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
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
Wax nostalgic about and learn from the history of early electronics.
See articles from Radio &
Television News, published 1919 - 1959. All copyrights hereby acknowledged.
Aficionados of vintage oscilloscopes might want to visit the
Museum website, and Simpson Electric fans can visit the Simpson260.com
website for just about any
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 examine.
The 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
Why 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 negative-peak voltage.
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
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.
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.
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.