Questions and Answers on Oscilloscopes
April 1957 Radio & TV News Article
Electric is a name most RF Cafe visitors are probably familiar
with as being the maker of high quality analog multimeters, with the
Simpson 260 line being the most famous (it is still manufactured
today). Not as many people, however, know that Simpson also used to
make oscilloscopes. This article from Popular Electronics was
written by a Simpson Electric engineer whose job was, in part, to respond
to questions asked by users. It covers basic operations like how to
calibrate the display, adjust
horizontal time base and vertical amplitude scales, and how to synchronize
the display with the input signal. Some explanation of how to interpret
periodic and pulse type waveforms is provided as well as tips on how
to avoid overloading and possibly damaging the instrument.
April 1957 Radio & TV News
of Contents]These articles are scanned and OCRed from old editions of the Radio & Television
News magazine. Here is a list of the
Radio & Television News articles
I have already posted. All copyrights
(if any) are hereby acknowledged.
of vintage oscilloscopes might want to visit the
Museum website, and Simpson Electric fans can visit the Simpson260.com
website for just about any
Simpson user's manual.
See all available vintage
Radio News articles.
Questions and Answers on Oscilloscopes
By Robert G. Middleton
Chief Field Engineer, Simpson Electric
to the queries the manufacturer most often gets from technicians on
this important instrument.
Of the many questions manufacturers
of oscilloscopes are asked, the most common is, "Which is the right
pattern? I can get nearly any pattern I want on the scope screen." The
question is too complex to be answered simply. What is involved here
is a basic understanding of the operation and application of the instrument.
The "right" pattern, of course, is obtained only when operation and
application are correct. Still, the high degree of frequency with which
the question is asked, even in this era, justifies a fundamental review
of the instrument and the nature of the phenomena which it is used to
Basic Scope Display
of a 60-cycle sine-wave signal is very basic, and an instructive point
from which to start. Turn on the scope and adjust the intensity and
focus controls to obtain a bright and well-focused horizontal trace
(see Fig. 1). The vertical and horizontal centering controls are adjusted,
as required, to center the trace on the scope screen.
vertical-gain controls of the scope are advanced to maximum, a pattern
is obtained on the scope screen even though there is no signal input
to the scope. This is a very puzzling point to the beginner, and is
explained as follows: the scope input system has a very high impedance.
For this reason, the exposed input terminals pick up stray 60-cycle
fields about the bench, producing substantial vertical deflection. Note
that, when a 1-megohm resistor is shunted across the scope input terminals,
the pick-up is eliminated. This is the reason that stray fields do not
enter the scope in normal circuit testing - the circuit impedance shunts
the scope input, so that any stray pick-up is not observable.
To display the 60-cycle pattern in proper form, the coarse frequency
control is set to a position which includes 60 cycles. For the scope
illustrated in Fig. 1, the control would be set to the 14- to 250-cycle
position. The fine-frequency control is then adjusted to obtain one,
two, or more cycles, as desired, of pattern.
To lock the pattern
on the screen, so that it does not "run" horizontally, the function
switch should be set to a suitable sync position, such as line-sync
in the case of this 60-cycle wave, and the sync-amplitude control is
advanced just sufficiently to lock the pattern tightly. This 60-cycle
sine-wave pattern may appear very elementary, but it has some important
properties which are worthy of note.
Fig. 2 shows such a sine-wave pattern, with the important" values depicted.
The average value of the symmetrical sine wave is zero, and falls along
the zero-volt axis, or resting position of the trace when no input signal
is present. The total excursion of the waveform is a measure of its
peak-to-peak voltage. The peak-to-peak voltage is made up of a positive-peak
voltage and a negative-peak voltage, as shown in Fig. 2. The r.m.s.
voltage is equal to 0.707 of the peak value; this r.m.s. voltage is
the value which is indicated by an a.c. service voltmeter.
Fig. 2. A sine wave is used to illustrate various ways to measure
these three values? The first a.c. voltage value which was recognized
was the r.m.s. value. This has its origin in power work, and is still
used in the measurement of line voltage, transformer voltages, and heater
voltages. Power work started out with d.c. power sources (even in some
areas today, power lines still supply d.c. voltage). When a.c. power
became common, it was desired to measure a.c. voltage in units such
that the amount of light, or heat, or power obtained from a 117-volt
a.c. line would be the same as that obtained from a 117-volt d.c. line.
Thus if a 117-volt (r.m.s.) a.c. line is connected to a soldering iron,
just as much heat will be obtained as if a 117-volt d.c. line is connected
to that iron.
However, the advent of TV brought in a new requirement:
vacuum tubes in most cases respond upon the basis of the applied peak-to-peak
signal voltage. In some cases, the tube responds to the positive-peak
or to the negative-peak voltage which is applied - this depends upon
operating bias. In any case, these newer units of voltage are of chief
significance in servicing electronic circuits. The peak voltage of a
sine wave is equal to 1.414 times its r.m.s. value, and the peak-to-peak
voltage of a sine wave is equal to 2.83 times its r.m.s. value. It is
evident that the peak-to-peak voltage of a sine wave is double the value
of either the positive-peak or of the negative-peak voltage. This is
so only because a sine wave is symmetrical.
Most television waveforms are not symmetrical. A simple pulse waveform
like the one shown in Fig. 3 has a positive-peak voltage which is unequal
to its negative-peak voltage as shown in Fig. 4. The peak-to-peak voltage
of the pulse is equal to the sum of its positive-peak voltage plus its
Fig. 3. This asymmetrical pulse waveform is more typical of
those encountered in TV than the sine wave. Note the unequal
positive and negative peaks.
