HP 8340B Manual page 174

The other outputs
of the A49 phase detector are:
•
FM Coil Drive
—
The high
frequency
(>100 Hz) phase/frequency error voltage is
output to the
YO
FM
coil.
•
High Unlock YO
—
Outputs
a
TTL high when the YO loop
is
unlocked. The status of the
YO
loop is
monitored by the instrument microprocessor.
• Low Overmod
—
Outputs
a
TTL
low when the phase difference between the sampler
IF and
the
20-30 synthesizer is greater than approximately 300°.
Frequency Modulation
A23 FM Driver. This assembly receives
the
±
1V or
±
8V frequency modulation
input from
J22
on the
front panel. Frequency modulation
voltage information
is
converted to
a
corresponding current that
goes directly to the A44 FM
coil via
the A62 motherboard and A44A1 YO
bias
assembly.
The FM
driver outputs
a
low signal
(LOMD)
if the the peak deviation of the FM input signal
exceeds
10
MHz.
SWEEP GENERATOR
-
YO LOOP FUNCTIONAL GROUP THEORY OF
OPERATION
This functional group can best be described by
considering the two main instrument modes
of
opera¬
tion: CW/manual,
and swept frequency. The
swept mode can be
in one of three conditions: single
band sweep, multiband sweep, or narrow band sweep.
CW/MANUAL Sweep Operation
When either mode is selected, the
sweep generator/YO loop circuitry phase-locks
the
YO
at the
appropriate frequency.
The
instrument microprocessor addresses and sends
a
number to the A54 YO
pretune/delay compensation assembly, which produces
a proportional voltage,
VDAC. VDAC
is sum¬
med with the
constant DC voltage, VREF, producing PRETUNE
-
a
voltage that tunes the
YO
to
approximately the desired frequency
(i.e.
to
within the capture
range of the YO phase-locked
loop).
VSWP
makes
no contribution to the
PRETUNE
voltage
in
CW operation.
The processor also sets the frequencies of the
M/N
and
20-30 synthesizer loops according to the
CW
frequency selected. The outputs of these two loops are used by
the
YO
loop to exactly
tune
and
phase-lock the YO.
The A45
directional coupler couples
a
portion
of the
YO RF output back
through
a
low-pass
filter to the A48
sampler, where
it is mixed with the Nth harmonic of the
M/N
output. This
generates
a
difference signal.
Sampler
IF,
in the range of 20-30 MHz that is
input to the A49
YO
loop
phase detector. There the phase is
compared to
the
20-30 MHz
output
signal.
The resultant
error
signal, YO
TUNE, phase-locks the YO. The
high
frequency
portion on
YO TUNE (>100
Hz) is applied
to the
YO FM
coil
by the FM
coil
driver on the A49 YO loop
phase detector assembly. The
low
frequency portion
(<100
Hz)
is summed
with PRETUNE on
the A55 YO driver and applied to the YO
main
coil.
This completes the YO
loop and phase-locks the YO.
Manual Sweep operation is identical
to CW
in
the operation of the
sweep generator
and YO loop
circuitry.
D-4
Sweep Generator
-
YO
Loop
Theory of Operation
HP 8340B/41B