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Basic Techniques of Waveform Measurement Using an Oscilloscope w

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                                                 SERVICE INFORMATION FROM HEWLETT-PACKARD
RI                                                                                                               MARCH-APRIL 1980
          Basic Techniques of Waveform Measurement
          Using an Oscilloscope
                         oe
          I i d i t d s N t : This article is based   Once these concepts are mastered,              CRT, an electron beam draws the
          on a three-prt uideotape series (HP         the only remaining hurdle for you is           waveform on a phosphor-coated
          pln 907410) originally developed to         to locate the controls on the scope’s          screen. T i screen presents three
                                                                                                              hs
          train customer technicians on the use       front panel. Most manufacturers try            types of information: voltage infor-
          and operation of a n oscilloscope.          to help you by grouping similar con-           mation on the vertical or Y axis,
          While most of the references EO eon-        trols together and separating the dif-         time infarmation on the horizontal
          trols are based on an HP 174OA              ferent gmups by color or lines on the          or X axia, and intensity information
          Dual-Trace Owilloscop, the infor-           front panel.                                   on the Z d s . All oscilloecopes have
          mationpresented is bagic and applies                                                       controb to @ust the voltage, time
          to the opernhn of other munufactur-         GETTING BACK TO THE                            and intensity idormation in order to
          em’ oscilloscopes as well.                  BASICS                                         present a meaningful picture on the
          I                                           The oscilloecope presents a voltage            CRT. Figure 1 shows a block &a-
          Generally speaking, a technician be-        vs. time display of a waveform on a            gram of the basic circuits that these
          coming familiar with a piece of test        cathode ray tube (CRT). Inside the             controls operate.
          equipment is concerned about three
          taiIl&:
          0   Knowing where and how to con-
              nect the test instrument,
!I   lF   0   Knowing how to adjust the
I
4         0
              controls,
              Andknowinghowtointerpretthe
‘L            data.



          oscilloscope. Why the oscilloscope?
          Because it is probably one of the
          most versatile troubleshooting in-
          struments you have on the bench.
          You can use it to measure voltage
          levels (from dc to microwave), phase
          differences, signal presence (or ab-                       POWBI                   Tngg%Aniro1s

                                                                             Figure 1. Block diagram of a basic oscllloscope
                      distortion, and complex
                       analysis (wave shape,          Part 1 of this article describes most of
                      etc.) to name a few. We         the basic controls of an oscilloscope                     IN THIS ISSUE
                       n’t show you how to use        and how to set them up to make a                      WAVEFORM MEASUREMENT
                        all these endeavors. We       basic measurement. Part 2 will con-
                                                      clude the series with                                     SERVICE NOTES
                                                      dual-trace oscilloscope
                                                      cal period, pulse rise
                                                      propagation delay, and time-delay
                                                      phase measurements.
                                                                                                 I              5420A OWNERS
                                                                                                        STIMULUS-RESPONSE ’TESTING

                                                                @Hewlett-Packard 1980

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The Vertical Input
As shown in Figure 1, the input sig-     trol incmatpea time interval meas-
nal is connected to the vertical input                                        exampie 1740A-sCape has four p i -
amplifier. The vertical amplifier                                             %ions: AC, GND,DC and Solb. The
either attenuates or amplifies the
signal for convenient viewing.
The next block the incoming signal
encounters is the delay line. The
delay line allows the sweep
generator circuitry time to start a
sweep before the signal reaches the
CRT vertical deflection plates. This
coordination of vertical and horizon-
t l timing by the delay line enables
 a                                       Now l&e look at the h n t panel of
viewing of the leading edge of the       an HP oacillowqk in Figwe 2 and
signal. This will be explained in        see how the controls that operate    low impedance sollIm?s.
greater detail later on. The vertical    these circuits are identified and
output amplifier provides additional                                          AC Position - The AC position
                                         grouped on this particular model.    couples the input signal through a dc
amplification that is required by the
CRT vertical deflection plates.                                               bl       capacibr, allowing only the
                                         VERTICAL INPUT CONTROLS              ac component t be viewed. AC mu-
                                                                                             o
The Time M s
Although precise horizontal deflec-
tion rates are not required in many
general purpose applications, the
more sophisticated scope applica-
tions require precise control of the

                                                                              quency digital-type signals. The
                                                                              internal dc blocking capacitor will
                                                                              distort the waveform as shown in
                                                                              Figure 3.




                                                                              GND Position - The ground
                                                                              (GND)  position is useful when you
                                                                              want to set a @;roundor zero volts
                                                                              reference level on the CRT screen

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                                                 and low level signals will
                                                 attenuationlmore          gain. The
                                                 VOLTSDW 6ont;mll allme you &o
                                                 change the v e r t h l mdtivity in cal- t u r n g d offin &hitit        ym would
                                                                                                                rruu~llss,
    This means that you can leave the            ibrated fixed steps,from 20 volts per see the retrace lines with every
    input signal connected to your scope.        division to 5 millivolts per divisionon sweep.
    You won’t short it out when you              the 1740A. The vernier portion of the
    switch to the ground position.               input attenuator provides continu-
                                                 ous sensitivity control between the
                       NOTE                      calibrated volk-perdivision ranges.
                                                 Whenever you move the vernier out
            High frequency signals can           of its detent position, the UNCAL
            create special problems for          light will be on, letting you know that
            switches in scopes as well as        the steps marked on the VOLTslDIv
            other instruments. Therefwe,
            when mearsuring HF signals, it




