Model 8340A - Service
C26, R43 and R44 are loop compensation components.
Reset Control Logic
s
At any phase-lock event LOW ENABLE RESET AMP 2 (Block
T)
goes
LOW. Then the instrument processor writes to the Reset Register
(Block
B).
This LOW-going strobe, WRDAC, comes to the Reset
Control Logic (Block S) and forces its output to go LOW. This
puts Ul3 in Reset Amplifier 2 (Block
T)
in the sample mode and
causes the output of U6 in summing Amplifier (Block
L)
to be
driven to zero. When a sweep is initiated, LOW ENABLE RESET AMP 2
(Block
T)
goes HIGH. This drives the output of U32B HIGH which
puts Ul3 into the hold mode, thereby opening the loop and
allowing the output of U6 to start sweeping.
Reset Amplifier 2
T
Since the inverting input of U7 is connected to ground, the reset
loop will force the non-inverting input to be at ground also. The
error voltage generated by U7 is fed to Ul3, which is a sample
and hold. C31 is the sample and hold capacitor. R83 limits the
charging rate of C31 and is required to make Ul3 stable.
When pin 14 of Ul3 is LOW, Ul3 is in the sample mode. This closes
the loop and forces the output of U6 {pin 6) in the Summing
Amplifier (Block
L)
to be at ground. When pin 14 of Ul3 goes
HIGH, Ul3 goes into the hold mode and the loop is opened. As the
sweep progresses and the output of U6 ramps up, the output of Ul3
will not change.
There is a guard trace around the node containing the sample and
hold capacitor and Ul3 pin 11. This trace is connected to the
output of Ul3 to keep it at the same potential as the sensitive
end of C31.
Oversweep Detector
U
When the instrument is operating correctly, the A57
Marker/Bandcross board will stop the sweep when the Marker Ramp
gets to +10 volts. However, if that board is not working and the
Marker Ramp reaches +12 volts, the OVERSWEEP DETECTOR pulls down
the LBX line which stops the sweep and wakes up the processor.
This allows the SWEEP GENERATOR to make repetitive sweeps even
when the MARKER/BANDCROSS board is not functioning. This makes
troubleshooting much easier. It should be noted however, that in
the case when the OVERSWEEP DETECTOR is stopping the sweep, no
bandcrossings will occur, and the frequency over which the
instrument is sweeping will be incorrect.
8-372
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