
The
2nd
LO
Signal
The
2nd
LO
is
obtained
from
the
A12
reference
assem
bly
.
In
high
band,
the
2nd
LO
is
xed
at
996
kHz.
This
is
pro duced
b
y
feeding
the
39.84
MHz
output
of
a
phase-lo c
k
ed
oscillator
in
the
A12
assem
bly
through
a
divide-b
y-40
circuit.
In
low
band,
the
2nd
LO
is
a
v
ariable
frequency
pro duced
b
y
mixing
the
output
of
the
fractional-N
V
CO
with
a
xed
39.996
MHz
signal
in
the
A12
assem
bly
.
The
2nd
LO
co
v
ers
the
range
of
0.304
to
16.004
MHz
(0.014
to
16.004
MHzif
Option
011
is
com
bined
with
Option
006)
in
t
w
o
subsw
eeps
that
corresp ond
with
the
source
subsw
eeps.
These
subsw
eeps
are
0.304
to
3.304
MHz
and
3.304
to
16.004
MHz.
The
Mixer
Circuit
The
1st
IF
and
the
2nd
LO
are
com
bined
in
the
mixer
circuit.
The
resulting
dierence
frequency
(the
2nd
IF)
is
a
constan
t
4
kHz
in
b oth
bands,
as
T
able
12-4
sho
ws.
T
able
12-4.
Mixer
Frequencies
Band
1st
IF
2nd
LO
2nd
IF
Sup er
Lo
w*
0.010
to
0.300
MHz
0.014
to
0.304
MHz
4.0
kHz
Lo
w
0.300
to
16.0
MHz
0.304
to
16.004
MHz
4.0
kHz
High
1.000
MHz
0.996
MHz
4.0
kHz
*
This
band
is
present
on
the
HP
8753D
Option
011
only
when
it
is
com
bined
with
the
Option
006.
A10
Digital
IF
The
three
4
kHz
2nd
IF
signals
from
the
sampler/mixer
assem
blies
are
input
to
the
A10
digital
IF
assem
bly
.
These
signals
are
sampled
at
a
16
kHz
rate.
A
fourth
input
is
the
analog
bus,
whic
h
can
monitor
either
an
external
input
at
the
rear
panel
A
UX
IN
connector,
or
one
of
31
in
ternal
no des.
A
m
ultiplexer
sequen
tially
directs
eac
h
of
the
signals
to
the
ADC
(analog-to-digital
con
v
erter).
Here
they
are
con
v
erted
to
digital
form
and
sen
t
to
the
A9
CPU
assem
bly
for
pro cessing.
Refer
to
\Digital
Con
trol
Theory"
for
more
information
on
signal
pro cessing.
DRAFT
3/21/106
15:14
Theory
of
Operation
12-21
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