        allows you t view both dc and ac
                     o
        compoaents of the input signal. For      40 MHz. The vertical magnifier is
        example, if you have set the 0 volts     umhl when you’re tryingto measure
        reference level at the center of the     low-level signals such as power sup-
        screen (using the GND position) and      ply ripple.
        then switch to DC, the waveform
                                                 HORIZONTAL INPUT
I                                                CONTROLS
I
        nent, if any, and the signal will
        offset either up or down depending       The sweep generator, sometimes
        on whether the dc component is posi-     called the time-base generator, pro-
        tive or negative. DC coupling is also    duces the sawtooth wavBform which        Sweep Speed Control
        used when you are measuring                      s the rate the beam is drawn
        digital-type signals or quare waves.     horiaontdly           the face of the    The sweep generator’s sawtooth
                                                 CRT. The generator‘s most impor-         waveform is controlled by a front
                                        n is     tant function is to ensure linear        panel control called TIMEDIV or
        a dc input (no blocking capacitor)       beam movement, meaning the beam          SEC/DIV. This calibrated control
        with the Xc of the input amplifier       moves at the same rate from start to     lets the operator select many M e r -
        very large compared to 50 ohms. The      finish. Without this precise rate, ac-   ent sweep speeds in order to view
        5 input is used to measure high
          W                                      curate time measurements are not         waveforms that vary from a few
        speed pulses and square waves from       possible. Another factor of accuracy     Hertz up to the bandwidth limit of
        50 ohm sources with minimum dis-         depends on the delay line. Its func-     the scope. The control is usually
        tortion and VSWR reflections. Most       tion is to delay the vertical input      divided into steps i n a 1-2-5
        oscilloscopes with a built-in SOCi       signal just enough so that the trace     sequence covering the ranges of sec-
        input have internal compensation         being displayed is the signal that       onds, milliseconds, microseconds,
        that make it a better match than an      started the sweep (see Figure 4).        and nanoseconds. These ranges cor-
        external load.                                                                    relate to how faet the beam is drawn
                                                 Another function of the sweep            across the CRT. The faster the beam
        The Input Attenuator Control             generator is CRT unblanking. An          is drawn across the CRT, the faster
        Most modern scopes use a combina-        unblanking pulse is a positive           the time reference (i.e., the shorter
                                                 square wave that turns the trace on      the scale). For example, if the


,
    a   tion of variable attenuation and ad-
        justable vertical amplifier gain to
        control input signal levels. High
        level signals will require more
                                                 in relation to the rising portion of
                                                 the sawtooth. What this means is
                                                 that the trace is turned on during its
                                                                                          TIME/DIV control is set for 0.5
                                                                                          seconds-per-division, the time refer-
                                                                                          ence over the full 10 major divisions
I
        attenuatiodless gain so that the         left-to-right movement across the        (vertical graticule on the CRT face)
I       trace is not deflected off the screen,   screen and then turned off during        is 5 seconds. If it’s set at 5

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milliseconds-per-division,the full
scale time reference is 60 milli-
                                         steps. Whenever you move the ver-
                                         nier out of its detent CAL position,
                                                                                  factor the magnifier is labeled. For
                                                                                  example, if yo& scope has a 10 divi-
                                                                                  sion time arris (10 squares on the
                                                                                                                          a
seconds. Figure 4 shows how the          the UNCAL light will be on letting
sawtooth waveform produced by the        you know that the steps marked           horizontal axia) and the magnifier
sweep circuit develops a sinewave        on the TIME/DIV dial are not             has a factor of ‘Xlo’, you would have
pattern on the CRT.                      calibrated.                              an effective 1OO-division wide signal
                                                                                  and a lo-division window. This also
P r of the TIMEDIV control is a
 at                                      Another control that interacts with
                                                                                  means the signal has 1 times the
                                                                                                            0
s e p vernier control that provides
 we                                      the sweep speed control is the hori-
                                                                                  horizontal resolution as before.
continuous adjustment of the sweep       zontal magnzier. This control ex-
speed between the fixed TIMEfDIV         pands the sweep time by whatever

   Rise Time and Bandwidth
   OsciIIoscoDe Amplifier Considerations
                               m
   Oscilloscope users generally            frequency components will reach          requirements for scope band-
   consider a scope’s bandwidth            the deflection plates at the same        width using the 5 times faster
   and rise time as its primary pa-        time. This results in minimum            and 0.35 factors together can be
   rameters. And rise time is usually      distortDon of complex waveforms.         estimated using the following
   considered the more important           Note that this Gaussian re-              rule of thumb:
   parameter when working with             sponse is always falling in gain,                            1.70
   faster waveforms. This is mainly        therefore, accurate voltage              (minimal)    Fastest Rise Time
   because the primary axis of the         measurements can only be
   scope’s display is the horizontal       made at dc. The frequency re-            But remember, very accurate
   or time axis, and it offers the         sponse will be down 1.5dB at             absolute rise time meas-
   greatest rmolution - less than          20% of the 3dB bandwidth, so             urements are not always i m p -
   2% for timing f n e a w r m t s .       3% accurate amplitude meas-              tent. When simply comparing
      Why is t horkontal or time
               b                           urements of sinewaves can’t be           the rise times of two signals,
   axis txmkfwd the major axis?            made for frequencies greater             scopes with a tfse time equd t  o
   ComMar that the vertiial axis           than 20 MHz on a 100 MHz                 the rise time of the signals
   has an 8cm window, whereas              oscilloscope. However, the               applied am usually consided
   the horizontal axis has a lOcm          amplitude of a pulse is dc so ac-        adequate.
   window. 10cm provides more              curate pulse amplitude meas-                In conclusion, it can be said
   resolution than 8cm. Also, the          urements can be made up to the           that the modern oscilloscope
   range of the vertical axis (the HP      full bandwidth of the scope.             with its Gaussian response is
   1749 for example) is 2,000 to 1                                                  designed for pulse parameter
                                               Constraints make bandwidth           analysis but not sinewave
   or from 1 mv/cm to 20 v/m. The           and rise time numerically related
   time axis has a range of                                                         analysis. The characteristicsof a
                                            in well designedgeneral purpose         sinewave can be better meas-
   4O,OOO,ooOto1from 2 W c m to            oscilloscopes. Bandwidth in
   50 ndcm. This is 20,oOO times                                                    ured with instruments other than
                                            megahettz multipliedby rise time        the oscilloscope. For true RMS,
   greater than the verticai axis of-       in nanoseconds is approximately
   fers.                                                                            a voltmeter can give better
                                           0.35. Therefore, if your oscillo-        amplitude measurements, a
      Signal bandwidth is of course        scope needs are defined in
   defined as the frequency range                                                   counter better frequency meas-
                                           terms of one factor, for example         urements, and a spectrum
   in which signals are handledwith         rise time, Widing it into 0 3 will
                                                                       .5
   less than a 3dB loss compared to                                                 analyzer better distortion meas-
                                            produce bandwidth.                      urement$. However, for a com-
   midband performance. However,
   the wertical system of an oscilb            In terms of rise time, scopes        plex waveform such as a pulse,
   scope is not flat like that of a        ideally should have a vertical           the oscillosocpe is clearly the
   voltmeter - it is Gaussian.             system capable of responding at          best choise. The voltmeter can’t
      What does Gaussian re-               least three to five times as fast as     respond fast enough to make
   sponse mean? It means that the          the fastest applied step signal. In      this measurement. The trigger
   vertical system of the scope al-        such a case,the rise time of the                        f
                                                                                    uncertainties o a counter mask
   ters the input signal and delays it     signal indicatedon the scope will        its accuracy for pulse meas-
   in such a way that it produces a        be in error by less than 2%.For          urements, and nothing but a
   linear phase response. The              example, if you are going to ac-         scope can measure parameters
   linear phase response has a             curately measure ‘X’ micro-              such as overshoot, droop, and
   constant group delay so all the         second pulses, the minimal               ringing.
WAVEFORM MEASUREMENT
                           I




