HP 8340B Manual page 179

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.
Figure
D-l.
A44A1 YIG
Oscillator Bias Assembly Drawing
HP 8340B/41B
Change 10
Sweep Generator
-
YO
Loop Theory of
Operation
D-9