It should be noted that the average value
of a pulse waveform - as of all complex waveforms - is zero. This average
level falls along the zero-volt level on the screen; that is, it falls
along the resting position of the beam when no input signal is applied
to the scope. It is this basic property which is utilized in measuring
the positive- and negative-peak voltages of the waveform. Note that,
in any pulse or complex waveform, the area of the pattern above the
zero-volt axis is exactly equal to the area of the pattern below the
zero-volt axis. This is a necessary consequence of the fact that the
average value of the waveform is zero.
When a pulse waveform
is applied to the input of a d.c. voltmeter, the pointer indicates zero
volts. Again, this observation is the result of the fact the average
value of the waveform is zero. However, when the pulse voltage is applied
to the input of an a.c. voltmeter, the indication obtained depends upon
several factors. In general, the indication will be largely meaningless.
The pulse waveform does have an r.m.s. value, but this is somewhat difficult
to determine, and is of little interest to the service technician. The
indication obtained will depend upon the frequency characteristics of
the test instrument, which way the test leads are applied to the pulse
source, and other factors. Of course, some voltmeters have a peak-indication
function, or a peak-to-peak indication function. In such case, a useful
measurement can be obtained unless the pulse repetition rate is low
and the pulse is narrow. Because of these various considerations and
reservations, the use of the techniques that do not involve the scope
for waveform examination can be seen to have serious shortcomings.
If the oscilloscope
is to be the instrument for reliably measuring waveforms, as well as
observing them for appearance, how can dependable measurements be made
on an instrument with which, by adjustment, we "can get any pattern
we want?" Indeed, these questions pertaining to measurement, and especially
to the measurement of peak-to-peak value, fall into the most-frequently-asked
Most scopes nowadays provide a source of calibrating
voltage for reference. The scope shown in Fig. 1, for example, makes
an 18-volt peak-to-peak sine-wave voltage available through a binding
post on the front panel. (Where a calibrating standard of this kind
is not built in, a sine wave of known amplitude may be introduced externally
with no change in the remainder of the measurement procedure.) When
a lead is connected from this binding post, or other source, to the
vertical-input terminal of the scope, a known voltage of 18 peak-to-peak
volts develops a sine-wave pattern on the screen.
If the vertical-gain
controls are adjusted to make the sine wave occupy a total height of
18 squares on the calibrated grid or screen, each square will evidently
measure one peak-to-peak volt. Now the calibrating lead can be disconnected
and, provided that the vertical-gain controls are left untouched, another
signal voltage can be measured. After this unknown signal is applied,
a count is made of the squares of vertical deflection it achieves. This
is its peak-to-peak voltage.
Of course, signal voltages subject to measurement vary widely in amplitude.
Some may be so large that they will drive the beam off-screen when applied
to the scope with the same vertical-input settings used for the reference
voltage; on the other hand, some of these voltages may be so small as
to fail to produce measurable deflection. To meet this situation, a
step attenuator, or coarse vertical-gain control is provided. Generally
this step attenuator is arranged in decimal steps, which are most convenient.
Fig. 4. Positive and negative peak amplitudes are unequal for
the pulse, but areas enclosed by each peak are equal.
The continuous, fine vertical-gain control, or the vertical
vernier, is still left untouched, but the coarse control is turned in
either direction the number of steps required to obtain a satisfactory
deflection on the screen for the voltage being measured. If the coarse
attenuator has been turned up one step to increase the sensitivity of
the scope by ten times as compared to its former position, then each
vertical square will represent .1 peak-to-peak volt, instead of 1 volt.
On the other hand, if the attenuator has been turned down one step,
to reduce an oversize waveform, each square will now represent 10 peak-to-peak
volts. If the attenuator has been turned down two steps, each square
will represent 100 peak-to-peak volts.
Thus the utility of a
decimal step attenuator lies in the fact that the basic calibration
of the scope is unchanged - only a decimal point is shifted. Some scopes
may have a step attenuator that changes sensitivity by some factor other
than 10. These can be just as accurate, but they are not quite as convenient,
as they may involve a small amount of arithmetic.
While a relatively
simple pulsed waveform has been chosen to illustrate the technique of
measurement involved, the procedure is used unchanged with the most
complex wave-shapes. In fact, most technicians prefer, while taking
peak-to-peak measurements, to reduce the horizontal gain or width control
to zero. This reduces all waveforms, no matter how different or confusing
in shape, to a common denominator - a single vertical line. Since the
length of that line is the true peak-to-peak value, irrespective of
shape, measurement procedure is simplified.
Another question which is often asked concerns distortion of
the displayed waveform which results from application of excessive signal
voltage to the scope input. It must be recognized that it is quite possible
to overdrive a scope amplifier, just as may be done with any other amplifier.
When overload occurs, the waveform is clipped on the top, or bottom,
or both. The resulting distorted pattern can be very misleading.
To avoid scope overload, a simple operating rule should be followed
at all times: Adjust the vertical-input controls so that the continuous
attenuator is operating on the upper portion of its range. If necessary,
the coarse attenuator can always be advanced a step or two, to permit
this condition. The reason for this precaution is that the output from
the coarse attenuator is generally applied to the grid of the scope
input stage, while the continuous attenuator works in the cathode circuit.
It is grid overdrive which provides overload and clipping.
Posted November 8, 2013