HP 55330M User's manual

HP 55330M User's manual
A Complete Self-Contained Audio
Measurement System
This automatic, autoranging audio analyzer has the signal
source, distortion analyzer, and counter to make the
measurements most often needed In audio-frequency
by James D. Foote
Analyzer (Fig, 1) 18 à complete audio measurement
syalem for quick and accurate characterization
of eystems and signals in the frequency range 20 Hz to 100
kHz. The starting point For the BA0AA is the claësical distor-
tion analyzer. Added to this are microprocessor control, a
reciprocal frequency counter, rms detectors, and & pro-
grammable audio source. These provide accurate mea-
surement of ac level, distortion, SIAL" signal-to-noise
ratlo, and de level. The audio source and the measurement
circuits cran work independenthy or together. The soumce is
programmable in frequency and level and has very low
distortion The measurement circnils can monitor This in-
temal sourca or any other independent input Wwavetorm.
Together the source and measurement input can bé used For
swept TRSpOnse measwreman ts,
a o ag Fadl
E Me FLEE aaa
rem pray CTA A MAA №
All measurements ara avallable at the push of a button.
No knob adjustment OT operator interaction 18 Necessarv.
Ono simply applies the signal and selacts the measurement
made. All control and processing are handled by the inter-
nal micro processor. The microprocessor monitors the inpul
signal and makes internal gain and frequency adjustments
as required.
[n automatic measurement srstema, the 09034 18 capable
of rapid and straightforward remote control. Analyzer op-
erations can be controlled and all measurements can Бе
transferted via the Hewlett-Packard Interface Bus [HP-IE],
Hewlett-Packard's implementation of IEEE Standard 44H:
1978. Om the banch, the B00TA allows rapid and accurate
circuit characterization when many repetitive measure
ments are necessary.
Major application areas for the B90G À Audio Analyzer are
general audio testing. transcelver testing, and automatic
avstems, In general audio testing, the 89074 measures the
Fig. 1. Model! 54034 Aud
Analy 2ar mares the accurate
measurements nésded fà charsc-
Maries EST ario Sigurals im ke
frequency range of 20 Hx do 100
kHz. has applicatons мт селена
Bo resing, Fanscaver désir,
and aufevratic systems, Micro
DOCcRSsor CONO Mames 4 alo
mario and easy 10 use
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Fig. 2. The BA03A Audio Analyzer is hasicaly a-distoeriion amayzer, with Ba maten filler io
ramove Ihe fundamental frequency component of Ma saya and a detector to measure war
remains, vivo sonsisls of noise and DST
ov. Added ho (hs are à micréprocessar Lased
controller. a reciorocal lrequencr Dévoisr, a pregrammarbre audio Source, and olher resources.
Лали a complele audio mas LASTEN SY SNE.
frequency response and distortion characteristics of filters,
high-quality am plifiers, 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 for easy generation of
plats using an X-¥ recorder.
For transceiver applications the most common recebver
measurements are SINAD for FM receivers and sigmal-to-
noise ratio for AM recelvers. À psophometric filter is io-
cluded for making measurements to CEPT standards.
Common transmitter measurements such as audio distor-
tion and squelch tones are made using the 58034 with its
companion Instrument, the 8001 À Modulation Analyzer.!
In automatic systems. the 89034 provides me frequently
needed audio functions, doing the work of an audio syn-
thesizer, digital multimeter, [requency counter, and tun-
able notch filter. More details on specific applications are
presented on page 6-
Control Philosophy
Eront-pañel control of the audio analyrer ls simple. yet
powerful. Most functions can be used and understood with
vary little training. The casual user can select amplitude,
frequency, measurement mode, and filtering simply by
reading the labels on the controls, More details are avai lablr:
on the instruments pull-out card.
A great deal of measurement sophisticallon is built into
the 800A software. Measurement routines are structured
to optimize measurement speed and accuracy, As a rule,
— SR
measurements triggered from the bus or initiated from the
keyboard are accurate from the first reading, The operator
needn't wail for successive measurements to verify that the
reading has stabilizad. The software algorithm monitors
key voltages in the audio chain and waits until they
slabiliza before taking data, Not only does the software
perform these functions mu ch more rapidly than the
operator, but it can also ensure the optimal convergence of
the measurement with & repeatable, well defined technique.
Distortion, SINAD, and signal-to-noise ratio, in particular,
are examples of measurements that in the past required à
significant amount of setiling time and operator interac
tion. A classical distortion analyzer requires repeated ad-
justments to achieve an accurate distortion reading. Mara
recent analvzers have offered semiautomatic tuning and
leveling, but response time is often Lang. and operator 10-
teraction is required if the frequency, amplitude or relative
distortion of the signal changes significantly. With the
BAA, some delay still exists, but the delay invalved is
minimized by careful circuit design and microprocessor
Special functions extend user control of the instrument
beyond that normally available from the front panel. These
functions are intended for the user who knows the instru.
ment and the service technician who needs arbltrary con-
trol of thé 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 ol 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 instrament.
To enter a special function, the user enters the special
function code (usually à prefix, decimal, and suffix) then
prasses tha 3PCL key. The special function code appears on
the display as it iz being entered. If a mistake 1s made during
entry of the special function code, the user can press the
CLEAR key and start over, When a special function Is 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 lrowblesome ground loops, both the source
output and the measurement input of the 8003A are Moat-
ing, This is balplul in low-distartion or low-level ac mea-
surements when it is necessary 10 reject potential differ
ences belween the chassis of the 8803A 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 nf
high EMBL All of the analog circuitry is shielded by an
internal EMI-light box. The output and input lines extend.
ing from thiz 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 ar twisted-wirs connection can be
made to the instrument when EMI shielding is not critical.
Analyzer Architecture
The 89034 Audio Analyzer combines (ree instruments
nto ore: 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-bazed 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-1B programmability, Fig, 2 is a simplified
Block diagram.
The amplitude measurement path flows from the INPUT
jacks [HIGH and LOW) to the MONITOR output on the rear
panel, and includes tha input and output rms detectors, the
dc voltmeter (the voltage-to-time converter and counter],
and the SINAD meter circuitry, Measurements are made an
the difference between the signals at the HIGH jack and the
LOY jack. Differential levels can be as high as 300, Signals
that are common to both the HIGH and LOW jacks are hal:
enced out. Signals applied to the LOW jack must not ex-
ceed 4V.
The input signal is ac coupled for all measurement modes
except di level, The signal is scaled by the input attenuator
to à level ol AV pr less. To protect the active circuits, the
overvoltage protection circuit quickly disconnects the
input amplifier if its inpul exceads 16%.
