A M P L I F I E R S Q U A R E W A V E T E S T
introduction:
A square w a v e generator and the oscilloscope,
such as this oscilloscope can be used to display v a -
rious types of distortion present in electronic
circuits. A square wave of a given frequency
contains a large number of odd harmonics of that
frequency. If a 5 0 0 Hz square wave is injected into
a circuit, frequency components of 1.5 kHz, 2.5
kHz and 3.5 kHz are also provided. Since vacuum
tubes and transistors are non-linear, it is difficult to
amplify and reproduce a square wave which is
identical to the input signal. Interelectrode
c a p a c i t a n c e s , j u n c t i o n c a p a c i t a n c e s , s t r a y
capacitances as well as limited devices and
transformer response are a few of factors which
prevent faithful reproduction of a square wave sig-
nal. A well-designed amplifier can minimize the
distortion caused by these 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
injecting a 5 0 0 Hz sine wave 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 determine how the amplifier would
response to input signals from 5 0 0 Hz up to the
15th or 2 1 s t harmonic.
The need for square wave evaluation becomes
apparent if w e realize that some audio amplifiers
will be required during normal use to pass
simultaneously a iarge number of different
frequencies. With a square wave, w e have a
controlled signal with which w e can evaluate the
input and output quality of a signal of many
frequencies (the harmoniques of the square w a v e
which is what the amplifier sees when amplifying
complex wave forms of musical instruments or
voices.
The square w a v e output of the signal generator
must be extremely flat so that it does not
contribute to any distortion that may be observed
when evaluating amplifier response. The oscil-
loscope vertical input should be set to DC as it will
introduce the least distortion, especially at low
frequencies. When checking amplifier response,
the frequency of the square w a v e input should be
varied from the low end of the amplifier bandpass
up toward the upper end of the bandpass; however,
because of the harmonic content of the square
wave, distortion will occur before the upper end of
the amplifier bandpass is reached.
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
bandwidth amplifier, (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. The square wave also will
reveal some deficiencies not readily apparent when
using a sine w a v e signal. Whether a sine w a v e or
square wave is used for testing the amplifier, it is
important that the manufacturer's specifications on
the amplifier be known in order to make a better
judgement of its performance.
Fig. 28 Equipment set-up for square wave testing
of
amplifiers
25
S Q U A R E W A V E G E N E R A T O R
A D J U S T S W E E P
. S P E E D F O R 1
C Y C L E D I S P L A Y
A D J U S T V E R T G A I N F O R
C O N V E N I E N T V I E W I N G H E I G H T
O U T P U T
A M P L I F I E R
C I R C U I T
B E I N G T E S T E D
INPUT
