractical Many resistively tunable active con figurations are feasible However determining the op AUGUST 1980 HEWLETT PacKaan Juana 11 To provide the greatest versatility in both benchton and systems applications the BBOGA Audio Analyzers bult in source is floating This Sets the user elerinate ground loop errors sum signals and add dc offsets to the source olfput Previous G signs used a separate isolated power supply tor the source circuits This method is straghtforward and offers very good lowlrequancy Common mode rejection However there are several reasons why ihe arrangement is net used in the B90GA The biggest problem with the floating power supply approach is Interfacing with tha digital programming lines Since only three of the shineen attenuator lines Save talay isolation and none of the ningieen oscillator lines can be floated over thirty lines must be coupled in same menner One solution isto float only t
3. Although the programmed frequency does mat have the accuracy of a synthesizer sufficient resolution is available 10 permit firmware tuning to within 20 3 of the programmed vaue Ts oscillator design Can be Gasocribad as a stala varadie liter in Driver 1 of 8 which the resisior that determines the Q 1s replaced by an analog multiplier The control signal for the multipier is provided by an automatic leval control ALC circuit The ALC circuit compares the oscillator amplitude wih stable de reference obtained irom a temperature compensated reference diode The resulting error sig nal is processed through two paths One path carries the cycle by cycle proportional ero to the contralling multiplier This provides wery tast settling The other path inciudes an integrator in the loop 10 aliminate nearly all of the steady state error This design is theoreti gally capable of satling the output amplitude within two cycles atter smat signal amphude disturbances There are two important refinements in the 89034 oscillator The first is the use of a Special two stage peak detecter This consists ot track hokd and samplenold amplifiers to lminale any cistortion Causing ripple on the detected peak output The second refinement is the addition of an ALC icap gain control to compensate the ieveting joop cycle by cycle tar changes in oscillator ampldude Thes greatly decreases tho large sgnal settling tme of the oscilator which is
4. analog meter and digital display and switchable detector litering alow the user to Make unusual or special measurenents wilh Convenience and itte auxiliary apparatus 10 dBY ment modes It is also used to set the gain of the two pro grammable gain amplifiers The detector can be configured internally to respond to the average absolute value of the signal instead of the true rms value This option is provided because some measurement specifications for detection of distortion and noise specify the use of an average respond ing detector Average responding detectors do not give an accurate indication of signal power unless the signal wave form is known If the waveform is Gaussian noise the read ing will be approximately 1 dB less than the true rms value In the SINAD mode the outputs from the input and output rms detectors are converted to logarithms subtracted and converted to a current by the SINAD meter amplifier ta drive the SINAD panel meter Since SINAD measurements are often made under very noisy conditions the panel meter makes it easier to average the reading and to discern trends The voltage to time converter converts the dc inputs intoa time interval which is measured by the counter The 8903A uses a reciprocal counter To measure fre quency it counts the period of one or more cycles of the signal at its input Then the controller divides the number of cycles by the accumulated count The reference for the counter i
5. coarse tuning and a four quadrant multiplier for fine tuning between the discrete steps of the switchable resistors Since the four quadrant multiplier is coupled into the circuit only enough for 7 tuning it does not contribute significantly to the overall noise and distortion A resistor switching network is best implemented if one end of the network is dynamically kept at ground potential this relieves many constraints on the switching network To this end a state variable notch configuration is used see page 14 With this design low distortion tuning over a 10 1 frequency range is achieved The nyme Matevenah Mier a a COteequese tw Lac thal Ine equmore that de Acre Ihe Ry Bern Can do eter A a form that Sto ino acca stte qnane Seanad of airearsynem ie R Ax Ou weve gis fet reponse dor Sate vname of the sytem 1a i pat u Capacitors are switched into the network to change fre quency in three octave bands and provide complete cover age of the frequency range of the analyzer Tocomplete the fine tuning path asynchronous detector mixes the filter output waveform with the fundamental waveform If any fundamental exists in the notch output a de current is generated to fine tune the notch The critical parameters here are 100 dB dynamic range and rapid opera tion A FET double balanced mixer was selected The input mixing signal that drives the FET is a square wave rich in odd harmonics so the circuit responds to odd order
6. devices are relatively linear but are slow and have a limited dynamic range Thermistors diodes and other nonlinear devices would all be useful only for fine tune applications The drive and compensation circuitry for all of these alternatives would be complex and the overall performance marginal The tuning elements selected were switchable resistor 1091 1t Ai R3 R5 T Fig 2 Elinmwnating the iverting site of the circuit of Fig 1 by Shorting the low output to ground results in is reduced cw cut in practice it was found thal parasitic effects and the eloctromagnetic interference EMI filters degrade the crcull dal ance when the frequency approaches 100 kHz However each bosd is tested for a minimum of 50 GB common mode rejection at 1 kHz Atypical unit has greater than 40 dB rejection at 100 kHz Also a 10 k N resistor internally ties the low output to the chassis ground This provides a reference for the output when no extemal load ls com nected Without this resistor the common mode output voltage is indeterminate One mtial concetn about the circuit was dilfoulty ol troubleshoot ing problems such as one of the resistors crifting out of tolerance causng poor common mode rejection Tres problem was solved by implementing the following test procedure First the input to the floating amplifier is set to exactly 1 00V rms using the special tunc tons built inte the 89034 Then the tectwnician shorts the low side to
7. har monics as well as to the fundamental This is the classical solution A complete null may aot be achieved if third fifth or higher order odd harmonics are present Total error can AUGUST t980 HEWLETT PACKAAD JOURNAL 13 A Digitally Tuned Notch Filter by Chung Y Lau The notch filter in the amp S034 Audio Analyzer rejects the tundamen fal frequency Componers of the incorm ng sgnal This filter consists of a state variable active filter ang fine tune and fine balance control crcuts A smplitied schematic of the filter minus the control circuitry amp Swn in Fig 1 Conventional notch filters bridged T atc wsed in distortion anslyZers are not wel sulled for digital control because they require expensive relays or analog switches The state variable fitter ap proach has the following advantages The Q of the filter tiked and incependent of frequency Tuning is accomplished by switching resistors and capacitors in the Iwo integrators Inexpensive JFETS are uSed as switching slements and the switch drivers are simple because bath the gate and the channel of the JFET are virtually at ground potential when the switch is on Three filter outputs low pass bandpass and high pass are available simultaneously The low pass and high pass autputs are phase shitted from the input by 90 when the fiter is tuned to the input frequency These two outputs are used in the control circults 0 xira phase shifting circuits are Aol nee