The differential signal ls 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 keap the
signal level at the input rme detector between 1.7 and AV
Ems to oplimize 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 af the distortion
measurement. The gain of the input path is determined by
measuring the de level. The input ms detector also
monitors the ac component [if there is ona) and lowers the
gain of the input path ifthe ac signal will overload the inpat
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 filtar is often used to suppress line
hum or the low-frequency squelch tone used in some
mobile receivers. The psophometric filter hos a band pass
frequency response that simulates the “average” response of
human hearing. М is often used to condition a receiver audio
outpul when determining the recelver's input sensitivity,
During SINAN. 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 filler 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 dc voltmeter.
During distortion or distortion level measurements. the
notch filter iz tuned to the frequency counted at its input,
Coarse tuning is done by the controller, and internal analog
circuitry fine tunes and balances the notch filter. During
SINAD measuremonts, the controller coarse tunes the natch
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 determing
the input frequency). If an external source is used in the
SINAD measurement mode, the source freguency 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 lo maintain a signal level of 0.25
to 3V at the output detector and monitor. The 30-kHz and
B-kHz low-pass filters are selected from the keyboard.
With no low-pass Filtering, the band width of the measare-
ment system is PH kHz. The filters are most often used to
remove the high-frequency nolse components in low-
frequency 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 polat,
the counter monitars this output in ac level and signal-to-
noise modes.
The output rens detector is read by the do voltmeter in the
ac level, SINAD (the denominator], distortion (the
numerator), distortion level, and signal-to-noise measura-
The 6304 Audio Analyzer 5 measuremen| capabilites reach lar
beyond conventional detortion analzars. Much olf this paricamance
results from microprocessor contr! and HP-IB programmability
Numerous hardwara features such as a fast coundar, bath analog
meter and digital display, and switchable detecter ilfening flow he
user io maxe musual or special measurements wilh Comeniencs
ard bte auxiliary apparatus
10 dev —
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4 64 kip 12.1 kik |
—10 dav 3. Input 4700 _L 100 |.
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= $ 511 kil
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—50 dB+
Total Harmon Distanion
Audio Analyzer Applications
Ceorsider fhe d9034 used af a fest and calibration station in he
manufacture of audio power ampdiliers. Atypical sequence of events
might incude an pulg offset null, frequency response check, distor-
tion test, and nose messurament. The 69034 can pertorm а! ева
Mméssurements quickly with à sevgle test setup. lé an X-Y proiiar Is
connaciad to the rear-panel outputs, the results of swept frequency
measurements can be recorded on standard ogllog oor hogrlin
graph paper
+ Noise
—70 d8 Ï
10 kHz 20 kHz 30 kHz
Flg. 1: Swepr osiorhon ang fre
ueno; response of a hvo- poa ar-
Mee Mar, measured by the B24
ment modes, [His 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 spéciflcations for detection af
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.
torm ig known, [If the waveform is Gaussian noise the read.
ing will be approximately 1 dB less than the true rms value, |
In the SINAD moda the outputs from the input and output
rms detectors are converted to logarithms, subtracted. and
converted io a current by the SINAD meter amplifier to drive
he SINAD panel meter. Since SINAD measurements are
aften 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 de inputs into a
time interval, which is measured by the counter.
The BADIA uses à reciprocal counter. To measure fre-
quency, IE counts the period of one or more cycles of the
signal at its input. Then the controller divides the number of
evcles by the accumulated count. The reference for the
counter isthe 2-MHzx time base, which also iz the clock tor the
controller, The counter has four inputs and three modes of
1. Voltage measurement. The time interval from the
voltage-Lo-Lime converer is counted, The accumulated
count is proportional to the de vollagé. For direct mea-
surements [ac level and distortion level], the count is
processed directly by the controller and the result is dis-
played. For ratio measurements (SINAD, distodion, and
signal-to-noise), the counts of two successive measure-
ments are processed and displayed. For SINAD and dis-
tortion. the controller computes the ratioof the culpots of
thé input and output rms detectors, For signal-to-noise
measurements, the output of the output rms detector is
measured with the pecillator on and off and the ratio of the
two measurements is computed.
3; 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 tn make i compatible with the counters input. The
period of the signal is then counted, the count ig pra-
cessed by the controller, and the frequency is displayed.
3. Source fraquency measurement. The counter measures
the frequency of the oscillator during tuning and when
BO kt
Fig. 2 Tes! setup for screaming operations! amedfiers.
Tha emeptireguency measurement with plol capability nds many
applications in the laboraony, Ag. 1 ehgws ihe swept distlomon and
frequency responsecla [wo-pole active lilter. Thé upper curve Encwes
the magnitude response ahile thé ler shaws distortion, Notice hat
fe analyzer input magniludé covers a 30-08 dynamic range. During
he: sweep. the ÉSCGA is automatically salting in put gain before per-
farming the disfortion measurement,
ltan HP-IB controñar ts available there are many mors apphéations
tor Me S034. Fig. 2 shows a simple fest salup far screening opera-
tonal ampliigrs WIEh nú other instrements in the system, a
comouter-controlied 33034 can rapidly and accurately measure
np oliset voRage. input noise voltage, and distortion. It can also be
Usd By maasune the gain-bandwidth product of the op-amp. pro-
vided il ismofgrealerihan 30 Mz Tha controlas tesi programan be
written fa provide aithar 8 gono- 30 outgutor a hating Ol measwement
Many of he special measurement modes are avallable through the
Use ol special finctiona. For example, the B903A car be Zed as a
lest amplifier wih gain settable from —22 de to +94 de. Signal
Mtenng can be added by selecting the 8 ppropriste front-panel con-
mls, or the special Functions can be usedio paí e instrument into a
notch or bandpass Alter mode. OF course, signal frequency and
amplitude will be measured and displayed iHhe operaling logde are
| ‘Chosen Correctly. This moda of operation migkes if possibéa 10 count
Audio Analyzer
Fig. 3. Transmiller Jest sedup using Me 9014 Modulahon
Analyzer and the 89034 Audio Ansiyzer
the frequency of a signal whose amplitude Es in thé low lens of
Transceiver Testing
Much gffor has gone into the design ol tho B903A to laciitale the |
Audio measurements required lor automatic, programmed,
Iransteser lasting. For example, tha worst-case source frequency
error ol 0.3% allows Me SICIA source lo replace a synthesizer for
aquetch-torw generation. In addition. à binary programming modé is
availabe ough the HP-IB thal causes tha 89034 to gener ate a tone
burst secuencá that can be used lo unlock a coded receer. À
refaled function allows tha BARDEM lo measure burst tones gererated
Dy an exlamal source, such as e ransmibler uncior test. The packed.
four-byte output allows the 83034 10 ouput Trequency measuremants
as often as every sight miliseconds
When the 80004. usad in conjunciion weh ihe 8901 A Modulation
Analyzer. almóst all iransmitler testa car be automated. Fig. 3 shows
Ina block diagram. "th the source fumed off and the transmitter
keyed, he squalch-tone frequency can ba counted, Then, tha
400-Hz high-pass filter can be swiched in to allminate the scguelch
tone din the remaining measurements. With the source outout lurmes
on, the 89038 can easily be programmed to make the necessary
Measurements io determine distortion and microphone sensitivity
verifying that the oscillator frequency is within tolac-
ance. This frequency is normally dot displayed.
