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Adjustments
Model 8902A
5.
Connect both channels of an oscilloscope through two 1 0 1 divider probes
to A54TP5.
See
Figure
5-12. Set the coupling of both channels to ac. Set both channels to 10 mV per vertical division
(combined with the probes the total vertical sensitivity
will
be 100 mV per division). Set the
triggering to channel A. Set the sweep time to 20 ns per division (use a magnifier if needed).
6.
On the Measuring Receiver, key in 48.7 SPCL.
7. On the oscilloscope, adjust the horizontal position to locate the zero crossing of channel A. Set
the input switch for channel A to ground. Adjust the vertical position of channel A
to center the
trace of channel A. Set the input switch for channel A to ac.
8.
Set the input switch for channel B to ground. Adjust the vertical position of channel B to center
the trace of channel B. Set the input switch for channel B to ac.
9. Note the time difference of the zero-crossing of the t w o traces. This difference represents the
phase error of the two channels. (By momentarily grounding one of the input switches, note
which channel leads.)
10. Move the probe of channel B to A54TP4. Adjust the vertical position of channel B to bring the
trace of channel
B on the display.
11. Adjust A54L4 to center the peak of channel B directly over the zero crossing of channel
A plus
the error noted in step 9. The centering must be within f0.5 horizontal division (that
is,
f 1 0
ns or f1.7O). See Figure 5-12. (Momentarily ground the input switch of channel A
to check the
zero crossing of channel A.)
12. On the Measuring Receiver, key in 48.1 SPCL
to
display the synchronous detector offset voltage.
Adjust A54R76 (SYCH DET
OFS)
for a reading between -0.0010 and 0.0010.
13. If the A54U7 Loop Bandwidth Control DAC has been replaced, perform the following procedure.
a. 'hm the Measuring Receiver o f f . Unplug the A54
IF' Synchronous Detector Assembly.
b. Measure the resistance between pins
1
and 16 of A54U7.
c. Calculate the resistance (in
ohms) of A54R24 by the formula
- 1 ) .
resistance of step b
7736
R24 = 2150
x
(
d. Replace A54R24, if necessary, with a resistor of a value within f10% of the resistance
calculated in step c.
Optional Steps
5a On the Gainphase Meter set channel A and B input range switches to "2-2OV".
Set the Frequency Range switch to "100-1M".
Set the Phase Reference switch to "A".
Set the Amplitude B/Phase switch to "Phase".
6a. Connect channel A, through a
1:l
probe, to A54TP5.
7a. Adjust A54L4 for a 90.0°f0.50phase difference reading. (Continue with step 12.)
Connect Channel B, through a
1:l
probe, to A54TP4.
5-44
rev.
15JUL87
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