  width




                       .       %




                           o€s;hie series will show you
                how to make the Same measurement
                using other controls t o obtain
                greater accuracy.




                Frequency is the d p m d of the
                time period for one cycle. For exam-
                ple, the time period of the signal
                shown in lFigtve 7 is obtained by
                counting fbjg.nam&erof h&&d


                ma). Then take the reciprocal.
                   1
                f=-=
                                   1      1
                                        3 =
                                         -      1.0
                   t       6X      0.2m8 10-0
   . I               I
                I :.-.
                     .-e
                                 WAVEFORM MEASUREMEN 1



                                         of horizontal tangent points, and di-
                                         vide this number by the number ob
                                                                                 Figure 9 represents a frequency re-
                                                                                 sponse curvi and is obtained by con-
                                                                                                                        e
                                         vertical tangent points. If you u e
                                                                           s     necting a sweep generator to both
                                         this method, always make certain        the input of the circuit under Cest
                                         that the tram contains visible cross-              X
                                                                                 and the ' 'axis. The output of the
                                         overs, that they are not masked                                      Y
                                                                                 circuit is connecW to the ' 'axis.
                                         by trace coincidence; that is, the      The oscilloscope becomes a simple
                                         horizontal tangent points don't fall    network analyzer that is a swept re-
                                         together.                               ceiver that pmvi?es a visual display




The        ode of operation is a two-
dimensional representation of two ac
                             Y
voltages. The vertical or ' 'input
signal deflects the beam up and
down while the horizontal or ' '   X
input signal replaces the scope's
sweep generator and deflects the
beam horizontally. A third dimen-
sion can be added by modulating the
beam's intensity through the ' '   2


One of the more co         868   of
X-Y mode is to generate Lissajou
patterns to check phase. For exam-
ple, the transistor checker discussed
in the Sept.-Oct. and Nov.-Dec. 1974
issues of Bench Briefs provides a
Lissejous pattern that indicates the
voltage-to-current characteristic of a
diode junction. Another more sophis-
ticated use is in the area of circuit
freauency response where you turn




Figure 8 shows wme of the various
Wajous patterns you can expect
using the X-Ymode. Note that Fig-



                            o
ing a 1 2 frequency ratio. T obtain
       :
the ratio of vertical and horizontal
deflec%ion frequencies from any
Lissajous pattern, count the number

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                                                                   WAVEFORM MEASUREMENT
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                                                                                                                   derence. It also


                                                    %weep  gemmator in order k, produce
                                                                       -
                                                    a reference bamline if there is no       Normal Mode -The Normal


            between the horizontal and




      ”
                                                                                                                 signal connected to
  ::t,,
...
                                                                                                                ut of the wow, you
                                                                                                                gger signal -hence
  .
  :
  1




                                                                                               trace. This loss of trace with loss



                                                                           Podtionr                           Functions

                                                                            AUTO               Connects sweep to multivibratar so
                                                                                               sweep free-runs in absence of adequata,
                                                                                               trigger signal.



                                                                                                                                             I
                                                                           NORMAL              Connects trigger circuit to one o three
                                                                                                                               f
                                                                                               sources: input signal, external W l or
                                                                                                                                 a,
                                                                                               line.

