Oscilloscope Traces: Square Waves
November 1957 Popular Electronics
November 1957 Popular Electronics
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This easy testing method helps us uncover a multitude of electronic
By Howard Burgess
Square wave testing can be called the "buckshot"
approach. One shot covers a lot of territory, and can bring down a whole
flock of fast clues. In many kinds of testing, a single frequency or
tone is put into the input of an amplifier or system and the output
waveform is checked for distortion and level. But when an amplifier
is to be checked over a wide band of frequencies, this method can be
long and tedious. It would save considerable time and provide a better
overall test if a number of the desired frequencies could be checked
That's just what actually happens in cases where
we employ a square wave as a test signal. A quick look at the structure
of a square wave shows why this is so.
What Is In the
Wave. The oscilloscope pattern in Fig. 1 is an example of a
sine wave. This is a simple sinusoidal waveform which we will call F1.
The square shown in the broken line is the desired shape of a "square
In Fig. 2, we still have F1 but the third harmonic F3 (or F1 times 3)
has also been added. This combination provides the waveform labeled
F1+F3, which fills out a little more of the square-wave box. By adding
the fifth harmonic, we get the wave F1+F3+F5 as shown in Fig. 3.
A low-capacity probe such as this one is needed for square-wave
observation. Finished probe is shown in top photo, circuit and construction
details in the two lower illustrations.
Even a simple square-wave generator used in conjunction with a 'scope
will quickly show up defects in an audio system. Primarily it serves
as a good indication of frequency response.
Using our imagination,
we can see what is happening to the original waveshape. With each harmonic
added, the shape comes closer to that of the dotted line square. If
the process of adding odd harmonics is continued, we finally arrive
at a fairly acceptable square wave by the time about 10 harmonics are
thrown in with the fundamental.
first four figures illustrate the relationship between the
square wave and its constituent sine waves. Fig. 1 compares the sine
wave and square wave. In Fig. 2 is a sine wave and its third harmonic.
In Figure 3 is a sine wave plus its third and fifth harmonics, which
together begin to fill out the shape of the square wave. Figure 4 shows
an ideal square wave containing a large number of harmonics.
Yet, in many cases, 100 or more harmonics may be needed to produce
the desired waveshape with the filled-out corners, as shown in Fig.
4. Suppose that a 1000-cps square wave which includes the 10th odd harmonic
is used to test an amplifier. The amplifier must then be able to respond
up to 21,000 cps or better to pass the waveshape without distortion.
By using a square wave as a test signal, it is not only possible
to test the complete frequency response of an amplifier, but you can
also show up troubles such as phase shift and instability resulting
in oscillations and parasitics.
"Square Deal" Probe.
When using a square-wave generator and oscilloscope in a test setup,
keep these items in mind: (1) the generator must be properly matched
to the input of the amplifier; (2) the amplifier output must be properly
loaded; (3) the oscilloscope must be connected across the output of
the amplifier under test in such a way that the 'scope leads themselves
do not distort the waveshape of the signal. In most cases, simple leads
to the 'scope are not adequate and will cause serious distortion. A
simple probe, easy to make, is almost a necessity.
circuit for such a probe is shown at left, and the photos will give
a general idea of its construction. The low-capacity shielded line to
the 'scope should be less than two feet long and the entire probe must
be kept well-shielded. The ceramic trimmer is adjusted by feeding a
known square wave from a generator into the tip of the probe and tuning
for the squarest wave possible on the 'scope. Once adjusted, this type
of test lead is also excellent for use on video circuits. The probe,
because of its method of operation, will normally attenuate the input
signal somewhat, but you can compensate for this.
of the square-wave generator and 'scope are very much like those suggested
for testing with a sine-wave oscillator, but the interpretation of the
pattern is very different.
Which End Is Up?
When an amplifier is driven by a square-wave generator and the oscilloscope
connected to its output displays a pattern like Fig. 4, the amplifier
is probably passing up to the 25th or higher harmonic. However, if the
trace more nearly resembles Fig. 5, the slope to the right indicates
a loss at the lower frequencies while retaining good high-frequency
A slope in the reverse direction, as shown in Fig.
6, indicates just the opposite: good low-frequency response with a dropping
off at the highs. Figure 7 is a curve indicating that an amplifier is
lacking in both low and mid-range response.
The curve in Fig.
8 bears little resemblance to a square wave and shows an extreme case
of high-frequency attenuation. When using square waves, it can be said
in a generalized interpretation that the left-hand edge of each half-cycle
indicates the high-frequency conditions existing in the tested amplifier
while the right-hand edge of each half-cycle indicates the low-frequency
conditions. Superimposed ripples on the leading (or high-frequency)
edge as in Fig. 9 indicates the presence of oscillation or "ringing."
Complete books have been written about square-wave testing,
and very limited ground can be covered in a few hundred words. However,
even with the simplest kind of square-wave generator, such as the one
shown, used only for the simple patterns given here, one can gain much
experience and knowledge.
Square-wave patterns indicate conditions
within the amplifier under test. The waveform in Fig. 5 indicates good
high-frequency response but poor lows, while the waveform in Fig. 6
indicates good low-frequency response but poor highs. Figure 7 illustrates
a case of poor low- and medium-frequency response, and Fig. 8 indicates
serious attenuation of high frequencies. The pattern in Fig. 9 betrays
the presence of high-frequency instability or "ringing" in the system.