8. hogh frequencies Low droop is impor fant to maintain low distortion a low lrequences Also this SCreme has no steady state ripple in the samoted output which would cause distortion 10 appear on the multiplier output and in tur on the main oscillator output in practice the two stage scheme is aso easier to Iinplement than a very fast single stage samplehold circuit The required circuit functions are accomolishad with simple JFET switches and unty gain butlers as shown in Fig 2 For sampled data systems in general settling time is strangly dependent upon loop gain For this circur the ideal megiator gain is nearly proportional to frequency This ettect is achieved by switch ng the integrator resistor on once per Cycle t r 4 duration 0135 us Ths process increases the integral error signal with Irequericy The duty cycle increases with fraquancy undi the ntegrator swach S3 is latt closed comtinusesty lor frequencies greater than 25 kHz Above 25 kHz the oscillator easily settles in less than one millisecond 83034 oscillator performance is largely limites Dy the quality of commercally available reasonably priced anabo mutipliers The multiplier used is decoupled slightly from optimum to reduce its contribution to TMD and noise This extends the osciliatorsatting time to a Period Of tour to live cycles Reference l E Vornerson and Smeh A Low Tastoron Ositany wits Fasi Arty tcl Stabeiew tain Injermatanal journal of Elscironcs Vol
9. important when switching fom one range to ancther Fig 1 shows the oscillator integraters The gain constant of these Miegraters is changed in Theee octave steps by selecting the leed back capacitor This gives us range sweching Coarse tuning within each range is dona by switching the input resistors The eight binary weighted resistors provide 255 usable steps Placing the sweching devices at the virtual gound pare permits the use of JFETs with low Grainto sauroe on resistance Ags oh An incivideal transis tor switch conducts wher its gate is allowed to rise to gound pater tai and tumsoll when its gate is pulled tothe nagative supply 15V Fig 1 The gain switched inte gato used in the 8903 s inter nal oscilator Capacitors are Switched to change ranges Resis tor switching NOKIOS CORSE WN ing within each range reset occurs within ten milliseconds if the overload ts st vere This protects the input from burning out or blowing a fuse and allows for rapid overload recovery 10 HEMLETT PAC RARD JOURNAL AUGUST t36 Notch Filter The notch filter design challenge was twofold First it was necessary to design a low distortion low noise filter that was also programmable Second to minimize overall Tne switching scheme is economical because multiple resistor packages and quad comparators can be used to interface to TTL levers Fig 2 is a block diagram of tne oscillator integrators UT and U2 an
10. necessary to reject potential differ ences between the chassis of the 8903A and the device under test The 8903A also has EMI electromagnetic inter ference protection built into the source output and mea surement input lines so that it can work in the presence of high EMI All of the analog circuitry is shielded by an internal EMI tight box The output and input lines extend ing from this box to the front panel are shielded and termi nated in BNC connectors For user convenience the BNC connectors are spaced so that BNC to banana adapters can be attached Thus a banana or twisted wire connection can be made to the instrument when EMI shielding is not critical Analyzer Architecture The 6903A Audio Analyzer combines three instruments into one a low distortion audio source a general purpose voltmeter with a tunable notch filter at the input and a frequency counter Measurements are managed by the microprocessor based controller This combination can make most common measurements on audio circuits au tomatically To add to its versatility the analyzer also has selectable input filters logarithmic frequency sweep X and Y outputs for plotting measurement results versus fre quency and HP IB programmability Fig 2 is a simplified block diagram The amplitude measurement path flows from the INPUT jacks HIGH and LOW ta the MONITOR output on the rear panel and includes the input and output rms detectors the de voltmeter the voltage
11. on the cabling feed ing the cireuits special precautions were taken First from the inner box to the front panel shielded cable is used Second BNC connectors are provided on the front panel The BNC connectors allow the attachment of shielded ca bles directly to the instrument if desired thus preventing EMI pickup The instrument s digital circuitry also gener ates EMI related to harmonics of its 2 MHz clock This problem was minimized by means of RF gaskets on some of the cabinet seams and by installing an EMI suppressing filter on the power line input As a result the instrument will not disturb sensitive receivers operating nearby and yet will perform well near a powerful transmitter Frequency Measurement A key feature of the 8903A is its ability to measure fre quency automatically even when the input waveform may have a significant amount of noise and distortion and the amplitude may vary from6 mV to 300V Part ofthis problem is solved because the instrument is autoranging and keeps the leveled waveform within 6 dB over most of the input amplitude range But before the signal can be accurately counted it must first be converted into a binary signal hav ing the same period as the major frequency component in the waveform and herein lies a problem If a zero crossing circuit is used noise may cause multiple crossings and a false indication of the frequency Hysteresis in the detector will help but if the hysteresis is too Jarg
12. received his BSEE and MSEE degrees from Stanford University in 1974 and 1976 With HP since 1974 he s contributed to the design of Ihe 4964 Power Mater the 86624 Syri thesized Signal Generator the 8911A Modulation Analyzer and the 8903A Audio Analyzer He s taught rmecr procassor design at Foathill Collage and served as chief engineer of Stan ford FM station KZSU Bom in Boston Massachusetts Cory now lives in Mountain View Caltorna His interests include FM radio broadcasting photo ES SS graphy backpacking and the rt ot com dy he s an avid fan of radio and television comedy groups To summarize the gain of the input amplifiers is mod ified in three ways First the microprocessor monitors the input detector If the detector voltage is too high or too low the microprocessor varies the gain of the attenuator amplifier chain to bring the level within bounds Second the overvoltage protection network limits if the input signal exceeds 15V Third any sustained input overload trips the input overload detector This detector monitors the input rms detector and the differential amplifier and if either exceeds a certain absolute voltage limit the overload detector trips resetting the gain of the entire amplifier chain to its minimum value maximum attenuation This AUGUST 1980 HEWLETT PACKARD JOURNAL 9 Design for a Low Distortion Fast Settling Source by George D Pontis To fulllf the requ
13. response ts achieved in two ways First when an overload occurs the circuit gain is quickly reduced by the output amplifier circuit minimizing the transient and thus minimizing the recovery time of the synchronous detector from the overload Second the response time of each fre quency band has been optimized Theoretically it is impas sible to fine tune a filter as rapidly at 20 Hz as at 1 kHz It simply takes a much longer time to detect a null at 20 Hz but even more important is that the time response of a 20 Hz notch filter is much greater In fact a fundamental burst 44 HEMETI PACKARD JOURNAL AUGUST 1950 tone applied at the input of the notch will not immediately be nulled at the output The entire input will first appear at the output and then decay away in proportion to finotch Q where Q is the Q of the notch Thus as the notch frequency is increased so is the speed of the fine tuning circuitry The bottom frequency band 20 200 Hz is relatively slow but nulls optimally at 20 Hz For higher frequencies the instru ment response time improves Output Amplifier For the amplifier following the notch filter gain accuracy tine balance circuit insures that Vap Vy 1 atthe input Irequency no ampitude error The fine tune cirout is described in gome detail here the fine balance circuit operates in a similar manner Fig 2198 simplitied schematic of both circuits The fine tune circuit operates as follo