                                                         Mode
          TRIGGER CONTROLS                                                 SINGLE              Sweep will start only upon the occur-
                                                                                               rence of the trigger signal that meets the
                                                                                               condiths of level and slope after the

          accomplishd by
          sweep signal discussed earlier so                                  INT               Uses sample of input signal to start
                                                                                               sweep.
          that each trace is written right on
          top of the previous one. You see one
          single trace, but it is actually being                             EXT               Uses sample of some external signal to
          refreshed on each sweep.                                                             start sweep. External signal is usually
                                                                                               related to input signal.
          Several controls allow you to select
          the source, positive or negative
                                                         Source             EXT   t   tO       Same as EXT except attenuates external
          mode, and level of the synchronizing                                                 signal by a factor of 10.
          trigger signal as shown in the
          simplified diagram Figure 1.The fol-                               LINE              Uses sample of power source. Useful for
          lowing table is an abbreviated de-                                                   viewing events related to power line
          scription of the basic controls and                                                  frequency.
          their functions.
                                                                           VARIABLE            Permits selection of triggering at any
          AutoLNormal                                    Trigger
                                                                       -1.5~to +1.5~(EXT)                                      r
                                                                                               point (level) on the posithre- o negative-
                                                          level      -15v to +15v (EXT+10)     going edge of the displayed waveform.
          This switch is probably the greatest
          source of ‘bilot error” in oscilloscope
                                                                           pas (+)             Sets up the t r i m n g circuit so that the
          operation. In simple terms, Normal             Slope             NEG (-)             displayed signal is triggered on the
          m d requires a trigger signal to
           oe                                                                                  positive- or negative-going edge.
          generate a sweep - Auto does not.                                                                                                  1




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                                                     UREMENT



of input can be a valuable trou-           (depending on how you have set the        One method of viewing the time re-
bleshooting aid. Say, for example,         Slope and Level controls).This            lationship between the input signal
you are probing a circuit looking          allows you to view a time event           and external trigger signal is with a
only for presence or absence of a sig-     related to the input signal. If you       dual-channel scope. U e one input to
                                                                                                           s
nal. If you adjust the trigger level       are using a dual-channel scope, you       look at the signal and one input to
control for an optimum level, and          must know which input channel will        look at the trigger. You must know
then probe a point in the circuit that     trigger the sweep circuit and use         which input channel will trigger the
has no signal present, there will be       that channel for your input. Part 2 of    sweep circuit and use that channel
nothing to trigger the display and         this article will explore some of the     for the trigger input. Then set the
the screen will be blank. Figure 1         various triggering options available      source switch for INT.
shows a simplified representation of       for dual-channel viewing.                 If you are going to use an external
how the trigger controls are               If you are using the internal trigger     trigger signal, it is advised that you
interlocked.                               mode for troubleshooting, you may         first look at that signal on the input
                                           have to re-adjust the trigger level       of your scope. You must determine if
Trigger Level and Slope                    control to maintain a trace as you        it has a dc component or noise
                                           probe different points in the circuit     greater than the trigger level you
Trigger Level and Slope controls           under test. The reason this occurs is     are trying to set up (or possibly ex-
allow you to select any point on the       because the trigger circuit has been      ceeding the limit of the input). For


                                                                     i
positive or negative edge of the dis-      initially adjusted (by you1 to trigger    example, the trigger level range of
played waveform to trigger the             the sweep at some positi e or nega-       your scope may be 2 1 5 volts (215
                                                                                                              .
sweep circuit (see Figure 10). Usu-        tive voltage level. There re, as you      through EXT + 10). If you try to use
ally, when the scope ia in the Inter-      move the probe from pomnt-to-point        an external trigger signal with a dc
nal Trigger mode, the level control        monitoring different si$nal levels,       component greater than 1.5 volts,
will select any point on the vertical      the voltage level to the trigger cir-     you won’t be able to trigger the
waveform displayed. With external          cuit is also constantly changing. To
trigger signals, the control has a &
voltage limit (refer to the operating
manual).
                                                                    r
                                           eliminate this inconve ience, use
                                           the External Trigger m de and con-
                                            nect the external trigger to a low rep
                                                                                     sweep unless you block that dc.
                                                                                     Some scopes have ac coupling
                                                                                     (selectable) built in - other do not.
                                                                                     At any rate, you must use dc cou-
                                           rate timing signal from the circuit       pling for trigger signals below about
Internal Trigger                           under test. In a digital circuit, use a   20   Hz.
                                            sub-multiple of the clock pulse rate.
When the switch is set for Internal                                                  Your external trigger signal may
Triggering, it means that a portion         External Trigger                         also have power line pick-up or
of the input signal is tapped off, as       When the switch is set for External      possibly RF noise. In either case, you
shown in Figure 1, and sent to the          Triggering, you must provide a sig-      need to filter out the unwanted por-
trigger circuit. The CRT will display       nal to a connector on the scope          tion in order to obtain a stable dis-
aportion of the input signal related        marked EXT TRIGGER. If the sig-          play. Some scopes have built-in fil-
to the first occurrence of a positive or    nal voltage exceeds the input volt-      ters while others do not. The point is,
negative slope of the input signal          age limit (refer to your manual),        if you use external triggering, make
                                            then use the EXT + 10 trigger input.     certain the signal is clean.
                                            A good rule-of-thumb is use a 1O:l
                                            probe on EXT and no probe on EXT         Line Trigger
                                            + 10. This will help prevent satura-     In the Line mode, the display is
                                            tion of the trigger cornparitor. The     triggered by a sample of the power
                                            External Trigger signal is usually       line which is usually 50 or 60 Hertz.
                                            derived from a low rep-rate timing       Line triggering is often used when
                                            signal related to the input signal.      you want to determine if there’s any
                                            The CRT will display the input sig-      relationship between the displayed
                                            nal on each occurrence of the trigger    signal and the line frequency (often
                                            signal. This allows you to view an       called power line hum).
                                            event time-related to the trigger
                                            source. The Trigger Level and Slope      Trigger Holdoff
                                            controls work the same for an exter-
                                            nal triggered signal as an internal      Some oscilloscopes may have this
                                            triggered signal.                        specialized variable control that is

