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S t e r e o Amplifier S e r v i c i n g :
Another convenient use for dual-trace oscilloscope
is in troubleshooting stereo amplifiers. If identical
amplifiers are used and the output of one is weak,
distorted or otherwise abnormal, the dual-trace os-
cilloscope can be efficiently used to localize the
defective state. With an identical signal applied to
the inputs of both amplifiers, a side-by-side com-
parison of both units can be made by progressively
sampling identical signal points in both amplifiers.
When the defective or malfunctioning stage has
b e e n
l o c a t e d
t h e
troubleshooting and repair methods are employed
can be observed and analyzed immediately.
improving the Ratio of Desired to Undesired
Signals:
In some applications, the desired signal may be
riding on a large undesired signal component such
as 6 0 Hz. It is possible to minimize or for practical
purpose eliminate the undesired component.
1 4 shows the oscilloscope control settings for such
an application. The waveform display of Channel 1
indicates the desired signal and the dotted line in-
dicates the average amplitude variation correspon-
ding to an undesired 6 0 Hz component.
Channel 2 display indicates a waveform of equal
amplitude and identical phase to the average of the
Channel 1 waveform. With the M O D E switch set
to A D D , and the CH2 signal inverted, and by ad-
justing the C H 2 vertical attenuator control, the 6 0
Hz component of the Channel 1 signal can be
cancelled by the Channel 2 input and the desired
waveform can be observed.
S I G N A L
S T A R T W I T H
6 0 Hz
C H A N G E T O S U B
Fig. 14
Improving desired-to-undesired signal radio
e f f e c t s
of
w h a t e v e r
Fig.
The
A D J U S T F O R O N E
C O M P L E T E
C Y C L E
A T 6 0 Hz
" S L O P E +
D U A L
Amplifier Phase Shift Measurements:
Phase measurements can be made by several
methods using oscilloscope. Typical applications
are in circuits designed to procedure a specific
shift, and measurement of phase shift distortion in
audio amplifiers and networks.
In all amplifiers, a phase shift is always associated
with a change in amplitude response. For example,
at the — 3 dB response point, a phase shift of 4 5 °
occurs. Fig. 15 illustrates a method of determining
amplifier phase shift directly.
measurements are being made at approximately
5 0 0 0 Hz. The input signal to the audio amplifier is
used a s a reference and is applied to the Channel 1
input jack.
The V A R I A B L E control is adjusted as required to
provide a complete cycle of the input waveform
displayed on 8 div horizontally. A waveform height
of 2 div is used. The 8 div display represents 3 6 0 °
at the displayed frequency and each centimeter
represents 4 5 ° of the waveform.
The signal developed across the output of the audio
amplifier is applied to the C H 2 input jack.
The vertical attenuator controls of Channel 2 are
adjusted as required to produce a peak-to-peak
waveform of 2 div as shown in Fig. 15B. The
Channel 2 POSITION control is then adjusted so
that the Channel 2 waveform is displayed on the
same horizontal axis as the Channel 1 waveform as
A D J U S T A S R E Q U I R E D F O R
C O M P L E T E C Y C L E IN 8div
AF
INPUT
SIGNAL
GENERATOR
Fig. 15 Measurering amplifier phase shift
In this case, the
OUTPUT
AUDIO
• LOAD
A M P L I F I E R
15
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