HP 8341b - 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