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    used in coqjunction with the Trigger            Set the input coupling switch ac-    Axiom #2: If the trigger signal has
    Level control. Trigger Holdoff                  cording to the foIlowing criteria:             a large dc component, it
    increases the time between sweeps
    and helps stabilize the display when
                                                    -       ifthe source is a pulse or             must be blocked by a
                                                                                                   capacitor (e%., 0 1 p )
                                                                                                                      . F.
                                                       signal generator.
    internally triggering off a complex                                                            If your sc#p hae d d c
    digital signal or RF signal.                     - DC if the source is a low-                  s e l d o n built into the
                                                        frequency digital signal                   trigger input controls,
    PUTTING IT ALL TOGETHER                             (square wave).                             use the ac position to
    Now that you have an idea of what               -AC ifthe source has a large DC                block t b unwanted dc
    all the basic controls are for, let‘s put          companentthat needs blocking                    compo.lkent.
t
I   them all together in step-by-step                  or for general purpoge probing.
L                                                                                            Select the trigger murce. You can
I   order to actually set up your scope.            Connect the input eignal and ad-
4                                                   just the input atbwtor control           trigger the sweep from an exter-
    1 Turn-On and Preset,(beforethe
    .                                               to obtain a reasonable display.          nal, internal or line frequency
                  COM??Cbd)                                                                  signal.
                                                0   Adjust the sweep speed control
    0   Turn on the power and allow ap-             until you get a display you can
        proximately 30 secohds for warm             recognize.                           0   If the frequency of the trigger sip
                                                                                             nal is less than approximately 40
        UP.
                                                2.Fine Tuning                                H r z change the mode switch to
                                                                                              et,
    0   Preset the trigger mode switch to                                                    Normal.
        Auto and turn the intensity con-        The following steps are contingent
        trol up.                                on the type of oscilloscope that you         If you have selected external trig-
        If there’s still no display, use the    have. A lot of brands won’t have all         ger, select either ac or dc trigger
        beam finder together with the           of the controls that we have been                       s
                                                                                             coupling. U e ac if the trigger sig-
        horizontal and vertical position        discussing, and wme brands may               nalcontains-a l e e dc component.
        controls to bring the trace to cen-     have more. “he point is, the theory          Uee de if the trigger eignal fre-
        ter meen.                               is the same regardlees d what the            queney is less than tlJOi%rt%.
                                                control is caned or even if you have
    0   Adjust the intensity control to a       Om.
        comfortable viewing level. Adjust                                                0   Always u ~ the EXT + 1 input if
                                                                                                          8          0
        the focus control for sharpest                          NOTE                         you are not using a divider piwb
        trace.                                    The following axioms apply                 to connect the external trigger
    0   Adjust the input attenuator con-          when you are using an external             signal to the scope.
        trol to its highest setting. This         trigger signal.
        will prevent the trace from being       Axiom #1: The trigger signal must         at
                                                                                         P r 2 will go into more detail on
        deflected off screen if the signal                  be clean and free of         triggering and probe equaliza-
        has a large dc component or is a                    noise. If your soope has     tion in preparation to making
        very large ac signal.                               built-in filters,use them.   measurement&


                                     345549 Digital Voltmeter
c   Does your 3455A “hang-up” in its
    turn-on mode? Or &oes it eometimea
    display out-of-spec low frequency
c   readings whea in normal AC, or in-
    termittently read incorrect data
    when autoranging? If so, there are
    some resistodcapacitor changes
    listed in service notes 3455A-7A and
    3455A-15 that can correct and im-
    prove the voltmeter’s reliability.
    Order the 3455A service notes with
    the order form at the rear o Bench
                                 f
    Briefs.


                                                         WWW.HPARCHIVE.COM
 I * l
       *I e
             :
                   a

                                                                                                                                              1
                                                                             B OBClLLOSCOPE                                   seride1622AW3Wandbelcrw.
                                                                             ala PrefemdreplacementforhQh                    imprarebUeRMSbWhqUOnCy
                                                                              A m .                                          a reliaMuty.
                                                                                                                  -16.       Serials 1622AO53W and below.
                                                                         SAIB OSCILLOSCOPE                           to replace relay AlOKl .
                                                       1                  senals. Preferred replacementtor high
                                                                         storm                                           34
                                                                                                                  3465A-5.
                                                                     12Q7AIBO ~ l L L O s c o p E                     3465A
                                                       1207AlB5A. All serials.Preferred mphcmmt for high            80024.
                                                         voltage oscillator A                                     34656-2. AU serials. R
                                                                                                                    34656 b e t t m Battery P/N tor HP uI)(1: OOO91-
                                                       1208A OPTION H11                                             80013. Customer use: Accesllay 82(M3A
                                                       H11-1208A-12. serlals                                      3581C (C MODEL ONLY) WAVE ANALYZER
                                                         repkroement for high voltage oscillator A604.            3581AIC-5. Sdals 1411A01107 and below, and 88
                                                       Hll-1208A-13. Serials 1330A arxl below. Preferred            rids 1411A01110, 1411A01 113. 141lAOl114,
                                                         rwbment for hlgh voltap OsOHlptor A m .                    1411A01116. Audlo amplifier modification to
                                                                     121SAIB OSCILLOSCOPE                           p a ~pertormaw.
                                                                                                                           t
                                                       1215AIBlA.AllWwiak. Preterredreplacementforhigh
                                                         VOItage oscillator A m .                                        3582A SPECTRUM ANALYZER
                                                                                                                  358ZA-2. Serials 03582-00151 to 035BUMBlD. Mdi-
                                                                     1                                              tionofdiodestopratecttrandstorQlOonA13PCB.
                                                       1217AJE2A
                                                         voltage oscillator A6Q4.                                            3745NB SELECTIVE LEVEL
                                                                                                                                 MEASURING SET
                                                                                                                                     1812U and b.low. R
                                                       11
                                                        -.           serials 184344
                                                             replacementto inxcwsB the relicrbi#tyofthe X and Y                   1812U and bebw.
instruments you own, remove the                              amplifiers.                                              f& mlopUons H15 and H16.
                                                                                                                          a
                                                                                                                  374WB-298. SerlaJs 1815U. 1831U and 11t;98u,
order form and mail it to the HP                                          1304A DISPLAY                             Modificatlon to prwnt ptmft&a remote SlMtUp
distribution center nearest you.                                           1715A and bebw.                          problem.
                                                                                                                  374wB-30. serials 192Ou and below. Preoanrrd re-
                                                                                                                    placement of Memory Assembly A109.

