Model 8340A - Service
(Block
K)
continues to sweep up to +lOV. This pause in the ramp
will occur once at each bandcrossing. CR3 keeps U35B from
sourcing current through CR2 and into C30.
Because any stray current flowing into C30 will cause an error, a
guard trace surrounds the entire node comprised of C30, pin 3 of
U9, the cathode of CR2, and the source of Q6. To keep the voltage
on the guard trace always at the same potential as the node that
it protects, the voltage at the inverting input of U9 is buffered
by UlOB and applied to the guard trace.
R4 is the SWEEP TIME adjustment. It varies the gain from C30 to
the output of U9. This varies the slope of Marker Ramp. Since
Marker Ramp is always at +10 volts at the end of a sweep, varying
the slope of the ramp will vary the time that it takes to
complete the sweep (i.e.,· it will vary the sweep time).
summing Amplifier
L
During single band sweeps the output of the summing amplifier is
a 0 to -lOV ramp. During multi-band sweeps, however, Marker Ramp
will pause at each bandcrossing. At this time VSWP resets to zero
for the next phase-lock point. This resetting is accomplished by
summing the output of the Reset DAC (Block
C),
the Ramp Generator
(Block
k)
, and the Reset Amplifier 2 (Block
T)
via the summing
Amplifier (Block
L)
. At any particular lock point the instrument
processor knows what value the Marker Ramp voltage should be. The
processor then programs the Reset DAC (Block
C)
to sink a current
that is subtracted from the current generated by Marker Ramp, RS,
and R9. This drives the output of U6 toward zero. Since the Reset
DAC has finite resolution, it is also necessary to provide analog
circuitry to bring the output of U6 to exactly O volts. Reset
Amplifier 2 (Block
T)
generates an error voltage from the output
of the summing Amplifier (Block
L)
which, when applied to the
summing Amplifier, will force that output to zero. Rl3, SWP GAIN,
varies the gain of U6 slightly to adjust for any gain error in
the path from MKR RMP to VSWP.
Refer to Figure SD-17, MKR RMP, RST RMP, and VSWP Waveforms. The
output of the summing Amplifier (Block
L)
is a series of voltage
ramps, each ramp corresponding to one of the bands being swept.
During single band sweeps the output of the summing amp will be a
single 0 to -lOV ramp. During multi-band sweeps the output
duplicates the slope and delta-voltage of Marker Ramp, but is
inverted and resets to OV at each bandcrossing. When the first
bandcrossing occurs, MKR RMP signal pauses and the output of the
summing amplifier increases to OV. When the MKR RMP signal
resumes, the summing amp output matches the slope and
delta-amplitude of Marker Ramp from this bandcrossing point to
the next. This process repeats until the end of sweep. If the
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