HP 8340B Manual page 439

A23 FM Driver Theory of Operation
The front panel FM
input
can accept an
AC signal that
is
sent to the A23 FM driver. This signal
can be
of two different ranges:
±8
Vac in
1 MHz/V
FM
sensitivity
or
±
1
Vac
in
10
MHz/V FM
sensitivity.
The
FM
driver scales this signal and
converts
it
into
a
current that drives the YIG oscillator's FM
coil.
The FM
Driver provides:
•
FM
input range attenuation
•
Harmonic number (band number) attenuation
•
FM overmodulation detection
•
FM
coil
drive.
A44/A44A1 YIG Oscillator/Bias Assembly Theory of Operation
NOTE: The A44
microcircuit and A44A1 bias assembly are not field repairable and are not separately
replaceable.
The FM
coil in the instrument
is
connected to chassis ground at
the bias
assembly.
During testing,
however, the
coil
must be floating so
a
jumper has been provided to connect ground after testing.
YO Switchable Filtering. The
filtering required at the
YO main
coil
depends upon whether the
instru¬
ment is in
the CW mode or
in
swept modes.
A
switch circuit is provided to accomplish
this filter
change.
A signal from the
processor (HFIL) goes high
in
CW mode. This
signal
is
buffered
on the
A50
YO loop interconnect assembly. The resulting
signal (LFIL) goes to the A44A1 YO bias assembly
where
it
provides
a very
low impedance
path for the CW
filter circuit.
The CW
filter is
connected
directly
across the YO
main coil in
CW
mode. The filter reduces residual
FM of the YO. A
swept mode
filter
matches the delay characteristics of the YO main
coil
to those of the YO main
coil driver circuitry.
NOTE
Refer to
Sweep Generator
—
YO Loop Troubleshooting Block
Diagram for schematic of A44
Figure
D-l.
A44A1
YIG
Oscillator Bias Assembly Drawing
HP 8340B/41B
Sweep Generator
-
YO
Loop
Theory of Operation D-9