14. to ten volts peak The entire operation of the instrument is under control of the microprocessor based controller which sets up the in strument at turn on interprets keyboard entries executes changes in the mode of operation continually monitors instrument operation sends measurement results and er tors to the front panel displays and interfaces with the HP IB Its computing capability is also used to simplify circuit operation For example it forms the last stage of the Attenuator Transmitter MIC Input AF input Modulation Output SECTA Modulation Anatyzer 89038 Audio Analyzer Source Output Analyzer Input Fig 3 Transmitter test setup using the 901A Moculanon Analyzer and ihe 89034 Audio Analyzer the frequency of a signal whose arnpitude is in the low tens of microvolts Transceiver Testing Much etfon has gone into the design ol the B9G3A to taciitate the audio measurements required for automatic programmed transcewer tasting For example the worst case source frequency error of 0 3 allows the 8903A source to replace a synthesizer for Squeich tone generation In addition a binary programming mode is available through the HP 1B that causes tha 8903A to generate atone burst sequence that can be used to unlock a codad reveier A telated function allows the B9034 to measure Durst tones generated by an external source such as a transmitter uncer test The packed sour byte output allows the 89034 to ou
15. to time converter and counter and the SINAD meter circuitry Measurements are made on the difference between the signals at the HIGH jack and the LOW jack Differential levels can be as high as 300V Signals that are common to both the HIGH and LOW jacks are bal anced out Signals applied to the LOW jack must not ex ceed 4V The input signal is ac coupled for all measurement modes except de level The signal is scaled by the input attenuator to a level of IV or less To protect the active circuits the overvoltage protection circuit quickly disconnects the input amplifier if its input exceeds 15V_ The differential signal is converted to a single ended signal referenced to ground and amplified The signal is further amplified by a programmable gain amplifier which is ac coupled The gain of this amplifier and the differential to single ended amplifier are set to keep the signal level at the input rms detector between 1 7 and 3V rms to optimize its effectiveness and accuracy The output from the first programmable gain amplifier is converted to de by the input rms detector and measured by the de voltmeter The output of the detector is used to set the gain of the input circuits and becomes the numerator of the SINAD measurement and the denominator of the distortion measurement The gatn of the input path is determined by measuring the de level The input rms detector also monitors the ac component if there is one and lowers the gain of t
16. A Complete Self Contained Audio Measurement System This automatic autoranging audio analyzer has the signa source distortion analyzer and counter to make the measurements most often needed in audio frequency testing by James D Foote Analyzer Fig 1 is a complete audio measurement system for quick and accurate characterization of systems and signals in the frequency range 20 Hz to 100 kHz The starting point for the 8903A is the classical distor tion analyzer Added to this ara microprocessor control a reciprocal frequency counter rms detectors and pro grammable audio source These provide accurate mea surement of ac level distortion SINAD signal to noise ratio and dc level The audio source and the measurement circuits can work independently or together The source is programmable in frequency and level and has very low distortion The measurement circuits can monitor this in temal source or any other independent input waveform Together the source and measurement input can be used for swept response measurements k j EWLETT PACKARD S NEW MODEL B903A Audio aA antail t sao pia nome plus dzien to naso pind Apamon in a reoepeer s AH measurements are avalloble at the push of a button No knob adjustment or operator interaction is necessary Ono simply applies the signal and selects the measurement mode All control and processing are handled by the intar nal microprocessor The microprocessor monito
17. MHz The controiler test prograen canbe wiittan ta provide either a go ne ga output ora listing of measurement results Many of Ihe Special measurement modes are available through the use of special functions For example the B90GA can be used as a test ampbfier with gain settable from 24 GB to 94 dB Signal fitering can be added by selecting the appmpriate front panel con trols or the special functions Gan be used to put the instrument into a n amp ch or bandpass filter mode Of course signal frequency and amplitude will he measured and displayed if the operating loads are Chosen correctly This moge of operation makes it possible to count verifying that the oscillator frequency is within toler ance This frequency is normally not displayed The source covers the frequency range of 20 Hz to 100 kHz It is tuned by the controller to the frequency entered from the keyboard using a tune and count routine Note that the frequency is not obtained by frequency synthesis The switch following the oscillator is closed except in the signal to noise measurement mode or when an amplitude of OV is entered from the keyhoard The source output amplifier and output attenuators pro vide 77 5 dB of attenuation in 2 5 dH steps and 2 5 di of attenuation in 256 steps This gives an open circuit output from 0 6 mV to 6V The floating output amplifier converts the ground referenced input to a floating output Either output HIGH or LOW can be floated up
18. a Ka Ae sono IA TRCTOR Tra ma ja naga ae YAR Dy OE HOC Ses aoe FRAO Am D Ay CAD toed SE Pa dee 3 Oe ery Sem fe te aparat halt EO Cnt Sat Reg d renee ayena Full roana eA Cy eng oe Mees enn spana ME 75 L 2s hee ise AOD ETO Ace m BANAD ie Hem Nee BARRE paee WEN ne D ot cay SON mame Minn knee arenes pre OAE oarceton PUNDAMENTAL FACOUENI MAME 2D he mh Ok re OUTPUT Lek RE RCLLITION BFS ow mere LOPLAT SAVER OFTTE Oe 1A ORM Ow Oly ACEURACY of da DO He be Oihe cae SH atic hs HD abe PUT WONT MAME Se Pw Be ae a SUA VOAS NED ISITE The gree HRAS O R we A av OM IAM NDI M eID Mane OME ey SES HOR ob os Shee ee ee BOO OU a DOPLAVED HEEL ATOM CINIA lt 07 Sane Gt Cram Pe aeone SOUS JAM Sete BEN A terete FORAY MUSE GO to woe ante PUT WR FADE MANOS S oe we doy PERCU MORE The npor I0 dhama KE SOA EW OB or oe ARASON Mew mayre pian te wr OF he roat aap m de stama 8 ROWAN eT ia at Dalal SN tet OUT SENEIIRTY DO 7 es cheers emt BALD nonea SE OW a i Dee ed ABO He 1008 FARE PLTN 220 act TERA key ARE nt a ec CORT Pecomreeranne Paa Lal MEO LIma 6 OE Ber Sete oR HOTS Ow ee SOLOW FEREPE TED JD tot Bacal FER Table Paint HOME teow OO ELLE PA SEP TEP 1 eet beany Biart i SUSE ARIT DR daaa James D Foote A native of Madison Wisconsin Jm Foote earned his BSEE degree at the H University of Wisconsin i 1972 and joned HP n 1973 in 1975 he obtained his MSEE at Stanford University He has Served as a d
19. aconds The slope checking gives us a rapid check on signal quailty which eliminates the need to delay every time Attar tne slope check and possitve delay we assume the output amplifier is well settled and ready for leveling We then measure the vollage al ts output and it teo high or low adjust Ihe gain acoord ingly After adjusting the gain we make another pass through the leveling algorithm unless we ar now in the higheat gain range To keep irom getting Caught in indinite loops we require that after the first pass the gain never be reduced and assume that if tis we have an unstable signal which causes us to start the entira measurement cycle tune source evel input tune nateh level output over again The mal protection trom infinite loops comes from counting the number of times we restart without displaying each time we pul a pattarn on the display and ater 128 times we display Error 31 which also goes out to the HP I8 Because the 89034 has the ability to generate accurate and com paratively rapid measurements automatically with no need for the user to insert delays in HP IB routines or wait for the display to sette before taking a reading it is possible to have the instrument sweep itself over a range of frequencies without user or computer interven tion Here the calculating power of the microprocessor is brought to bear on the problem of determining the frequency increment for each new point in the l
