A P P L I C A T I O N S
Analysing the Waveforms:
The short rise time w h i c h occurs at the beginning of the
half-cycle is created by the in-phase sum of the medium
and high frequency sine w a v e components. T h e same
holds true for the drop time. T h e reduction in high frequen-
cy components should produce a rounding of the square
corners at all four points of one square w a v e cycle (see Fig.
1 2 ) .
Distortion can be classified into the following three
categories:
1 . T h e first is frequency distortion and refers to the change
in the amplitude of a complex w a v e f o r m . In other
w o r d s , the introduction in an amplifier circuit of reso-
nant networks or selective filters created by combina-
tion of reactive components will create peaks or dips in
an otherwose flat frequency response curve.
2. T h e second is non-linear distortion and refers to a
change in w a v e s h a p e produced by application of the
waveshape to non-linear elements such as v a c u u m
tubes, an iron core transformer or a clipper network.
3 . T h e third is delay or phase distortion, which is distortion
produced by a shift in phase between some components
of a complex w a v e f o r m .
In actual practice, a change in amplitude of a square
w a v e component is usually caused by a frequency
selective network w h i c h includes capacity, inductance
or both. T h e presence of the C or L introduces a dif-
ference in phase angle between components, creating
phase distortion or delay distortion. Therefore, in square
w a v e testing of practical circuitry, w e will usually find
that the distorted square w a v e includes a combination
of amplitude and phase distortions.
In a typical wide band amplifier, a square w a v e check
reveals many distortion characteristics of the circuit.
The response of an amplifier is indicated in Fig. 1 3 ,
revealing poor low-frequency response along with the
overcompensated high-frequency boost. T h e response
of 1 0 0 Hz square w a v e applied to the amplifier will ap-
pear as in Fig. 1 4 A . T h e figure indicates satisfactory
medium frequency response (approximately 1 kHz to
2 kHz) but s h o w s poor low frequency response. Next, a
1 kHz square w a v e applied to the input of the amplifier
will appear a s in Fig. 1 4 B . This figure displays good fre-
quency response in the region of 1 0 0 0 to 4 0 0 0 Hz but
reveals a sharp rise at the top of the leading edge of the
square w a v e because of overcompensation at the fre-
quencies of more than 1 0 kHz.
A s a rule of thumb, it c a n be safely said that a square w a v e
can be used to reveal response and phase relationships up
to the 1 5th or 2 0 t h odd harmonic or up to approximately
4 0 times the fundamental of the square w a v e . It is seen
that wide-band circuitry will require at least t w o frequency
check points to properly analyze the entire bandpass.
In the case illustrated by Fig. 1 3 , a 1 0 0 Hz square w a v e
will encompass components up to about 4 kHz. T o analyze
above 4 kHz and beyond 1 0 , 0 0 0 Hz, a 1 kHz square w a v e
should be used.
F i g . 1 2 .
S q u a r e w a v e r e s p o n s e w i t h h i g h f r e q u e n c y
l o s s
F i g . 1 3 .
R e s p o n s e c u r v e of a m p l i f i e r w i t h p o o r l o w
a n d h i g h e n d s
100 HZ
S Q U A R E
W A V E
A
1 KHZ
S Q U A R E
W A V E
B
F i g . 1 4 . R e s u l t a n t
1 0 0
H z a n d
1
k H z s q u a r e w a v e s
f r o m a m p l i f i e r in F i g . 1 3 .
1 6
RESPONS
E