The source covers thé frequency range of 20 Hz to 100
kHz. It 1s tuned by the controller to the frequency entered
from the keyboard. using à tune-and-count routine. (Note
that the frequency is not obtained by frequency synthesis.)
The switch following the oscillator is closed except in the
signalto-noise measurement mode or when an amplitude
of 0V 1s entered from the keyboard,
The source output amplifier and output attenuators pro-
vide 77.5 dH of attenuation in 2.5-dR steps, and 2.5 dB of
attenuation in 256 steps. This gives an open-circuit output
from 0.6 mV to 6V. The floating output amplifier converts
the ground-referonced input to a floating output, Either
Output. HICH or LOW, can be floated up to len volts peak,
The entire operation of the instrument is under control of
the microprocessor-based controller, which sets up the in-
strument at lurn-on, interprets keyboard entries, executes
changes in the mode of operation, continually monitors
instrument operation. sends measurement results and ee-
вотв te the front-panel displays, and interfaces with the
AP-18, lts computing capability is also used to simplify
circuit operation. For example. it forms the last stage of the
unter, converts measurement results into ratios (in % or
dB], and soon. It also executes routines For servicing the rest
oËthe instrument as well as itself.
Input Circuits
Numerous design constraints were imposed on the input
attenuator protectionamplifier circuitry. An input imped.
ance of 10M kf} is necessary Lo prevent the input circuitry
from unnecessarily loading the device under test, On the
other hand, maintaining a good signal-to-noise and distor
tion ratio, 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. It is possible for a 300V signal to appeer suddenly at
the input while the instrumenl is measuring a 50-mV input
level, Not only must no damage occur, but also the overload
recovery must be quick, and the input protection circuit
must not be allowed to degrade the input noise floor when
measuring 50 mW. The 300Y 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, ar even blowing a fuse.
Making the Most of a Microprocessor-Based
Instrument Controller
by Corydon J. Boyan
In tha 25034 Audio Analyzer mosi dl ie tasks that Bad ho Mme
display ol measurements ará coordinaled by a micnprocassor. The
procgesor fan FE with 18K bytes of ROM and 192 bytes ob RAM)
counis and tunes the miemal audio source, sels the inpat ampliiers
gain, nas the notch liar, sets the oupat amplifiers gan, and
measures the volages that will be used lo generate each reading,
This means that the 24034 can. among othar ihngs. automalically
lee digtoton readings withoul calling upon tha user Turm Several
kñoña wen seghng a Mil This ability alone is ample justification for
basingihe instrument eontroder on a microprocessor, bul the 80034
goes far beyond this in applying the power of 15 «catrober.
“Guaranteed” Accurate Measurements
The 490534 5 HP-18 programmable, and this brought some impor
land factors into consideraton during the design. The performance of
he internal scurce when setiling from une frequency ar léval Lo
another, (or example, 15 a key factorin assumio ihe valichty dl the lira?
measurement taken after changing ihe frequency or level. Similarty,
Me setting perormance of the input and output amplifers and the
malo iitar under various adverse signet condilinons Decomas very
important hen he instrimentis ingng to deleer an accurate firgl
measurement after a change in operaing naramalars (6 q, altar
Mungia s new frequency], Ade fo ihes Me desire to Make MaasUne-
ments as rapid ly as posstve and you have a very imerésiing problem
fora mcroprocessor lo solve.
For exami, avery firma tha imernal source 16; tuned, The processor
spends atoul 170 milkseconds iuning 0 ard ven ying ha fraguency
This translates to over BE O00 operations. Of equal complauty is (he
ob: of salting the correct gain fof the cul put amplifier and allowing he
circuits to seitls before the outpul ampifer voltage is read, wih the
Pobject of never giving the uses aninvalid reading while at the same
lima delverng he reading in as shor a hme as possiole. To ac-
| complih this, the routine that ooniols the output amplifies makes use
of such techniques as siope checking and frequency dépendent
delays to ensure rapid, valid readings. These lechnigees make pos-
sible accurate readings in half the time it might otharwse take.
Rafarrisvg ле ом chart Fig. 1, note that the key 10 his routing is ne
lechnigue ál measuning the rate af charge of the signal on he cunpul
ampliler Mie sopa) before altemptag 10 check the sigral level Lo
| datermine if the output amgéiier gain is properky ses. This is done
| because € is quite comman. aller the signal level has suddenly
| changed (far example, when the notch ilar is sucdanly swilcned in
He La Yes
[Na LL Take Heading
Cain LL all
Fig. 1. ВВОЗА feveling algorithm. Tie MCromCes so ahiecks
ihe rate of change of Ihe need signed as wall 5 ds eval fo maka
carta that the spon Aas sedtied batons the oop ampliar
qa 5 sel
wher gong rom ac level ia distortion mode], 1o nave a rapidly falling
output ampiter voltage pass Inmugh ihe accoptable region. and
Ius have he gain apeear io be properly Ser henintaciihésigna! is
on As way 10 a evel that will require mare gaintorina proper reading.
Ta keen tha cartralie: [rom being fooled by this phanamenon, wa (sa
bec voltage readings a raped succession. and fmm hem calculate
the rabe of change of the signal. The time over which we megsura this
shops e longer at lower frecuencias, andis a factiookad upinatable
ROM based onihé lregueney 81 which we Te operating. li me rate
of change i ton fas! durng this pariod. we delay before: checking tho
level (ees Fig, 21. This delay is also Trequency dependent and tame
driven Note that f we simply delayed each cysle io keap rom: risking
De inputs and low-level differential amplification for
common-mode rejection are necessary features for an in-
strument like the 8003A, 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 perfar-
mance and require the use of compensation capacitors.
For de operation the first part of the input signal path is do
emipled with the Input blocking capacitor bypassed, The
output of the differential-to-single-ended amplifier cam
then be monitored to obtain an accurately scaled represen-
tation of the input de level.