20. ain for the proper reading To keep the controller from being fected by this phenamenan we take two voltage readings on rapid succession and trom them calculate the rate of change of the signal The time over which we measure this slope i longer at lower frequencies andis fact looked up in a table in ROM based on the trequancy at which we Te operating It ne rate of change is tog fast during this period we delay before checking the lavel see Fig 2 This delay is also frequency dependent and table driven Note that if we simply delayed each cycle to keap from risking Input voltages larger than 4V are attenuated by the input attenuator a network of resistors that divide down theinput signal The appropriate tap point is selected by a reed relay network If an overload occurs the maximum attenuation setting is enabled To protect the sensitive input amplifier following the attenuator from short term transients an overvoltage pro tection network is used For low level signals the transter impedance is low and signals applied to the input are coupled to the differential input amplifier However for input signals large enough to damage the amplifier the output of the protection network is limited to a safe value A A a te aN shows need to delay to let signal settle Fig 2 An exame of shope checking by the 5993A micro processor displaying invalid readings every measurement wouid take longer perhaps by several hundred millis
21. changing the frequency or vel Similarly the settling performance of the input and output amplifers and the naten ditar under various adverse signal conditions becomes very important when the instrament is tring to deliver an accurate first measurement alter a change in operating paramalars 69 alter 1uning 10 3 new frequency Ada to ths the desire to make measure ments as raped ly as posswie and you have a very interesting problem for a microprocessor to solve For example every tima tne internal source is tuned the processor spends about 170 milliseconds tuning to and verifying tne frequency This translates to over B6 000 operations Ot equal complexity is the pb of setting the correct gain forthe cutput amplitier anc allowang the circuits to settle before the output amplifier voltage is read with the object of revar giving the user an invalid reading while at the same time delverng the reading in as shori a time as possible To ac complish this the routine thatoontrots the output amplifier makes use of such techniques as slope checking and frequency dependent Gelays to ensure rapid valid readings These techniques make pos sible accurate readings in half the time it might otherwse take Referring 0 thefiow chart Fig 1 note that the key to this routineis me technique at measuring the rate al change of the signal on the output amplifier the sope before attempting to check the signal level to determine if the output ampiitier gain is pr
22. circuit see page 12 takes the oscillator output level and translates it to the selected floating output amplitude To minimize cost and achieve overall output accuracy goals the attenuation is done in two stages Coarse amplitude steps are implemented with a 2 5 dBistep at tenuator network Smaller steps are provided by a resistive ladder network that adjusts the amplitude linearly in small discrete steps The combination can adjust the amplitude within a nominal 0 15 worst case Computation of the proper switch settings is an easy job for the computational skills of the microprocessor EMI Design Meeting the required electromagnetic interference EMI and susceptibility goals was a bit more challenging than initially expected Large amplitude RF flelds tend to gener ate voltages on exposed cabling and circuits These voltages overdrive many of the active circuits causing nonlinear operation and distortion To avoid direct exposure to these fields the analog circuits are housed in an internal EMI tight box The box has an aluminum frame around the sides The bottom cover is the ground plane of a printed circuit board and the top cover is a removable EMI tight lid Re moval of the lid which is held in place by only two screws makes all the circuits available for service The micro processor boards are sufficiently shielded by the instru ment cabinet and do not require the extra shielding To keep the RF fields from developing voltages
23. controler a reciprocal raquency counter a programmatie audio source and other fesources forming a complete audio measwament System frequency response and distortion characteristics of filters high quality amplifiers audio integrated circuits and other devices The frequency of the internal source can be swept while making measurements in all modes The analyzer provides recorder outputs and scaling foreasy generation of plots using an X Y recorder For transceiver applications the most common receiver measurements are SINAD for FM receivers and signal to noise ratio for AM receivers A psophometric filter is in tluded for making measurements to CEPT standards Common transmitter measurements such as audio distor tion and squelch tones are made using the 8903A with its companion instrument the 8901A Modulation Analyzer In automatic systems the 8903A provides many frequently needed audio functions doing the work of an audio syn thesizer digital multimeter frequency counter and tun able notch filter More details on specific applications are presented on page f Control Philosophy Front panel control of the audio analyzer is simple yet powerful Most functions can be used and understood with very little training The casual user can select amplitude frequency measurement mode and filtering simply by reading the labels on the controls More detailsare available on the instrument s pull out card A great deal of measurem
24. d inverter U4 form the state variable Miter Structure Fine tuning is done by U4 This siege uses a switched resistor network similar to that usad for coarse tuning in the integrators As the resistors are switched the gain cf U4 changes effectively altering the amount of signal transmitted tom the output of U2 The gain is proporianal to WA BA where Ris the parait combination of the selected input resistors and A and B are constamts that provide a 5 fine tuning range Ths gives the oscillator enough resolution to tune within 20 2 of any trequency within the range of the instrument Itis ganerally true for sinusoidal oscifators that purity and settling time are chatty functions of the ALC circuits or mechanisms used The two stage peak detector circuits are the key te the perlormance of the 89094 oscillator Oscillator amplitude data is obtained by the track hold and sampie hold amplifiers inthe tolowing manner Switch 51 closes during the time the output is atits negative peak Capacitor C1 rapidly charges following the sine wave amplitude upto as pasi tne peak At this time SI opens holding the peak voltage on Ct Switen S2 then quickly clases and opens again thus updating the sampled peak level held by capacitor C2 The two stage scheme has several advantages For one the first Sage may be optirnized tor fast data acquisition while the sacond state is optimized tor long boki time Fast acquisition is essential for good amplitude accuracy a
25. ded Distortion generated in the fine tune circuitry is filtered by two integrators betore appearing at the notch output In distortion mode the fiker is tuned in the following manner The MIcroproaessor Counts te input frequency and tunes the notch fiter 40 the Same frequency by switching in the proper capac ors and RI Correction Signals from Control Circuits resistors When the fiter is tuned 1o the fundamental frequency of th input signa the bandpass output jp in Fig 1 inverts the funda phase and amplitude characteristics of Vae the harmonics present i Vin are relatively unsttenuated Analytically we have Yap w0 his PA r a Vin S sw o and Vee Yn s wi gs r Viy S e sw O a where w is the center frequency in radis Therefore there is zer iransmisson al S tjan The state variable filter alone can provide only about 15 dB o fundamental rejection because of the coarseness ol the frequency tuning use of discrete resistor values resistor mismatches etc Fine tune and tine Dalance control circuits are necessary to achieve anotch depth greater than 99 dB The fine tune circus insures that the ilter i exactly tuned to the input frequency ao phase error and the vy Fig 1 89034 notch fitter Correc tion signals from the control cir cuits of Fig 2 provide fine tuning and fine balance be as much as 0 46 dB but this is considered reasonable Rapid
