A P P L I C A T I O N S
Amplifier Square Wave Test
Introduction
A square w a v e generator and the oscilloscope can be used
to observe various t y p e s of distortion present in electric cir-
cuits. A square w a v e of a given frequency contains a large
number of odd harmonics of that frequency. If a 5 0 0 Hz
square w a v e is injected into a circuit, frequency com-
ponents of 1.5 kHz, 2 . 5 kHz and 3 . 5 kHz are also provid-
ed. Since v a c u u m tubes and transistors are non-linear, it is
difficult to amplify and reproduce a square w a v e which is
identical to the input signal. Inter electrode capacitances,
junction capacitances, stray capacitances as well as nar-
row band devices and transformer response are the factors
which prevent faithful response of a square w a v e signal. A
welldesigned amplifier can minimize the distortion caused
by t h e s e limitations. Poorly designed or defective
amplifiers can introduce distortion to the point where their
performance is unsatisfactory. A s stated before, a square
w a v e contains a large number of odd harmonics. By injec-
ting a 5 0 0 Hz sine w a v e into an amplifier, w e can evaluate
amplifier response at 5 0 0 Hz only, but by injecting a
square w a v e of the same frequency w e can understand
how the amplifier would response to input signals from
5 0 0 Hz up to the 1 5th or 2 1 s t harmonic.
T h e need for square w a v e evaluation becomes apparent if
w e realize that some audio amplifiers will be required during
normal use to pass simultaneously a large number of dif-
ferent frequencies. With a square w a v e , w e can evaluate
the quality of input and output characteristics of a signal
containing a large number of frequency components such
as complex w a v e f o r m s of musical instruments or voices.
The square w a v e output of the signal generator must be
extremely flat. T h e oscilloscope vertical input should be set
to D C as it will introduce the least distortion, especially at
low frequencies. Because of the harmonic content of the
square w a v e , distortion will occur before the upper end of
the amplifier bandpass.
It should be noted that the actual response check of an
amplifier should be made using a sine w a v e signal. This is
especially important in an limited bandpass amplifier such
as a voice amplifier.
The square w a v e signal provides a quick check of amplifier
performance and will give an estimate of overall amplifier
quality. T h e square w a v e also will reveal some deficiencies
not readily apparent w h e n using a sine w a v e signal.
Whether a sine w a v e or square w a v e is used for testing the
amplifier, it is important that the manufacturer's specifica-
tions on the amplifier are based on in order to make a better
judgement of its performance.
Testing Procedure (refer to Fig. 1 1 ) :
1 . Connect the output of the square w a v e generator to the
input of the amplifier being tested.
2 . Connect the vertical input probe of the oscilloscope to
the output of the amplifier.
3 . If the D C component of the amplifier output is low, set
the A C - G N D - D C s w i t c h to D C position to allow both the
A C and D C components to be v i e w e d . However, the A C
position may be used to observe the A C component on-
ly, though this will reduce the audio frequency content
of less than 5 Hz.
4 . Adjust the vertical gain controls for a convenient view-
ing height.
5. Adjust the s w e e p time controls for one cycle of square
w a v e display on the screen.
6. For a close-up v i e w of a portion of the square w a v e , use
the X 5 magnification.
S q u a r e w a v e
generator
I N P U T
Amplifier
circuit being
tested
O U T P U T
A d j u s t s w e e p
s p e e d for 1
c y c l e display
F i g . 1 1 . E q u i p m e n t s e t - u p f o r s q u a r e w a v e t e s t i n g o f a m p l i f i e r s
1 5
I N T
+ S L O P E
A d j u s t V. gain
for c o n v e n i e n t •
v i e w i n g height
DC