Input voltages larger than 3V are attenvatad by the inpul
attenuator, a network of resistors that divide down theinpul
signal. The appropriate tap point is selected by a reed relay
network. IF an overload occurs. the maximum atlenuation
selling ls enabled.
To protect the sensitive input amplifler Following the
attenuator from short-term transients, an overvollage pro-
tection network is used. For low-level signals the transfer
impedance is low and signals applied to the input are
coupled to the differential-input amplifier, However, tor
input signals large enough to damage the amplifier the
output of the protection network is limited to a safe value.
Signal while setting
Level looks good here Window of
Acceptable Level
Signal Level
| | Time
ina slope measured in this imerval
shows reed bo delay to lel signal settle
Flg. 2. An examye oF sod checking Ly He S90ZA micro:
displaying invakd readings, every measurenent woud eke longer
peñars Ey severa! hundred milksaconds. The slope checking gives
us a rapid chack on signal guakty which aliminatas the raad to снегу
every lima
Attar the slope check (and posabde delas we aaeiinne the output
| amplifier is wali settked and ready far lesaling. We than maasure thie
volaga al ES output, 2nd if too high ar lg adsl ie gan acord
ingly. After edjuisting the gain, we make another pass though tha
leveling ОО (unless de ard nova in the monestgain range | Ta
keep from getbng caddghtin minte oops, we require that after ha lirst
pass the gain newer be reduced, and assume that if 1 is, wa hava an
упала signal, winch Gees us ho start the entira measurement
cycle (lune source, kevel input, tune noteh, level output] over again
Thé tnal protection from infinde loops comes from counting the
number of fimes ve restart wilhout displaying (gach time wë pui à
"eee pattern on the disclry!, and aber 128-hmes we de pray Ene 31,
which algo goes cul Eo The HF-IE,
Because the BI0IA has ihe abdily to generate accurate and com
| paratively rapid measwements aulomatically, with no need for tre
| user to inser delays in HP-1B routines or wait for ihe display to sells
before taking à reading, it is possible 10 have the insirument evweap
itself over à range ol freguencies wifhauf usér 6r computer imerven-
tion. Here the calculating power of ihe microprocassor is browghl to
bear orihe problem of datermining the frequency incresnent [or each
mew pond in tha logarthmic sweep, based on the sweep ranges and
the numbar of points per saeen (which can ba sal by the user), Al the
(ser need do lo gel a señas di megsurements spanning alreguency
range ¡Ss Ser the start ano stop aweep ragquanc igs and the somber of
pants peer Sweep, and press the SwEer button. This makes if easy 10
measure Ihe frequency response of an ampditer
X-Y Recorder Output
The calcutaties power of fhe microprocessor- based controller is
also apparent in the operation of а: ВОСКА X-Y reconder oulpuls
Thess pulputs are divers by digirs-to-analog convesters controllad
by the microprocessor, reherihan directiy from internal detectors. As
aresull; the user does not hava towory about he recordar ouput
volage aonplly changing when the analyzer autoranges. The mi
Croprocessor scales the recorder output accordirez lo he displayed
reading and he plo mils entered via he keyboard, The recorder
outpuis are aheays between zero and ten vote 23 Me recorder s Zem
and vérnier controls need be adjusted only once. Thus, a progarly
sealed ploi ls easily generated by узла the sweep and the KY
ou pits, without ny naed far an extemal control ir
Hidden behind {he basic measurements are neariy ony special
lunélions. which provide extended measurement! capabibiy and
many senvice alds. For example, the analyzer can be given a iad
resistance in ohms and commanded to displey es level an waits
Arather spacial funchon changes the number of points per decade in
a sweep, and several special functions modiby display operation
Normally he loft and right displays indicalé 1he recwency and level
for distorton, esc.) ol he signal apelied to he arstyzarinpul DOM
mes when using the ASOSA мая ава source, the user ray wantine
anatyzar to display the frequency and level of tha source. Special
fjunclion 10 provides thes display,
Service aids provide front-cañel display of many internal voltages
and setiirgs Milhaul microprocessor contri each Special tunchon
would require ons or morte switches an the Iron panel mstead ol one
SeECAL kay, Bnd would therelore probably not ba included. Thus, the
processor afous imglémentation ol useful feaburas the user woud
Pot ogherasse gel
Corydon J. Boyan
Cory Boyan received his BSEE and
METE degreaes tom Staniord Lines y
in 1974 and 1976 With HP since 1974,
he's contribubad to he design of The
A364 Power Mater, the 88624 Sym
thazized Signal Cnnerator, Ine B01 A
ModuaBo Analyzes, and ha 89034
Audio Analyzer. He's taught mmecra
processor design al Fodthill College
and served as chief anginear of Sian
га! РМО рабо КЗ! Вот in Boston,
| Massachisetts, Cory nov Ines in
| PM Mountain Mena, Cablorea. Hisimerests
include FA radio broadcasting, phatd-
coredy-—he's an avid fan of radio and television comedy Groups,
graphy, Dackpacking, and the a1 ol
The network consista 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. do
nlfset and frequency performance so that they do not de-
grade the slgnal quality. The total input amplifier chain acts
as a 4-dBistep amplifier with leveling hysteresis. which
maintains the post-am plifier signal level within 6 dB (3 to
1.5V 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 lo compensate,
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 ls too high or loo low,
the microprocessor varies the galn of the attenuator
amplifier chain to bring the level within bounds, Second,
the pvervoltage protection network limits if the input signal
exceeds =15Y, Third. any sustained Input overload trips
the input overload detector, This detector monitors the
input rms detector amd the differential amplifier, and if
aither excerdsa certain absolute voltage limit, the overload
detector trips, meeting the gain of the entire amplifier
chain to its minimum value {maximum attenuation], This
Design for a Low-Distortion, Fast-Settling Source
by George D. Pontis
TE ua tha raquirarants of the HSND3A Audio Analyzer, ihe baalt-im
séairée Must Reed good perormancs in oerain key areas. First ol ah
Bor swap measurements, i must be reach programmable and las
siting. 11 must also here dor disloricer a6d ros, ad E mel have
ve good armpliude Accuracy over he entiré frequency range of 20
The caminination of these contig ing requirements suggested the
use of a high-perorma nce AC oscillator instead of asynihesizar The
EyTINESTer ES Eat io program and setbes gustkly, butt is Sffcut to
bud a synthesizer with noise and distortion more Han 60 dE below
ihe fundamental Symhesized designe alec do not have sufficient
absolute level accuracy or fatness wihoul levabng, ard a leveling
oop thal does not unduly degrade the distortion and sailing firme
would be very difficult to design
Untaminatey, noné al the common AC audio cscilior designa
kok stable sihar Usially the amount of distorion is vebrsely
proportional to he satiing time, Also, the tung elements usually
fiat, making the circuit diticull io interlace with programming lines.