26. e smaller amplitude waveforms may not trigger the detector at all while large amplitude waveforms will have relatively little hysteresis protection when large noise components are present To alleviate this problem the 8903A employs vari able hysteresis As the peak amplitude of the signal varies so does the hysteresis level which is maintained at approx imately one half the positive peak for the positive portion of the waveform and approximately one half the negative peak for the negative portion of the waveform Hence noise immunity remains constant regardless of the incoming waveform Hysteresis is implemented with a bipolar peak detector followed by a dual comparator The waveform is transferred to reciprocal counter which measures the period of the signal and the microprocessor inverts this period to get frequency Acknowledgments Many people throughout HP contributed to the success ful introduction of the 8903A It must be stressed that the instrument s success is the result of the total contribution of many people from early investigation through production Sa first and foremost may I thank all of those who contri buted their time enthusiasm and support On the R amp D team I would like to thank Allen Edwards for contributing to the original project concept and leading the project through early development Chung Lau for his overall tech nical support and especially for his efforts in developing the notch and input circu
27. e the input frequency f an external source is used in the SINAD measurement mode the source frequency must be within 5 of the frequency of the internal source The two programmable gain amplifiers following the notch filter amplify the low level noise and distortion sig nals from the notch filter The overall gain of the two amplifiers is normally set to maintain a signal level of 0 25 to 3V at the output detector and monitor The 30 kHz and 80 kKHz low pass filters are selected from the keyboard With no low pass filtering the band width of the measure ment system is 750 kHz The filters are most often used to remove the high frequency aolse components in low frequeney distortion and signal to noise measurements The output from the second programmable gain amplifier drives the rear panel MONITOR output jack Taking advan tage of the increased amplification available at this point the counter monitors this output in ac level and signal to noise modes The output rms detector is read by the de voltmeter in the ac level SINAD the denominator distortion the numerator distortion level and signal to noise measure AUGUST 1950 HEWLETT ACKAAD JOURNAL Audio Analyzer Applications The 8903 4 Audio Analyzer s measurement Capabilities reach tar beyond conventional distortion analyzers Much of this pertormance reeuits irom microprocessor contro and HP IB programmability Numerous hardware features such as a fast counter beth
28. ent sophistication is built into the 2903A software Measurement routines are structured to optimize measurement speed and accuracy Asa rule 4 HEWLETT RACKAMD JOURNAL ALQUST 1960 measurements triggered from the bus or initiated from the keyboard are accurate from the first reading The operator needn t wait for successive measurements to verify that the reading has stabilized The software algorithm monitors key voltages in the audio chain and waits until they stabilize before taking data Not only does the software perform these functions much more rapidly than the operator but it can also ensure the optimal convergence of the measurement with a repeatable well defined technique Distortion SINAD and signal to noise ratio in particular are examples of measurements that in the past required a significant amount of settling time and operator interac tion A classical distortion analyzer requires repeated ad justments to achieve an accurate distortion reading More recent analyzers have offered semiautomatic tuning and leveling but response time is often long and operator in teraction is required if the frequency amplitude or relative distortion of the signal changes significantly With the RODSA some delay still exists but the delay involved is minimized by careful circuit design and microprocessor control Special functions extend user control of the instrument beyond that normally available from the front panel These functi
29. esign anginaer on both the 89014 Modulation Analyzer and the 88034 Audio Analyzer and as project manager tor the B909A Jim has just joined HP s Disc Memory Division and has moved to Boise Idaho with his wile and daughter Among his i imerests are reading walking rac d quetball skiing chess and doing odd E 2 jobs around the house instrument performance Peter Lee and Jim Stewart for in dustrial design and Bruce Creedy for initial product de sign Special thanks also to Ray Shannon and fim Stinchelfer who were instrumental in the early product definition Other key contributors include Rick Pinger and Jim Harmon in providing service and operating documenta tion Bob Stern and Bob Rands in product marketing sup port Bob Cirner and Ken McFarland in parts scheduling and procurement Greg Hoberg Bob Shatara Phillis Nakano Dana Kreitter and Rich Mills for production sup port and Charlie Sallberg and Chuck Clavell for reliability engineering test and support Reference t AP Edwards Precise Convenient Analysis of Modulated Signals Hewlett Packard Journal November 1979 SPECIFICATIONS Model aata Auto REAR PERE POUTECUTOUTS PES OUTPUT 9 01D rio nome CirwMoraN i wy a saom rahay TOn ve MANION CSTTUIT in pe ae eae ONAN MEI SUE eke et ont nap IN SRD Orton B ARID Nee dea praa ec mele ot Hepa wre 7 he cere erent Serer AMEE NOGA Vs emer by HF me tw tem at crore TOPA PD Fe OF OR Ror
30. ewe oaa ne Rem ex tenet read tr A ee yeg men AY PEALE a seat amibe 1 mopman tea TRPEPRARPS Qena P HC mown PC te EC POW DY LOW PEMENTE O CR IDC Tet LN Ay OMe LV PN AA aed ee 180 08 ree PERNT blew PF Wg LF GRE 1 h gt AmA ee ee RE RS Ty U Ot eat abemi een a ee n of mente ROS amt SHOP ow MAK STO ara WOE BOM art CHES paton 1 Contvaied OE CRS HARRY for he oare fi meters CR CS2 ane POD LE newer oF Am PLD EEA a Ym maan LA DUIM haein Sealy an VAT Oeon dat Rew paa candemosi mesed d kni pans ber mema nape owt ome vom mea BB mass Pob Aa Taere 9608 SA T NE etre r npr e he ee Y pa Ce rat a w R 10 h SO i 6 eat Went A DAD ae m Ne sata 6 spionas s ERA amp ma ii AUGUST t980 HEWLETT PACKARD JOURNAL IT
31. he final output stage This ebminates the need for couplers but requires a high performance differantialinoat amplifier to reject the common mode signal that appears al the input of tha floating stage Since a floating power Supply is sill required the cost of this approach is relatively high The 8903A solves ths probiern with a sngie ended to citterertial Outpul converter Ths c cuit shown in Fig 1 operates on the instru ment s ground relerencec 15V supplies and requires only Iwo op grational amplifiers A precise Combination of negatwe teecdoack positive feedback and cross coupling yields a symmetrical ditteren tial output with infinite common mode rejechon and a well defined Aput impedance A analysis of this Grout ss generally a tedious procedure because of the number of components involved However the high degree of syinmetry in the crcuit can be exploited fo greal advariage by using the relations R2 R1 AI2 R1t A3 A7 RE R10 and As F5 RB AG From these relationships one can derive the expression R2R1 26 A4 2A3 which is a necessary condition for achiev ing an intnite common mode output mpedance Then i s essy to Caku ale the diferenta oulput impedance and Ihe open circuit volt age gan Theresulting equations can be manipulatedto find suitable values For the resetor values used in the B9OGA the associated gain 1 125 and the output enpedance is 480 ohms The output is turther padded with a 120 ohm re
32. he input path ifthe acsignal will overload the input amplifier At this point either the 400 Hz high pass filter or the psophometric filter can be inserted into the signal path The 400 Hz high pass filter is often used to suppress line hum or the low frequency squelch tone used in some mobile receivers The psophomeltric filter has a bandpass frequency response that simulates the average response of human hearing It is often used to condition a receiver audio output when determining the receiver s input sensitivity During SINAD distortion or distortion level measure ments the fundamental of the signal is removed by the notch filter The output from the filter is the distortion and noise of the signal In the ac level and signal to noise modes the notch filter is bypassed After amplifying and low pass filtering the output from the notch filter is con verted to de by the output rms detector and measured by the de voltmeter During distortion or distortion level measurements the notch filter is tuned ta the frequency counted at its input Coarse tuning is done by the controller and internal analog circuitry fine tunes and balances the notch filler During SINAD measurements the controller coarse tunes the notch to the source frequency Thus a SINAD measurement is normally made with the internal source as the stimulus this permits measurements in the presence of large amounts of noise where the controller would be unable to determin
