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
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.
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. As time permits, I will be glad to scan articles for you. All copyrights
(if any) are hereby acknowledged.
Aficionados of vintage oscilloscopes might want to visit the
Oscilloscope Museum website, and Simpson
Electric fans can visit the Simpson260.com website for
just about any Simpson user's manual.
available vintage Radio News articles.
Questions and Answers on Oscilloscopes
By Robert G. Middleton
Chief Field Engineer, Simpson Electric Company
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.
Basic Scope Display
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.
If the 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 voltage.
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 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 category.
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
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.
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