Forihese reasons à Säte-varabéé oscillator was chosen, srnièr 16
Inal proposed by mih ando YVanmersonin 1975." Srcelhié OS ao!
5 Cuit amino à ceae-varialde lier siruciure, ing pn sve JFET
maitchies can be ised sai lo swilch iha tuning elements. Mere
mporiant e that Ine ALC des ign permits wiry rapid sailing «haut
rading ой good digiorbon performance. Although The programmed
frequency does not have the accuracy of a synihesizer, sufficien
résolution is available 1o permit firmware tuning to within 0.3% ol
the programmed wae
Tres osCillalor desigr canbe Joscnbad 45 a sala aran lier on
which the resistor thal desermines the O is replaced by an analog
multiplier. The control signal for ihe multiptar is provided Dy an
aviometio eval comtral (ALC) crcl. The ALC circuil Sompares tha
cecillator amplilude with a stable de reference obtamed from a
femperalurecompensated reference diedae, The resulting error sig-
nal is processed thowgh tao paths One palh caries te cycle-By-
cycle proportional- ere 16 [Hé controlling multiplier. This provides
very last setting. The other palñ incudes an iMagraton in he Cop 10
aliminale nearly all ol fie steady-state error This design 15 theorei-
caly capable of setiling Ihe citpat amoliude within two cycles after |
STE E-signal ampliude distu bances
There are two impodant refinements nthe B90%A -oscillaror, The |
first is the usé 4 apecial Two-stage pegk deteciór This consists of |
trackhold and samplenold empliiers to «+brinale any distortion-
casing poke on the detecred peak ouput. The second refinement is
tre addition of an ALC loop gain control to compensate the reveling
loop, cycle Бу ovale tar changes in oscillator smpldude. Thes greatly
desmases tho Large-sgnal setting bre of the cecilator, whch 18
important when Switéhing from ong range Lo -angéher
Fig. 1 ahows the oscillator ntegralors. The gan constant of these
niegrators is changed inihiec-cctave steps by celectno the feed-
back cepació? Tig dlves ds range swéching. Coarseduning mile
ssch rangé is cond Ey seiiching Me Input resissors. The 24
binan-weighted ressbos provide 255 usable sleps. Pacing the
swiching devices althé virtua! gound Sore parmits the use ol JFETS
with ko crain-te-sourcs en resatance (Apg An indvideal ransis-
torswiich conducts whan ils gate is allkowad 10 rise 19 ground palen-
kai and ums all whenits gates pulled tothe negative supply. —13V
+ ©
Vacr ©-
TT = From a Collector Drivers
3 10 kit
— {5%
Open Collector
Driver ¿1 el El TY
Input © 8 AAA
reset cccurs within fen milliseconds if the overload 15 50
vere. This protects the input from burning out or blowing a
fuse, and allows for rapid overload recovery.
Fig. 1. The gain-swilched Int
grafo sed nthe BES ине
| nal Osciafor. Capacuors are
SAIENeCto change ranges. Mes
for swe funn proses Coase dur
rg Meli Bach range
Notch Filter
The notch filter design challenge was twofobd. First, tl
was necessary to design a lov-distortion. low-noise filter
that was also programmable. Second, to minimize overall
The gwilching scheme is economical because multiple resistor
packages and quad comparators can Be used to interface to TTL
Fig. 2 15 a block chagram of he oscillalor. nlegrators UT and U2
and mvener Us-formihe siate-variable Aer Siruchuré Fine tening is
dore by U4. This stegé uses a swilched resistor network similar bo
(thai usad for coarse duning in the integrators. As the resislors ara
switched tha gam of Lid changes, eifectiely allenng the amount ol
signal transmitted ram the output of U2. The gan is prosanional [0
WA-HA, where R is the paralle! combination of ihe selected inpul
resisiors and À and 8 are constants that provide a + 5% dna tuning
range. Thes gives the oscillglor enough résolution 10 ture within
=U.2% of any frequency with the range of the Instrument.
ИН = ganarally rue lor sinusoidal oscdaiors thet purdy and ее
hme ara chesdly functions of the ALC circuits or mechaniems dsac
The we-stage peak-delector circults are the key tothe perormance
ofihe S90YA ascillalar Oscillates ampitude data = obtained by e
brack’hoed and sampie/hald amplifiers nthe down ma ree. Switch
al Closes dunng the time the oulpid 8 atits negative peak Capacditor
C1 rapidly charges, following the sine-wave amodiude upto 15 cosi-
tve pear. Al this Lime 51 opens, holding the peak woliege on CT,
aio 32 then quickly closes ard opens again, hus dpdanng the
sampled peak level held En capacitor C2,
The two stage Scherr has several advantages Forona, the firs
Sage May be optimized lor hast data acquisitron. while the acond
state is optmized for long hold time. Fast acquisition is essential for
Oacillæter Output
Coarse Tuning
State Variable Filter |
Range Swiiching
good amplitude accuracy a high frequencies, Low droop 1s impar-
lant to maintam kw distortion 21 Кому Пере ее Ass IES SC em
has nosteady-state ripofa mn the samoted oufcut which would cause
distortion 0 appear on ihe madtiplien output and in furs on the main
ОСН utp |
bn praciice, tha two-slage scheme is B50 easier to inpéemsant than
& very fast single-stage samplehold circuit. The required circuit:
unctons are accomglishad wilh simple JFET switchas and uniy- |
gain butlers as shown in Fig, 2
For sampled data systems In dóneral, satilimg time ie siranghy
dependent upon loop gain. For this circuñ the ideal miagralar gain is |
ineany proportional io frequency. This Her] 686er ty swikch-
по the integrator resistor on once ger cycle 1or g durdtion ol 35 gs
Tres process increases he integral error sagnal with frequency. The |
duty cychy increases with Fraguancy unbl the ntegraioe swich 53 is |
ET closed cominueueh; Inr insguencios greater than 25 kHz, Abova |
£5 kHz: the oscillätor easily seldeés in less than ong millisecond |
EAA osoillalor performance = largély limited dy he quality of |
commercially available, reasonably priced- analog mutipliere. The:
mMultipter used i decoupled slightly {Tram optimo to reduce its,
Coals ibubon fo THD and noise. This extends the pecilkaice salting fire
to à period ol gar ta live cakes.
| E Yaenesos and. 6 Smit C8 Li-Thstomod Osan ai Fa ee Si
bon” mermar! moral Elecimress Yo SA Ra 4 1975 de 4/570
= =
ALC Circuibr
Pouk Amplitude
o € >
El Prepertional
= 4.20 File
ov +4 = wT
424W 4 | | = Integral Feedback
|| Error
ADE lA ral” AAA —+
Oscillsior av 53 |
—a24V —
Cscilalor Poscive ALC Ouipui
Fig. 2. Back d'agrem of the 39034 $ eternal sfale-vavable osciistor. |
messurement time, itwas necessary to develop an accurate,
rapid fine-tune mechanism with quick recovery from over
loads and mistuning. Teach ieve good (С) over the frequency
range. an active KC filter is necessary, To tune this device, a
variable resistance or conductance device is needed, since
achieving the tuning range with variable capacitors or ln-
ductors is impractical. Many resizstively lunable active con-
figurations are feasible. However, determining the ор=
To provide the greatest versaliliy in both benchtono and sysiems
spelicationa. tre BB0GA Aude Analyzers buli-in source ls figating.
