corresponding to the undesired 50 Hz component.
The C H 2 display indicates a waveform of equal
amplitude and identical phase to the average of the
CH1 waveform. With the M O D E switch set to
A D D and the CH1 signal inverted, and by adjusting
the C H 2 vertical attenuator control, and 5 0 Hz
component of the C H 1 signal can be cancelled by
the C H 2 input and the desired waveform can be
observed.
Stereo Amplifier Servicing:
Another convenient use for a dual-trace oscil-
loscope is in troubleshooting stereo amplifiers. If
identical amplifiers are used and the output of one
is weak, distorted or otherwise abnormal, the
dual-trace oscilloscope can be efficiently used to
localize the defective state. With an identical sig-
nal applied to the inputs of both amplifiers, a
side-by-side comparison of both units can be made
by progressively sampling identical signal points in
both amplifiers. When the defective or malfunctio-
ning stage has been located, the effect of whatever
troubleshooting and repair methods are employed
can be observed can be observed and analyzed im-
mediately.
Amplifier Phase Shift Measurements:
Phase measurements can be made by several
method using oscilloscopes. Typical applications
are in circuits designed to produce a specific phase
shift, and measurement of phase shift distortion in
audio amplifiers and network. In all amplifiers, a
phase shift is always associated with a change in
amplitude response. For example, at the
—
3 dB
response points, a phase shift of 4 5 ° occurs.
Phase m e a s u r e m e n t s can be performed by
operating the oscilloscope either in the dual-trace
mode to measure amplifier phase shift directly.
Fig. 15
illustrates this method. In this case, the
measurements are being made at approximately
5 0 0 0 Hz. The input signal to the audio amplifier is
used as a reference and is applied to the C H 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 division
represents 4 5 ° of the waveform.
The signal developed accross the output of the
audio amplifier is applied to the C H 2 input jack.
The vertical attenuator controls of C H 2 are
adjusted as required to produce a peak-to-peak
waveform of 2 div amplitude as shown in
Fig.
15B.
The C H 2 POSITION control is then adjusted
so that the C H 2 waveform is displayed on the same
horizontal axis as the C H 1 waveform as shown in
Fig. 15B.
The distance between corresponding
points on the horizontal axis for the two waveforms
then represents the phase shift between the two
waveforms. In this case, the zero crossover points
of the two waveforms are compared. It is shown
that a difference of 1 div exists. This is then inter-
preted as means a phase shift of 4 5 ° .
Television Servicing:
Many of the television servicing procedures can be
performed using single-trace operation. These are
outlined later in the applications section covering
single-trace operations. One of these procedures,
viewing the V I T S (vertical interval test signal), can
be accomplished much more effectively using a
dual-trace oscilloscope. A s outlined in the
single-trace applications section and as shown in
Fig. 16.
the information on the Field 1 and Field 2
vertical blanking interval pulse is different. This is
shown in detail in
Fig. 16.
Also, because the oscil-
loscope sweep is synchronized to the vertical
blanking interval waveform, the Field 1 and Field 2
waveforms are superimposed onto each other.
With dual-trace operation, the signal information
on each blanking pulse can be viewed separately
without overlapping.
Fig. 18
indicates the oscil-
16
A D J U S T A S R E Q U I R E D
FOR C O M P L E T E C Y C L E
A F
S I G N A L
G E N E R A T O R
A U D I O
A M P L I F I E R
O U T P U T
L O A D
I N P U T
Fig. 15 Measurering amplifier phase shift