33. irements of the B903A Audio Analyzer the built in source must have good performance in certain key areas First of all for swept measurements i must be readily programmable ard last siling must also have low distartion and reise and must have very good amplitude accuracy over Ihe entire traquency range of 20 Hz 10 100 KH2 The combination of these conticting requirements suggested the use of a high perormance AC oscillator instead of asynthesizer The synihes er ts easy to program and settes quickly butt is difficult to buld a synthesizer wth noise and distortion more than 60 dB below the fundamental Symhesized designs aise do not have sutticient absolute level accuracy or flatness without Iovelog ard a leveling nop that does not unduly degrade the distortion and settling time would be very difficult to cesign Unforunately none of the common AC audio oscillator designs boked suitable either Usually the amount of distortion is nwersely proportional to the settling time Also the tuning elements usually float making the circuit dificult tointertace with pragrarmming lines For these reasons a Sate vanadie oscillator was chosen similar to that proposed by Smith and Vannerson in 1975 Since this oscitator i buit around a state variatte liiter siructure inexpensive JFET switches can be used easily to switch the tuning elements More important S that te ALC design permits very rapid settling without trading off good distortion perfomance
34. its and verifying overall instru ment performance Cory Boyan for the initial oscillator cir cuit investigation software development and coordination and digital circuit development Bob DeVries for product design Derrick Kikuchi for overall software development and latch board development George Pontis fat developing the oscillator attenuator output amplifier and power sup ply circuits as well as various special test fixtures to verify toona PIERE AAE r te oe meae vesa Ty PAULAM RARER LON LIA POEDUEMCY MOIORAGT Go a any IDAS LEVEL RAMCE QG mia m AY apat omaa DTU EELCO REY igan edt PS of arg AE OE re KAPUN JA F NG OWE Y RHE OTOU P otata ee ed re PAINE 1 0 remem E A ee ee DEIN OF ot Hen DORIEN S MONEE The reper AO a ME ol 201s wat OU Ae OO RE AAV oh DOME WOM REN A ON Mer bye Serer Mere RGTR Therm Se TO soei sao hw A PCLAW E Wai te DAR MIO Logay semen wim up 10 SD canavera a 78M cam Leer and set and ey ey enn a ae Ar Rae Oe eS Ac ve ACCURACY a1 at CULL RE PLA OV IR DTV LOBOY EY EAN LiG era OTARA 21 saing DA 208 iwe 22 ot matag 0 Van WY Diu RE UES rag Dore erry er et ee AC CONVERTS Ty ar CASADAS mg ne me sanita aon ay Me ee T Aamen BA A arte yoon seo we FRRRTTION tase on HRD De Lev PULL AMO DEP Lay MOF secey te amv anoe TOPE DN amag on KEY woe ACURA TEN A aag ME ow Oe DE Oi a oH PUNDIT AL Or WAGE Be New bwu LAY MAADE OAB a DD ORME BEL TOPE Ror gemng am y Vag Tete ADAME lt 1 DOE Ne RE ORE aOR Ow
35. nding the amplifier into overload This in turn generates dc offsets in the amplifier chain that can take seconds to decay even if the amplifier may start operating again much sooner Thus a large low frequency impulse appears at the amplifier output along with the signal being amplified The composite signal is transferred to the output rms detector which responds to a time weighted average of the total If the impulse is a significant fraction of the signal the rms detector will not give a true indication of the signal amplitude This can cause problems when the instrument ranges automatically In effect it forces the leveling algorithm to wait much longer to con AURUST 1880 WEWLETT PADK AAD JOURNAL 15 firm that the amplifier output is within the detector s range This in turn slows down the rate at which the instrument can determine the proper measurement range and display a reading Even if the impulse is 20 dB less than the signal the detector error can be as much as 0 5 This will also increase the amount of time required by the instrument to make an accurate reading from the detector once the proper range has been obtained To alleviate these problems the size of the transient Is minimized in three ways First operational amplifiers and circuit configurations are used that have better than average overload immunity Second an overload detector is placed at the output of the rms detector Ifthe signal level becomes too large
36. ogarthnic sweep based on the sweep range and the number of points per sweep which can be set by the user At ihe user need do to get a series of measurements spanning Irequency range is set the start and stop sweep frequencies andthe number of The network consists of two back to back diodes which open up under large signal conditions The differential amplifier consists of three high performance amplifiers These have the necessary noise de offset and frequency performance so that they do not de grade thesignal quality The total input amplifier chain acts as a 4 dBistep amplifier with leveling hysteresis which maintains the post amplifier signal level within 6 dB 3 to 15V rms Should the output level change with time and deviate from this range the gains of the attenuator the differential to single ended amplifier and the program mable gain amplifier are adjusted to compensate points per sweep and press the swrer button This makes it easy 10 measure the frequency response of an amplifier X Y Recorder Output The cakuiating power of the microprocessor hased controller is also apparent in the operation of the 69034 s X Y recorder outputs These outputs are driven by digite to analog converters controlled by the microprocessor rather than directly from interna detectors As a result the user does not have to worry about the recordar output voltage abruptly changing when the analyzer autoranges The mi croprocess
37. ons are intended for the user who knows the instru ment and the service technician who needs arbitrary con trol of the instrument functions Automatic tuning and ranging overvoltage protection and error messages protect the user from invalid measurements during normal opera tion When special functions are used some of these safeguards are removed depending on the special function selected and thus there is a degree of risk that the mea surement may be invalid However there is no risk of dam age to the instrument To enter a special function the user enters the special function code usually prefix decimal and suffix then presses the SPCL key The special function code appears on the display as it is being entered If a mistake is made during entry of the special function code the user can press the CLEAR key and start over When a special function ts en tered the light in the SPCL key goes on if it is not already on The readout on the display depends on the special function entered It may be a measured quantity an instrument set ting or a special code In some cases the display is unal tered Special functions can be entered from the HP IB by issuing the special function code followed by the code SP Floating Input and Output To eliminate troublesome ground loops both the source output and the measurement input of the 8803A are float ing This is helpful in low distortion or low level ac mea surements when it is
38. operly set This done because it is quite common alter the signal level has suddenly changad for example when the notch filter is suddenly switcned in De Inputs and low level differential amplification for common mode rejection are necessary features for an in strument like the 8903A One consequence of automatic ranging is that low capacitance mechanical switching techniques cannot be used effectively Needed are high voltage reed relays which affect high frequency perfor mance and require the use of compensation capacitors Fordcoperation the first part ofthe input signal path is dc coupled with the input blocking capacitor bypassed The output of the differential to single ended amplifier can then be monitored to obtain an accurately scaled represen tation of the input de level B HEMLETTAA KARG JOUSNAL AUGUST TaM Making the Most of a Microprocessor Based 7 Instrument Controller Measure Signat Level Take Reading for Dlegtay Fig 1 89034 leveling algorithm The mcragracesser checks the rate of change of the input Signal as well as its level to make certain that the signal Aas settled before the output ampiier gan ts set when gong from ac level ta distortion mode to nave a rapiely falling output ampilitier voltage pass through the acceptable regon ang thus have the gain appear to be properly set whan in tactthe signal i on as way to a level that will require more g