                                                                         1333A X-Y DISPLAY
                                                                                                                                - -
                                                                                                                  374WB-31. 3745A        S r a s below 193011-00347
                                                                                                                                          eil
                                                                                                                                                 . - .-
                                                                                                                    and 37458 Serials belaw 192AUaa201. Prefer-
                                                                                                                                                    -     . .
                                                                                                                    red replecemeot Capecitor N% CI
                                                       1333A-6. All prefixes. Preferred parts r              to   3745A/&32A. 3745A - s r a s below 1
                                                                                                                                           eil
                                                                 r l i of z-uds amplifier
                                                                   e t
                                                                     w  y                                           and 37458   - Serials belaw 1924uQ0201. P@W-
                                                                                                                    red replacement of Integrated circuit A211IC@,
                                                                                 ptetened   replacemenl to   im           3747AI0 SELECTIVE LEVEL
                                                                                                                               MEASURINGSET
                                                                                                                  3747AlB-W. Serials 1804U and below. #&Man d
                                                                1350A GRAPHtCS TRANSLATOR                             lOMHz crvstal tmar assemblv to improve
                                                       135OA-5. All serials.Program modication to eliminate
                                                         a bright spot which appears on screen when writing       3
                                                         into W o n 2047 or the data in location 2047 is not
                                                         displayed.                                               3

                                andbekw.ModMcp-
                                                             16OOA LOQlC STATE ANALYZER                           3
                                                       16WA-2. Serials 1714AOW llnd bekvr. ModiRcation
                                                         to prevenl inconed deta storage into “E” taMe.           3
                                                                                                                    tion to prevent unstable Input autorangin&
                                                                1 W A SERIAL DATA ANALYZER                        3747NB-10. 3747A - Serials belovv 1924U-00139
                                                       164OA-7. All serials. Prefermd parts replacement for                     -
                                                                                                                    and 37478 seriela below 1924U-00115. Prefer-
                                                         8251 model USARTs.                                          red replacement of cepacita A311c3.
                                                                                                                  3747-11.      3747A - Serials WW 1924U-00139
                                                                     1722NB OSCILLOSCOPE                             and 37478 - Serials below 1924U-00110. P ~ c -
                                                       1722A-4A. All serials. Extender board kit to facilitate       red replacement of integrated circuit A21 1IC%.
                                                         troubleshooting.                                         3747AlB-12. All serials. Installation of Option 022 to
                                                       1722A-6A. All serials. Preferred replacement for at-          facilltnteCMessage weighted filter and phaseJitter.
         W8UF SIGNAL GENERATORS                          tenuator detd wheel.
       F-20. -E:   Serials 1826A and below and 608F                                                                           3770A AMPUTUDUDELAY
       serials. Prefermi part replacement for series                  1744A OSCILLOSCOPE                                       MSTORTION ANALYZER
        w transistor 01.                               17-1.        SeriaQ, 1915A and below. Modiication to r e   377OA-37A. All serials. RetroRtting instructkwm for
           612A SIGNAL GENERATOR                             duce ‘‘writethrough’ and defocurJng.                   option 001, +l&JBm switoh.
612A-11. SeMs 1826A and below. Preferred pert re-                                                                        3771NB DATA UNE ANALYZER
  pkcammt for Wes regulator transistor 01.              2 W A QUARTZ THERMOMETER
612A-12. W’W 1914A and W. ModificaHon tor         2804A-2. All serials. Modification to prevent random            3 7 7 1 N M Serlals below 1926U. Modificatiantopre
                                                    reset.                                                           vent gain hits fmm causlng dropouts.
  impovcld~moduhtkn~~uacy.                                                                                        3771AlB-6. Serials bebw 1937lJ-00160 (3771A) end
        680 STRIP CHART RECORDER                                  3OWA BOARD TEST SYSTEM                             1937U-00123 (37716).           replacement of E l
                                                       306OA-1. All serials. Proper configuration to prevent         Mains Module.
-74.    All serials before 1200. Modification to im                                                               377WB-7. All serials. Retrofitting instructionS for
   p o safety grounding.
          ~
                                                          missing charactem on 26408 CRT.
                                                                                                                     OptlOn 061, Rack Mount.
6E0-8.AU serials. Preferred gear plate in tratmmhkm                                                               377lNB-8. All serials. Preferred replacement of
   mmnblbe.                                                        3437A SYSTEM VOLTMETER                            switchesA38s1andA4os1.
                                                       3437A-3. All serials. S r i e procedure to fix sticky
                                                                              evc                                 3771NB-9. All serials. Table of link positions for
            12oouB OSClLLOCOPE                           hwa PQnel pushbuttons.                                      377lA, 37716 and Options.
                                w-mentforhigh          3437A-4. SefW 1630-70           aml below. Recom-
                                                         m d d                 fa Aau124.                         3779NB PRIMARY MULTIPLEX ANALYZER
                                                                                                                  377W-10. serial 1919U-00155 and           Modwiorr-
                                SCOPE                              3466A DIGITAL VOLTMETER                                                               t
                                                                                                                    tkn to prevent keekdom o f m 2 a wlt&-on.
                                rep(ecementforh@       3456A-7A. Serials 1622A05650 and below. ModiRca-           3779A-11. Serials 191BU-00170 and below. Modlflca-
                                                         uon to improve tum-on reliebility.                         t h to pIII)v(Mt feilwe of Int~atad h d t k w 9 .
                                                                                                                                                       C



                                                                         WWW.HPARCHIVE.COM
                                       SERVICE NOTES




6




    opekn loo.




