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Frequency M e a s u r e m e n t :
1. Connect the sine wave of known frequency to
the C H 2 input jack of the oscilloscope and set
the S W E E P T I M E / D I V control to X - Y .
2. Connect vertical input probe (CH1 INPUT) to
the unknown frequency.
3. Adjust the C H 1 and C H 2 size control for a con-
venient easy-to-read size of display.
4. The resulting
pattern
pottern, shows the ratio between the
frequencies (see Fig. 27).
U N K N O W N
F R E Q U E N C Y
V E R T I C A L
I N P U T
S T A N D A R D
F R E Q U E N C Y
T O
H O R I Z O N T A L
I N P U T
Fig. 2 7 L i s s a j o u s ' w a v e f o r m s used for
frequency m e a s u r e m e n t
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
various types of distortion present in electronic cir-
cuits. A square w a v e of a given frequency contains
a large number of odd harmonics of that frequen-
cy. If a 5 0 0 Hz square w a v e is injected into a cir-
cuit, frequency components of 1.5 kHz, 2.5 kHz
and 3.5 kHz also are provided.
tubes and transistors arenon-linear, it is difficult to
amplify and reproduce a square wave which is
identical
to
the
input signal.
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
a s well
transformer response are a few of the factors which
prevent faithful reproduction
signal. A well-designed amplifier can minimize the
distortion caused by these limitations.
designed or defective amplifiers can introduce dis-
tortion to the point where their performance is un-
satisfactory. A s stated before, a square w a v e con-
called a
Lissajous
two
T O
R A T I O
O F
U N K N O W N
T O
S T A N D A R D
1 / 2 : 1
1 : 1
1 - 1 / 2 : 1
6 : 1
Since vacuum
Interelectrode
c a p a c i t a n c e s , s t r a y
a s limited device and
of a square wave
Poorly
tains 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 wave 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 w a v e evaluation becomes ap-
parent if we realize that some audio amplifiers will
be
r e q u i r e d
d u r i n g
simultaneously a large number of different frequen-
cies. With a square w a v e , w e have a controlled
signal with which w e can evaluate the input and
output quality of a signal of macy frequencies (the
harmonics of the square wave) which is what the
amplifier
sees w h e n amplifying
forms of musical instruments of voices.
The square w a v e output of the signal generator
must be extremely flat, so that it does not con-
tribute to any distortion that may be observed
when
evaluating
amplifier
cilloscope 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 wave input should be
varied from the low end of the amplifier bandpass
up toward
the
upper
however, because of the harmonic content of the
square w a v e , 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 wave
signal. This is especially important in an limited
bandwidth amplifier (voice amplifiers).
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 w a v e also will
reveal some dificiencies not readily apparent when
using a sine w a v e signal. Whether a sine wave 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.
Testing Procedure (refer to Fig. 28):
1. Connect the
output
generator to the input of the amplifier being
tested.
2. Connect the C H 2 probe of the oscilloscope to
the output of the amplifier being tested.
3. If the DC component of the circuit being tested
sufficiently low, to allow both the A C and D C
component to be viewed, use the DC position
n o r m a l
u s e
to
p a s s
complex wave
response.
The os-
end of the bandpass;
of
the
square wave
23
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