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In a typicaf wide band amplifier, a square wave
check reveals many distortion characteristics of the
circuit. The response of an amplifier is indicated in
Fig. 20. revealing poor low-frequency response
along with the overcompensated high-frequency
boost. A 1 0 0 Hz square w a v e applied to the input
of this amplifier will appear as in Fig. 31 A. This
figure
indicates satisfactory medium
response (approximately 1 kHz to 2 kHz) but s h o w s
poor low frequency response. Next, a 1 kHz square
wave applied to the input of the amplifier will
appear a s in Fig. 31 B. This figure displays good
frequency response in the region of 1 0 0 0 to 4 0 0 0
Hz but clearly reveals the over compensation at the
higher 10 kHz region by the sharp rise at the top of
the leading edge of the square wave.
A s a rule of thumb, it can be safely said that a
square w a v e can be used to reveal response and
phase relationships up to the 15th or 20th odd har-
monic or up to approximately 4 0 times the fun-
damental of the square wave. Using this rule of
thumb, it is seen that wideband circuitry will re-
quire at least two frequency check points to proper-
ly analyze the complete spectrum.
In the case illustrated by Fig. 3 0 , a 1 0 0 Hz square
wave will encompass components up to about 4
kHz. To analyze above 4 kHz and beyond 1 0 , 0 0 0
Hz, a 1 kHz square w a v e should be used.
Now, the region between 1 0 0 Hz and 4 0 0 0 Hz in
Fig. 3 0 shows a rise from poor low-frequency ( 1 0 0
Hz to 1 kHz) response to a flattening out from
beyond 1 0 0 0 and 4 0 0 0 Hz. Therefore, w e can ex-
pect that the higher frequency components in the
100 Hz square wave will be relatively normal in
amplitude and phase but that the lower-frequency
components in this same square wave will be
strongly by the poor low-frequency response of this
amplifier (see Fig. 31).
If the combination of elements in this amplifier
were such a s to only depress the low frequency
components in the square wave, a curve similar to
Fig. 3 2 would be obtained. However, reduction in
amplitude of the components is usually caused by a
reactive element, causing, in turn, a phase shift of
the components, producting
shown in Fig. 31 A .
Fig. 3 3 reveals a graphical development of a
similarly tilted square wave. The tilt is seen to be
caused by the strong influence of the phase-shifted
3rd harmonic. It also becomes evident that very
slight shifts in phase are quickly shown up by tilt in
the square wave.
Fig. 3 4 indicates the tilt in square w a v e produced
by a 10° phase shift of a low-frequency element in
frequency
the strong tilt a s
Fig. 3 0 Response curve of amplifier with poor
low and high ends
1 0 0 H z
S Q U A R E
W A V E
Fig. 31 Resultant 100 Hz and 1 kHz square
waves from amplifier in Fig. 3 0
Fig. 3 2 Reduction of square wave fundamental
frequency component in a tuned circuit
1 k H z
S Q U A R E
W A V E
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