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                           /I
                            I
                                5420A OWNERS
                                                                                                                      I

                                                                                                              \




Attention 5420A Owners
                                                                                                                          4
                                                                                                                          '



                                         This issue of Bench Briefs contains     AN 240-0 TIME AND
                                         several service notes that may be of    FREQUENCY DOMAIN
                                         interest to you. of primary impor-      MEASUREMENTS
                                         tance is service note 542OA-21 that
                                                                                 Defines, discusses, and illustrata a
                                         groups all necessary modifications
                                                                                 wide range of important meas-
                                         that are important to the reliability
                                                                                 urements made by the 5420A Digi-
                                         of the 5420A Digital Signal
                                         Analyzer.                               tal Signal Analyzer using sampled
                                                                                 data techniques. Includes Time
                                                                                 Record Averaging, Auto and Cross
                                         Owners of the 542011 can order the      Correlation, Auto and Cross Spec-
                                         service notes with the order form lo-   trum, Transfer Function, and more.
                                                                                 AN 240-1FEEDBACK CONTROL
                                                                                 SYSTEM MEASWEMENTS
                                                                                 Illustrates the application of the
                                         Also available are three application    5420A Digital Sigpal Analyzer to
                                         notes designed to help you obtain       both time and frequency domain
                                         maximum use from your 5420A.            measurements of feedback systems.


Safety-Related                           Hewlett-Packard Corporate Stand-
                                         ards have been changed to require
                                                                                         ar
                                                                                 4960A/BP i Identifier

Service Notes
Service Notes from H p relating to
                                         an instnunent to be able to pass 25
                                         amps through the safety earth
                                         ground. Instruments with serial
                                         numbers below those listed below
                                                                                                                  I           a
personal &ety and possible equip
ment damage are of vital importance
to our customers. To make you more
                                         were manufactured before this
                                         safety standard came into being.
                                         While there does not appear to be a
                                                                                                          Y
aware of these important notes, they
are printed on paper with a red bor-     hazard with the old circuit design,
der, and the service note number has     we are notifying our customers that
                                         a modification is available and can                   hock hazard may exist
a "-S"s e . In order to make you
immediately aware of any potential       be made at customer's expense, if
safety problems, we are highlighting     desired.
safety-related service notes here
                                                                                    bsEOW
with a brief description of each prob-
lem. Also,in order to draw your at-        204C - 0989A14605 and below              4960B serials 17371900750 and
tention to safety-related service          204C-H20, Option 02 -                    beI@V?
notes on the service note order form       0989A14855 and below
at the back of Bench Briefs,each ap-       2 4 - 1105A03415 and below
                                            0D
propriate number is highlighted by         209A - 1045A05846 and below
being printed in color.                    608 - All serials before 1200
 204C/D, 209A OSCILLATORS
 AND 680 STRIP CHART                                                             pear at the front panel fel+me
 RECORDER                                The modification consists of install-   line connectors. Installation d an
                                         ing a solder lug to a chassis point     insulator, HP Part #04960-00212,
                                         and connecting a 18 GA GRNNEL           removes thi~ hazard.
                                         wire from the power line connector
                                         earth terminal to the solder lug. For   For complete detailed instm&i   061181,
                                         complete detailed instructions, order   please order the approprirtte! wmim
                                         the appropriate service notes using     n o w using t e form at the
                                                                                               h                     &
                                         the form at the back of Bench Briefs.   Ben& Briefs.


                                                    WVWV, H PARCH IVE,COM
                                                                     STIMULUS-RESPONSE TESTING
                                                                                                  -        1
                                                                                                           1
                                                                                                           1
    0       Includes a technique for obtaining
            the open loop gain from data taken
                                                     STIMULUS-RESPONSE TESTING USING THE IC TROUBLESHOOTERS

            with the loop closed and the system
                                                                                                                        TEST MBTHOD
            in normal operation. Several practi-
            cal examples are illustrated.


            AN 240-2 IMPROVING
            ACCURACY OF STRUCTURAL
            RESPONSE MEASUREMENTS
                                                     SignalLineShortto
                                                            r
                                                       Vcc o Ground
                                                                              u
                                                                             Pw
                                                                                        I   P W
                                                                                            current
            Illustrates how to idencify and cor-
            rect mass loading and accelerometer
            loading errors. Shows the “ideal”
                                                                                        I   T-



            measurement system where the test
            system and exciter appear on sepa-
            rate sides of the load cell. Describes
            “real” measurement set-ups and how
            to correct for mass loading in both
            shaker and impact test systems.




                                                     1. A node is an interconnectionbetween two or more IC’s.
                                                     2. Uee the Pulser to pIovide stimulus, o use normal circuit signals, whichever is most convenienl
                                                                                            r
                                                     3. 547A Current Tracer        WlO!X?!jE,TLogic Probe
                                                        548A L q i i clip                        o$
                                                                                   5uw10526-r L ! c Pukef