This Gere the user eiminale grounc-Icop armors, sum signals. and add
de oléseds ho the source ouipeit
Previous désiors vaed à séparaté, soated power supply tor the
EQUIGE Circuits. Thié method fs stresghtforwisd and olfere very good
lov-[requaney commonmodé ropita, However ¡hen arg several
FESSOÑE Wity e arrangement 15 nel usadon Ihe FICGA
The biggest problem with the Hoating power supply approach 15
Interfacing with tha digital programming ines. inca only threg.of tre:
Ten alténualor Ines Tavo rafay rieclation and nore CF The Nineteen
gecillalor lings can be lloated, aver thirty lines must ba coupled in
sama manner. One soiuion is 10 foal only tha final output stage. This
ebminales the raed lor couplars, bud requires a high-perormance
aerea no ampliar to rejec the common-mode signal that
appears al the nprut ofre lnaling étage Since 4 loabng power
Supply 18 still required, (he cool of this approach: is mlalivesy Рост.
The ВВОЗА solves Ine probisr web a srgie-ended-10-difreremial
Qutpui commenter Tres cecuit shown in Fig. 1, opaersias on ima instru
ment’s ground-releren ac + 154 supplias and reguires only he ps:
grabenal amplifiora A precise combrnalióon of negalve feponalk
positive teeciback, and ciosa-coupknog yelds a symmaetrical difteren-
{ial output with infimite common-mode regchon and a well-detned
LE QU Impedance,
An añabsis ol this Giroud & qererally & tedious procedure because
of the number of comaoneats iImalved. Howawers, the high degree of
syrrEmetry in the cecull can be expioibed 1 greal advaniage by usng
te relations RAR = RISRIT,R3 = A7, 6 = R10 and Ad = F6 =
RE = RS From these ngalonsting, omg can derve Ihe expréssion
R2'A1 = [AE + A4]2R3, what E 4 necessary Condillon lar achnay-
ing ar inénite common-mode oulpod mpadance. Than | = easy 16
calcuéale he diflenen bal oulpud mpedance and Hé open-ireuit vot-
age gan, The resultag equations canbe manipulaled to brid suitable
values For he te5sior values usée in че ВОСКА, the &ssocialed
gan & 1.125 and he oulpul mpedance | 480 ohms, The output
is further padded with a 120-ohm resistor in yield the desired BI0-ohm
aulput impedance
would Have Essen possible 10 usa resistors thal gare an outpul
impedance of axictly 600 chms msiead ol 400 abms, Bug this would
have regquined Sefling up and siockng a supp ol sévérsl éxira cdd
estar values. As il is, (hé Créuil 16 reaized using 0.1%. 25-pom
resistors that are also Used esewhere in the instrumes
Floating a Source Output
| by George D. Pontis
Flg. 1. Séngie-endec-te-déferential tou! coteter provides
a fioshng output ‘or the H9094 8 creme! cccuvaes
Tre eases! way to gee Noé this CHOU works 5 to eliminate enfer |
he invaning fow; or norimeening (gh) half afihe eireuit by sharing |
te respeciive output lo ground. Fig. Z shows the reduced crouit |
whan the low haf is grounded If Rd is disconnected, the circud will |
have á forward gain of about one, 3nd an output mpedance of 276 |
phim, 4 works in conjunction wish AG fo provide voltage and current |
teegdback that causes 1he gain and the output mosdanca lo rise.
To deronstraje that the ouput is try listing, we ground thie pe |
and apply a lest source to both output, Healy, Ire curan: tig from
Ene hast source should be zero. Fig, 4 shows a Bock dagram and the
reduced circu for this test. Here 8 can be quickly calculated that the
autput of LIT will nga just enough over thal of the lest source Io makes
rhe current thiough FBS cancel the current through A4 and RE. Nota
eed the current fowng through sources V1 and V2 i supplied By he
other hall ol tre circa, which is nol shown
timum match between the filter configuration and the vari-
able resistive eloment 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-
thorn they add to a signal are greater than thé required level
of 40 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.
Four-gquadrant analog multipliers also do not have the
90-18 performance necessary, but they toc 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 is type of circuit, some of which can be obtained
in integrated form, Those most suitable use a differential
palr of bipolar transistors as a varlable galn element by
varying the common-mode current.
Light bulbs as variable resistive 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
200051 15004t
= E High
—— e ANN — B o Output
Poa Lk 24001:
Ave —
2491.01 ~~ RE+AT RY
Pig. 2. WTA thes esr hing soe of the urcol oF Fg. 1 iy
| Es the low ocdpul fo grocato ea in bs пе сен cir
In practice it was found thal parasitic eflects and the
pieciromagnatic-intarierance (EMI) filters degrade the erculf bal-
ance when Ehe frequency approaches 100 kHz. Howaver. aach
Deus 16 tested lor a mena ol 50 EB common-mode rajesion al 1
kHz. A pypical unit has graater than 40 dB repcnon at 100 kHz: Also, a
10-kA) resistor internally ties he low output ho the chasse ground. This
provides à reference for the output when no extemal oad ls con
nach Withour this resiste, the common-mode outpul voltage Is
Cae mtial concer abe circult Wes difculty dl incobles hen:
ing problems, such as one of imé resiciors deifing úl of tolerance,
CAUSING poo common-mode rejection: These problem was soived by
implementing the following test procedure. Fost, the Inpué to the
Igaling amplias le 562 10 exactly 1.00Y rms using the spacial fumc-
Bors Boel inté he ER Then (he lechmician shora e loa Side té
George D. Pontis
r Barn in Los Ángales, Calionmía, George
E Ponts altended me Ureversity ol
Color at Santa Barbara, receiving
hs ESEE degree nm 1976. Hejoned HP
1878 and contnbubed lo ma design
h of the 89034 Audio Anayzer. Former a
contributing editor of Audio Magazine,
es à member of the Audio Engineer-
М ng Society He lives in Palo ARG,
Calfomiæ and enpoys aubdoar
Spots, especially tennis, sking,
ano БМК
—{ — E a
Ec Le
la) a
200011 15001
pri AN —
= gt
: As
1051 £1 | 240001 | ‘оу С.) Water
Rl RE Ad ?