39. or scales the recorder output according to the displayed reading and the pilot limits entered via the keyboard The recorder outputs are always between zero and ten volts s the recorders zero and vernier controls need be adjusted only once Thus a properly scaled plot is easily generated by wena the sweep and the X Y outputs without any need for an extemal controller Special Functions Hidden behind the basic measurements are nearly torty special functions whch provide extended measurement capability and many senwics aids For example the analyzer can be given a lead resistance in ohms and commanded to display ac level nm walls Anather special function changes the number of points per decade in a sweep and several special functions modify display operation Normally the eft and right displays indicate the frequency and level or distortion etc of the signal applied to the anatyzer input Some times when using the 8903A just as a source the user may want the analyzer to display the frequency and level of the source Special function 10 provides this display Service aids provide front pane display of many internal voltages and settings Without microprocessor contro each special tuncton would requite one or more swilches on the trom panel msicad olone scal key and would therefore probably not be included Thus the processor allows implementation of useful featuras the user would fot orherwse get Corydon J Boyan Cory Boyan
40. otch filter apply equally to the oscillator In both cases tuning consid erations were the same with switchable resistor networks used as the decade tuning elements and four quadrant multipliers used for amplitude control In many ways the oscillator and notch circuits can be seen as duals The notch generates pair of zeros on the jw axis that reject the funda mental component while the oscillator generates a pair of poles on the jw axis that generate sustained oscillations The trick in the oscillator is to keep the poles exactly on the axis to maintain constant output amplitude This must be done continuously by the automatic leveling circuit If the fre quency of the circuit deviates from the desired frequency the circuit can be fine tuned by the microprocessor which monitors the output frequency on a sampled basis The major performance goals of the oscillator were low noise and distortion rapid amplitude and frequency settling and digitally programmable frequency control Again the state variable filter configuration along witha special level ing circuit offered the flexibility and performance required The oscillator design is described on page 10 16 HEWLETT PACKAI JOURNAL A GUST 1980 A It was determined during development that the oscillator would have to run at 4 constant output level to maintain reasonable settling and noise performance It was also de sired to have a floating output The attenuator and output amplifier
41. ratio of the two measurements is computed 2 Input frequency measurement The signal from the last programmable gain amplifier or the high pass bandpass filters is conditioned by the counter input Schmitt trig ger to make it compatible with the counters input The period of the signal is then counted the count is pro cessed by the controller and the frequency is displayed 3 Source frequency measurement The counter measures the frequency of the oscillator during tuning and when Analyzer input Computer Fig 2 Tes setup for screening operations ampibers The swept Irequency measurement with plot capability inds mary appkcations n the laboratery Fig 1 shows the swept distamon and frequency response ota two pole active filter The upper curve snows the magnitude response while the lower shaws distortion Notice that the analyzer input magnitude covers a 30 dB dynamic range During the sweep the BS0GA is automatically setting input gain before per forming the distortion measurement Itan HP IB controller is availabte there are many more appleations for the S90GA Fig 2 shows a simple test setup for screening opera tional amplifiers With no other instruments in the system a computer contolled 89034 can rapidly and accurately measure inpig offset volage input noise voltage anc distortion it can also be used to measure the gain bandwidth product of the opamp pro vided it ismot greaterthan 30
42. rs the input signal and makes internal gain and frequency adjustments as required In automatic measurement systems the 8903A is capable of rapid and straightforward remote control Analyzer op erations can be controlled and all measurements can be transferred via the Hewlett Packard Interface Bus HP IB Hewlett Packard s implementation of IEEE Standard 488 1978 On the bench the 89034 allows rapid and accurate circuit characterization when many repetitive meesure ments are necessary Major application areas for the 890A Audio Analyzer are general audio testing transceiver testing and automatic systems In general audio testing the 8907A measures the Fig 1 Vode 803A Audio Analyzer makes the accurate measurements needed fo charac tarize systems and Signals in ts frequency range of 20 Hz fo 100 kHz it has appications at generasi awo fesing vansowver testing and auformate systems Micro processor Canro mares t awto matic and easy t0 use AUGUST 1RROMEWLETT OACKARD JOURNAL J Input Attenustoe gt Migh Pase Bendoass Fiters Output Rms Avg o SINAD Meter gt o p Ea PS o Ampistier Output Fig 2 The 8903A Audio Analyzer is basically a distortion analyzer with a tunable matih filter to ramove the fundamental frequency Cornponent of the signal and a detector to measure what yamains wiyah cansisls of noise and distort ion Added to this are a microprocessor based
43. s the 2 MHz time base which also is the clock forthe 6 HEWLETT PACKARD JOURNAL SUGLGT 1980 o Consider the 39034 used at a test and calibration station in the manufacture of audio power ampiiliers A typical sequence of events might include an output offset null frequency response check distor tion test and noise measurement The 89034 can pertorm all thesa measurements Quickly with a single test setup If an X plotter is connected to the rear panel outputs the results of swept frequency measurements can be recorded on standard tog log or log lin graph paper Total Harmonic Distonian Noise Fig 1 Swept cistovhon and fre quency response of a two pole ac five filter measured by the 89034 Audo Analyzer controller The counter has four inputs and three modes of operation 1 Voltage measurement The time interval from the voltage to time converter is counted The accumulated count is proportional to the de voltage For direct mea surements fac level and distortion level the count is processed directly by the controller and the result is dis played For ratio measurements SINAD distortion and signal to noise the counts of two successive measure ments are processed and displayed For SINAD and dis tortion the controller computes the ratio of the outputs of the Input and output rms detectors For signal to noise measurements the output of the output rms detector is measured with the oscillatoron and off and the
44. sistor to yield the desired GO0 on pulpu impedance It would have been possible to use resistors that gave an ouput impedance of exactly 600 ohms ristead of 480 ohms bes this would have required setling up and stocking a supply of several exira odd resistor values As il is the cecuil is realized using 0 1 25 pom resistors that are aiso used elsewhere in the instrument timum match between the filter configuration and the vari able resistive element is not straightforward Let s go through the alternatives and the tradeoffs Switchable resistor networks have good distortion and noise characteristics but do not provide continuous tuning coverage and require extensive switching circuitry Photo resistors can be driven over a large resistance range and provide continous tuning However the noise and distor tion they add to a signal are greater than the required level of 90 dB below the signal level They can be used as fine tuning elements if coupled only partially into the circuit These devices can also be slow and are awkward to control rapidly reducing the tuning speed Finally they tend to vary significantly from device to device and with time and temperature making compensation difficult 12 WEWLETT PACKARD JOURNAL ALXQUST 1980 Floating a Source Output by George D Pontis fo ged Fig 1 Singic ended fo cifarential output corwerter orowides a floating output for ihe B90GA s wama sciaro The easest way