        d
                                                     practical terms the technician often              The logic pulser changed this be-
        i
        J                                            uneoldered and lif€ed an IC leg, or               atwe it delivers both high current
                                                     cut a circuit trace, then used a pulse            and low total energy by gelleratiag
                                                     generator to drive a gate’s input.                very short pulses sufficient to
            IC TROU6LESHOOTl)rso TOOLS                                                                 momentarily override TTL logic
P                                                    The “why” of such destructive tech-               LOW states. Pulse. width is never
            In the digital troubleshooting world     niques has to do with the internal                su&ciently long to degrade a gate’s
            the idea of stimulus-responsetesting     structure of standard ‘ITL gates. A               performance (the pulser will usu-
s           is a relatively new one. Tools to        ‘ITL gate in its LOW state is a satu-             ally generate a ‘ITL HIGH for only
            measure logic states have existed for    rated transistor to ground. To move               about 500 nanoseconds).
            quite some time, but forcing a state     a ‘ITL output HIGH requires a great
            change, especially on a line being       deal of current drive. The catch here             To help show how straightforward
            held LOW is something that was dif-      is that continuous high current                   digital stimulus-response testing
            ficult prior to the introduction of HP   tends to destroy the ‘ITL gate’s out-             can be, the following table outlines
            Logic pulsers. T accomplish such a
                              o                      put transistor. So, it was usually                seven node and gate troubleshooting
1           task meant disconnecting a device’s      safer and easier to simply disconnect             problems and how pulsers and other
!           input from the circuit, and then         a driver from a circuit and replace it            IC troubleshooters would be used to
            pulsing the input with a source. In      with a low current stimulus tool.                 pinpoint the fault.
1
                                                            WWW.HPARCHIVE.COM
                         -
                           STlM U L US-R ES PO NSE TESTlNG




                                                                  ening current density in   the
                                                                  €ollowinj$.
                                                            7. When you think you've located the
                                                               moving the pulsing pint to the
                                                               paths should be &etected el
                                                               you pulse directly mrom a short.
                                                            As nondestrudively as possible, remove the suspectsd
                                                            component and verify that the Vcc to ground short no
                                                            longer exists.




                              Node and Gate Troubleshooting Examples
Example 1                                                   b) Set the current tracer derence level by aitfi.
                                                                                  until the light just lighta
                                                                                  d,
                                                              U e the current tracer to follow the current pafh to
                                                             c)s                                                     .
                                                              the fault.
                                                            Example 2
A frequently o c d n g troubleshooting symptom is a
              this erample,the problem is to deter-
              er h dead, or ifa shorted i p t i clamp-
                 i                       nu,s                Here are two D-Flip-Flops. One w o r b n o d
                                                             other doesn't change output state although
                                                             ditions are identical for both.
                                                             a) U e the pulser at the D input ami probe Q
                                                                  s
                                                                 888 if the outputs &.-      (In this actuaL
                                                                 they di&t.)

                                                  vv\Ivw HPLLRCHIVF CnM
        r                                                     STIMULUS-RESPONSE TESTING



    0   b) The Reset line in the above case was found to be
           stuck in a LOW state by using a logic probe. Pulsing
                                                                    Example 6

           and probing simultaneously indicated the Reset
           line couldn’t be driven HIGH,indicating the line
           was shorted to ground.
        c) Further use of pulser and current tracer showed
           that the area near the Reset line drew current when
           pulsed and that the D-FlipFlop would not perform                        I t
           operations when the Reset line was pulsed.
h
        d) Using the pulser and tracer,the operator foun
           hairline solder bridge from the Reset line to
                                                                    over two PC bo
I       Example 3
                                                                    a) Use the HP-10
                                                                       node. The comparator is clipped onto the suspected
                                                                       IC and us88 the in-circuit signals to drive an identi-
                                                                       cal IC installed h i d e the comparator. The com-
                                                                       parator identifies AlU8-13 as bad allowing you to
                             ClmnNlT                                   troubleshoot further.
                                                                    b) Probing and           the node indicates it is stuck
        The node between U1 and U2 was stuck LOW when                  LOW.
        measured by a logic probe, although probing revealed
        pulse activity at Ul’s input.                               c) Pulsing and current tracing at A
        a) Probe U1 pin 2. If no voltage activity is present,          current is flowing toward PC board

    0      pulse pin 2 and see if the node’s state can be
           changed.
                                                                    d) A2U1 is sinking current andholding the node
                                                                       LOW. As a result, U2 is not being clocked. The com-
        b) In this case the state couldn’t be altered using            parator located the failure (A1 U8 pin 151, but it
           pulser so current was traced from U1-2 to U2-9, th          required the tracer to indicate current flow to veTify
           J-Kflip-flops input.                                        A2U1 as the c a m
        Example 4                                                   Example 6




        Outputs A, B, C, and D are LOW; other circuit inputs        U1 tests bad using the probe and pulser. The problem
        appear normal.                                              is to find out the nature ofthe Eault before h t i w h g
        a) Use the probe and pulser to make sure A, B, C, and       what appears to be an internally aharted IC.
           D aren’t grounded @robe and pulse each pin - if          a) Pulse pin 12, and obsem with the probe cluzt pin 13
           ungrounded the states will be changed by the                                              direction ClB and 13
    ~      pulses).
b
        b) Probe other pins on the IC and check for normal/
           abnormal indications.                                    b) Pulse pin 12 and
        c) Measure current at pins A, B, C, and D by pulsing
                                                                       verse the two instruments. Current is.identica1 in
           each pin, and tracing to see if current flow is indi-       both directions.
           cated from the pulser to the Shift Register.
    0   d) In this example all signals are normal except A, B,
           C, and D. They are stuck LOW, and are not indicat-
                                                                    c) Pins 12 and 13 are
                                                                       bridge on the back of the circuit board. Although
                                                                       originally located by pulser and probe, the tracer
           ing current flow, which suggeats an internal failure        adds important information that keeps you from
           in the IC, and not in the circuits connected to it.         removing the IC.


                                                        WWW. HPARCHIVE.COM
HEWLETT-PA
          1820 Em
         Pal0 Alto,
               muim
          vcNCti u n i u
           MAR-APRIL 1980
          Volume 20 Number 2

       Service lnformatlon from
       Hewlett-Packard Company

D   obtain a quallficatlonform for a fre
          Ion, send your rsquest to thi
             above address.

Peader comments or technical article
contrlbutlon8 are welcomed. Please
 send them to the above address,
      attention Bench Briefs.

      tor: Jlm Bechtold, HP Mt. View

				
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