2 240001 « RE
— 10611; ATLAS
Же Wa
dE т
Fig. 3. fa] To demonstrate mal Fe ourpul oF we cuout 5
Io both aLigóts. (E) Aéguceo Ci'cuir far fs fest cm all of the
Crean 1 Showa) Corren! Prough 6 canceds the current
Mróvo Af ato Ra, Corren through VT ang V2 #5 supple бу |
Me oinar REF 5! He Cancun (rot shown) Thus no current 1
arawr from he last source
ground and messues the potenbals al savoral-Circuil notés, The
measured values ¿an De compared tá the calculated vales paí |
lished ina abies in ibe 29054 menual A second sat of meazuremenés |
cen be made, | necessary, vih the high side grounded. Sy ohsary- |
ing the deviations batwean the maasurad and calculated values, if is |
easy to locate the faully component
networks for low-distortion 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
al ground potential; this relieves many constraints on the
switching network. To this end, a state-variable® notch
configuration |s used (see page 14), With this design, low-
distortion tuning over a 10:1 frequency range is achieved.
The rara reussi ei ob à Core Ci OF eu LC Hal ie rad [Ha O68
LOTO a y AAN Er Aa hor tal do ne Cacs-ca) sia0-space CO Fear: of
Ara RE ie. X= Ak + Bul wheres xis Mesones К АА vanabie) of the system
kl er ul
Capacitors are switched into the network to change fre-
quency in three-octaye bands and provide complete covers
age of the frequency renga of the analyzer,
To complete the fine-tuning path, a synchronous detector
mixes the filter output waveform with the {fundamental
wivedorm, If any fundamental exists in the notch output. a
de current is generated to fine tune the notch. The critical
parameters here are 100-d8 dynamic range and rapid opera-
tion. À FET double-balanced mixer was selected. The input
mixing signal that drives the FET is à square wave, rich in
odd harmonics. so the circuit responds to odd-order har-
mónics 89 well as to the fundamental. This is the classical
solution: A complete null may not be achieved if third, fifth,
or higher-order odd harmonics are present. Total error can
by Chung Y. Lau
The notch filter inthe 83034 Audio Analyzer resects the lundarmen-
lal frequency component of hheincomeno sagrál. This filler consists of
a etata-variable active fillér and fre-tune and fins-balenca control
circuts. Á smpliizo schematic of tha filter minus the control circuitry
5 seen in Fig. 1
Conventional notch filers (bricged-T. sic.) wsed in distortion
analyzers are nal wel suied for ohgital conirol Because thay regure
expensive relays ar analog switches. The state-variable filer ap-
proach has the folowing advantages:
= The О о! те filter = wed and independent of frequency.
Tuning is accomplished by switching resistors and capacilons in
[e [wo imeprators. Inexpensise JFETS are used as swilching
elements 2nd tha switch drivers are simple because bothihe gate
and The channel of the JEET are virtually at grousd potential whan
ne swith is on,
Three filer outputs (low-pass, bandpass, and high-pass) are
available simultaneously, The low-pass and high-pass aulputs ara
phass-ehifted from the про by SC" when Ihe fer is tuned tá tha
QUE Ireguancy. These two outpubs are gsed in the control cincuits,
so extra phèse-shifting circuits are 201 needed
Distortion generated in he fine-tune circuitry is filtered by two
integralors Defore appeanng at the noch cusp
Im distorion mode, the filter is tuned in the following manner. The
MIOS SOLE the input frequency and ones the notch fikar
10 the same Frequency by swilching in the proper capackars and
from Control Cirouits
Hz — zw]
AAA Gé bag 2 |
impar Ey
OANA — Vue |
— Vas
| РА AS a
———- =
te RY?
> Y
& A
R10 Hi
resstors When the fiter is turned toihe lundamental irequency cfthel
Input sige, the Bandpass output (Ge ПО Ра Ту лете the funda
mental component of the input signal, LS sums Vy, and Y=10 cance)
the fundamental component dl Vy, exactly. However, because of the
phase and ampliude characteristics of Ve the harmonics present in
Vy ara relatively unettenuated,
Andghtically, we have
Yar ) iy Si, |]
= бо =
Vin 5" + 5/0 + mi
Ey 5° r Sal, CJ + a
whare w,, is the center frequency in rad's. Theretore, Thang is zero
1Га ПОГИБ al 5 = jo.
The slate-variable filter alone can provide oráy aboul 15 dB of
fundamantal rejection because of the coarsenass of tha frequency
wning (use of discrede resistor values, resistor mismatches, atc),
Fine-1une and line-Lalance contról circuils are necessary to achieve
a noteh depth grealer than 99 dE. The fine-tune circuit insures thal ha
tilter 16 exactly tured to the input frequency {no phase error) and the
Correction Sigrals
FR :
# as
de + ==
| Cr
irl Fig 1. 89034 norch mier. Comec-
tia SIgaeis from Me control cir
culs of Fig. 2 provide fine fuming
and fine balance.
be as much as 0.46 dB, hut this Is considered reasonable,
Rapid 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 impos-
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 ime response of a 20-Hz
notch filter is much greater. In fact, a fundamental burst
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 есь) OQ
where (J is the (} of the notch, Thus, as the notch fregueney
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 instro.
ment response time Improves.
Qutput Amplifier
For the amplifier following the notch filter, gain accuracy
line-balance circull iInswes that Yoo = 1 atthe input frequency
fre ampituda armory. The fine-lune Grout is described in Some detail
hera; ihe fine-balance cir óperalés ina Similar manner. Fig, 2a
simpliied schematic of Doth crçuits.
