S I N E
0 = 1
A
Fig. 23. Phase shift calculation
Frequency Measurement:
1. Connect the sine wave of known frequency to the CH2
input of the oscilloscope and set the S W E E P
TIME/DIV control to X-Y.
2. Connect the CH1 probe to the signal to be measured.
3. Adjust the CH1 and CH2 for proper sizes.
4. The resulting Lissajous' pattern shows the ratio
between the two frequencies (see Fig. 25).
UNKNOWN FREQUENCY
TO V E R T I C A L I N P U T ,
STANDARD F R E Q U E N C Y
TO H O R I Z O N T A L I N P U T
S E E NOTE
R A T I O OF
UNKNOWN
TO
STANDARD
S E E NOTE
N O T E : ANYONE OF T H E S E F I G U R E S D E P E N D I N G
UPON PHASE R E L A T I O N S H I P
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 wave generator and the oscilloscope can be
used to display various types of distortion present in electric
circuits. A square wave of a given frequency contains a
large number of odd harmonics of that frequency. If a
500Hz square wave is injected into a circuit, frequency com-
ponents of 1.5kHz 2.5kHz and 3.5kHz are also provided.
Since vacuum tubes and transistors are non-linear, it is dif-
ficult to amplify and reproduce a square wave which is iden-
tical to the input signal. Interelectrode capacitances, junc-
tion capacitances, stray capacitances as well as narrow band
devices and transformer response are factors which prevent
faithful response of a square wave signal. 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.
As stated before, a square wave contains a lat
number of odd harmonics. By injecting a 500Hz sine wave
into an amplifier, we can evaluate amplifier response at
500Hz only, but by injecting a square wave of the same
frequency we can determine how the amplifier would
response to input signals from 500Hz up to the 1 5th or 21 st
harmonic.
The need for square wave evaluation becomes apparent
if we realize that some audio amplifiers will be required dur-
ing normal use to pass simultaneously a large number of
different frequencies. With a square wave, we can evaluate
the quality of input and output characteristics of a signal
containing a large number of frequency components such as
complex waveforms of musical instruments or voices.
The square wave output of the signal generator must be
extremely flat. The oscilloscope vertical input should be set
to DC as it will introduce the least distortion, especially at
low frequencies. Because of the harmonic content of the
square wave, 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 wave signal. This is
especially important in an limited bandpass amplifier such as
a voice amplifier.
The square wave signal provides a quick check of
amplifier performance and will give an estimate of overall
amplifier quality. The square wave also will reveal some
dificiencies not readily apparent when using a sine wave
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.
Fig. 24. Lissajous' waveforms used for frequency
measurement
21
WHERE
0 =
PHASE ANGLE
H
: 1
1 : 1
\h
: 1
6 : 1