45. t must not be allowed to degrade the input noise floor when measuring 50 mV The 300V signal may also have spikes or transients that far exceed 300V or the user may inadver tently apply a larger signal In all these cases the circuit must recover without causing a safety hazard to the user destroying internal components or even blowing a fuse AUGUST 1980 HEWLETT PACKARD SOLANAL T by Corydon J Boyan In the 89034 Audio Analyzer most of the tasks that ac to Ine display of measurements are coordinated bya microprocessor The procassar an FB with 18K bytes of ROM and 192 bytes of RAM counts and tunes the l mal audio source sets the input ampttiers gain tunes the natch filter sets the ouput amplifiers gain and measures the voltages that will be used to generate each reading This means that the 89034 can among other things automatically take distomon readings without calling upon the user to turn several knobs when seeking a null This ability alone is ample justification for basing the instrument controller on e microprocessor bul the 89034A goes far beyond this in applying the power of ts controler Guaranteed Accurate Measurements The 89034 is HP 1B programmabile and this brought some mpor tant tactors into consideration during the design The perfornance of the ternal source when settling from one fraquency ar level to arother for example 15 a key tactorin assuring the validity of the lirst measurement taken atter
46. the overload detector trips and the amplifier gain is reset to 0 dB Third a 13 Hz high pass filter is placed before the output detector This significantly reduces the duration and amplitude of any transient and hence keeps the transient from significantly increasing the total mea surement time The only delay factors that remain are the controllable and predictable settling times of the notch circuit and the rms detector The response time of the output rms detector is a com promise between rapid settling and low frequency accu racy A configuration was selected that settles to within 1 of a 10 1 step in 350 milliseconds and has a steady state error of 0 2 at 20 Hz This includes filtering in the detector and additional filtering following the detector to reduce excess ripple For leveling purposes the ripple is not sig nificant so the microprocessor uses the detector output when leveling and avoids the extra delay contributed by the additional filter The output detector and filters could have been designed with switchable time constants to respond more rapidly for higher frequency signals However the penalties would have been additional circuit complexity and the ambiguity of not knowing when to invoke the longer time constant A 20 kHz signal for example might still have a significant low frequency component which would cause excessive error with a more rapid time constant Oscillator Many of the design considerations for the n
47. to see Now this circuit works is to eliminate enher the inverting flow of noninverting high naif of the circun by shoning the respective output to ground Fig 2 shows the reduced circuit when the low hal is grounded If R4 is disconnected the circut wil have fotward gain of about one and an output mpedance of 276 ohms R4 works in conjunction with AG to provide voltage and current feedback that causes the gain and the output impadance to rise To dernonstrate that the output is truly floating we ground the npu and apply test source tp both outputs ideally the current flow from the test source should be zero Fig 3 shows a block diagram and the reduced circuit for this test Here can be quickly calculated that the output of UT will rise just enough over that of the test source to make the current through RS cancel the current through A4 and AB Note that the current flowing through sources V1 and V2 is supplied by the other hall of the circuit which is not shown Four quadrant analog multipliers also do not have the 90 dB performance necessary but they too can be lightly coupled into the circuit for fine tuning These devices are fast inexpensive and easy to drive There are many varia tions on this type of circuit some of which can be obtained in integrated form Those most suitable use a differential pair of bipolar transistors as a variable gain element by varying the common mode current Light bulbs as variable resistive
48. tput frequency measurements as often as every Bight milliseconds When the 89034 is used in conjunction wih the 49014 Modulation Analyze almost all transmitter tests can be automated Fig 3 shows the biock diagram With the source tumed off and the transmitter keyed the squelch tone frequency can be counted Then the 400 Hz high pass filter can be switched in to eliminate the squeich tone fron the remaining measurements With the source output turned on the 89034 can easily be pfograrmmed to make the necessary Measurements to determine distortion and microphone sensitivity counter converts Measurement results into ratios in or dB and so on It also executes routines forservicing the rest of the instrument as well as itself Input Circuits Numerous design constraints were imposed on the input attenuator protection amplifier circuitry An input imped ance of 100 kf is necessary to prevent the input circuitry from unnecessarily loading the device under test On the other hand maintaining a good signal to noise and distor tion tatio good frequency response to 100 kHz automatic operation and reliable performance with input signals as large as 300V is very challenging Consider the 300V con straint tis possible for a 300V signal toappear suddenly at the input while the instrument is measuring a 50 mV input level Not only must no damage occur but also the overload tecovery must be quick and the input protection cireui
49. utput rms detector Any error degrades the 8903 s performance The output amplifier allows the low level distortion products leaving the notch to be accurately detected by limiting the required dynamic range of the rms to de converter The rms detector is accurate over only a 30 dH range and the output amplifier boosts these signals into the range of the detector As with the input circuitry rapid recovery from overload conditions is crucial If the notch becomes mistuned be cause of a disturbance at the input the output suddenly Fig 2 89034 notch fitar fine tuning and balance circuits lune voltage is stable although oot necessarily zero and na dc current flows into the tune integrator Therefore there is no notch output component synchronous with Vim The fine balance circuit insures that there are no components synchronous with Vap inthe notch output Vep and Yip are bathal the fundamental input frequency and are in quadrature with each other Thus at steady state there is no fundamental trequency inthe notch output Chung Y Lau Chung Lau is a fative of Hong Kong He FeCeived his BSEE degree in 1975 and nis MSEE degree in 1976 trom the University of Calfomia at Barkeley With HP since 1976 he s worked on the B901A Modulation Analyzer and contr buted to the design of the 89034 Audio 2 Analyzer Chung lives in Cupertino gt Califorma and enjoys bridge and increases dramatically se
50. ws see Fig 2 The low pass filter voltage Vi p which is phase shittect 90 from the input drives the tune comparator UBA which turns JFET switch Q5 on and off When Q5is on point Ais essentially grounded and no current flows 110 the iMegrator capactar C3 When QS ls aff the natch amplifier feeds the tune integrator Because of this chopping action the de currant that flaws imo the integrator is caused by the notch output component that synchronous with Vyp The tune integrator output is a de voltage that can be changed only by tha do current flowing nto the integrator This voltage leeds one input of the multiplier USA The other input to the multiplier is Veo The product of the two inputs i summed into the siste variable ffer via U3 see Fig 1 The net resul is that the tune imegrator voltage can change the effective resistance Ry in Fig 1 and hence the center frequency of the notch filter The direction of change is such that any notch output components in synchronism with Vip are teduced Al steady state the Arual ted harmoors p the notch qutpul can also cause d current t flow n me Agio Thee error casera Snas amourtt of the tuntiamerts tequency COMpONert t Hass Trough the AWN Mie However Pe trauma eni c rirtulam t aelotan ree esemants fom thia effect is only ant 046 dB and good frequency response are important In ac level measurements the signal travels through the output amplifier chain and is detected by the o
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