The fire-tune circuit operates à5 follows [see Fig. #1, Tha low-pass
Alter voltage | . which is phase-shitted 20° from the ngut, drives ihe
he comparalor LIGA, wiich iums JFET ewitch 06 ds and of, When
Oison. port Ala essentiely grounded and no ciirrest Nows mio ire
miegrálor capacáór (2. When 16 15-01f, Me noch ampliar heads the
lune integrater. Because of this chopping action, he do currant that
Mowe mo the integrator ie caused by the notch oufpué component
that € synchronous with Мур"
The tuna integrator outpul is a de voltage that can ba changed only
by ha de current Ácweng mto the integrator, This voltage leeds one
input of tha multiplier USA, The other nput to he multiplier is Van. The
product of the vo mputs à Ssurmimed into fhe s1ste-vanable ег ма
US (ses Fig. 1), The nel resul rs (hat he lune integral? voltage can
change the effective résistance (Fr id Fig. 1) and hence Ihe center
frequency ol The notch filter
The directon of change is: such that amy notch output components
in syochronism with Y, are reduced Al steady slate, the
“Aia Hed mecs A ha Ao dub са alien Cause de col E Roa an ie
aro Tus Sor causes à Hal amour ol the ame Yara ¡ur ONO jn
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Vv 7 Integrater
Fig. 4. B9034 лей благ ng lung and balance cocos
+ 15%
lune «étage is stable, allhôugt nol necessadly zero, and nú de
current Ecws Inmo ihe fune intagrator. Theretore, ihera E mo nosch |
output component synchronous with Vo
The fine-balanmce circuit insures hal there are no components |
synchronous wilh Ye inthe evo output. Yee and Vp are balbal hs |
lundamental input frequency and are in quadrature with each olmer
This: at steady state, there is no fundamental frequency inthe noich
QuipuË |
Chung Y. Lau
= Chung Lay is a naive of Hong Kong He |
we (RECEIVED his BSEE degree in 1975 and
his MSEE degres in 1976 from the
Urnrvergiby of Calfomia at Barkalay, With
HP since 1976, he 5 worked on tha
B901A Madulahorn Analyzer and contr-
buted ta the design of the B03A Audio
Analyzer. Chung lives in Cupertino,
Califor, and enjoys bridge and
and good freguency response are important. In ac level
measurements the signal travels through the output
amplifier chain and is detected by the output rms detector
Anv error degrades the 80034's performance. The output
amplifier allows the low-leval distortion products leaving
the notch to be accurately detected by limiting the required
dynamic range of tho rms-to-de converter, The rons 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
increases dramatically, sending the amplifier into overload.
This in turn generates do 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 al the amplifier output along with the
signal being amplified. The composite signal 1s transferred
lo the outpul 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 ofthe signal amplitude. This can cause problems
when the instrument ranges automatically. In effect, it
forces the leveling algorithm to walt much longer to con-
AUST 19080 wEWLETT PACs ani j0uRsAL 18
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 à
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.
Ta alleviate these problems, the size of the transient Is
minimized in theese 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
ton large, 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 keaps
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 1x1 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 cutput detector and filters could have
been designed with switchable time constants to respond
mare rapidly for higher-fraquency signals. However, the
penalties would have been additional circuit complexity
and the ambiguity of not knowing when to invoke the longer
lime constant, A 20-kHz signal, for example, might still
have a significant low-frequency component, which would
cise excassive error with a more rapid time constant.
Many ol the design considerations [or the notch filter
apply equally to the oscillator. In both cases tuning consid-
prations were the same, with switchable resistor networks
used as the decade tuning elements ani fn ur-quadrant
multipliers used for amplitude control. In many ways the
oscillator and noteh cirouits can be seon as duals. The notch
generates a pair of zeros on the jw axis that reject the Fundas
mental component, while the oscillator generates a pair of
poles on the je 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 with a special level-
ing circuit offered the flexibility and performance required,
The oscillator design is described on page 10.
It was determined during development that the oscillator
would have to run at a constant output level to maintain
reasonable settling and nolse performance. It was also de-
sirad to have a floating output. The attenuator and output
amplifier circuit (see page 17) takes the oscillator output
level and translates it fo the selected floating output
amplitude, To minimize cost and achleve 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 пож! уе
ladder network that adjusts the amplitude linearly in small
discrete steps, The combination can adjust the amplitude
within 8 nominal =0.15% worst case. Computation of the
proper switch settings is an easy job for the computational
skills of the microprocessor,
EMI Design
Menting the required electromagnetic interference (EMI)
and susceptibility goals was a bit more challenging than
initially expected. Largo-amplitude RF fields tend to gener
ate voltages on exposed cabling and circuits. These voltages
overdrive many of the active circuits. causing nonlinear
opération and distortion. To aveld direct exposure to these
fields, tha analog circuits are housed in an internal EMI-
lig ht 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 on the cabling feed.
ing the circuits, special precautions were faken, First, from
the inner box to the front panel, shicided 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 ts 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 à 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 89034 is its ability to measure [re-
quency automatically, even when the input waveform may
have a significant amount of noise and distortion and the
amplitude may vary from& 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. Bul before the signal can be accurately
counted it must first be converted into a binary signal hay
ing the same period as the major frequency component in
the waveform, and herein lies à problem. IE à zéro-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 large, 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 emplovs vari-
able hysteresis. As the peak amplitude of the signal varies.
561 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 remalos constant regardless of the Incoming
waveform, Hysteresis is implemented with a bipolar peak
detector followed by a dual comparator. The waveform is
transferred to a reciprocal counter, which measures the
period of the signal, and the microprocessor inverts this
period to get frequency,
Many people throughout HP contributed to the success.
ful Introduction of the 803A. It rust be stressed that the
instrument's success is the result of the total contribution of
many people from early investigation through production.
50 first and foremost may [ thank all of those who contri-
buted their time. enthusiasm, and suppor. On the R&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 circuits and verifying overall instru-
ment performance, Cory Hoyan 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 ant power sup-
ply circuits as well as various special test fixtures to verity
James D. Foote
A native of Madison, Wisconsin, Sm
Foote earred his BSEE degree al the
Univarséy of Wisconsin п 1872 and
grad HP 0 1873. In 1975 ha obtained
his MSÉE ai Slanferd University, He
has served as a design anginaar on
bach the 830714 Modulation Anahyze:
| and the ВВОЗА Audie Analyzer, and as
project manager for the 9034 Jim
has just pined HP's Disc Memory
Cavisióon and has moved to Boise, Idaho
with his wile and daughter Among his:
iméresis gré readng, walking, rac-
quatitall, skiing, chess, and dong odd
jobs around the Agus.
instrument performance, Pater Les and Jim Stewart for in-
dustrial design, and Bruce Cresdy for initial product de-
sign. Special thanks also to Ray Shannon and [im
Stinahelfer who were instrumental in the early product
definition. Other key contributors include Rick Pinger and
lim Harmon in providing service and operating documenta-
tion, Bab 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.
LAP. Edwards, "Precise, Convenient Analysis of Modulated
Signale,” Hewlett-Packard journal, November 1679,
HP Mods] ERA Ado
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