
Since
the
c
hosen
V
CO
harmonic
and
the
source
dier
b
y
0.500
or
0.333
MHz,
then
another
V
CO
harmonic,
2
or
3
times
higher
in
frequency
,
will
b e
exactly
1.000
MHza
w
a
y
from
the
2nd
or
3rd
harmonic
of
the
source
frequency
.
The
samplers,
then,
will
also
do
wn-con
v
ert
these
harmonics
to
yield
the
desired
comp onen
ts
in
the
1st
IF
at
1.000
MHz.
Narrow
bandpass
lters
in
the
receiv
er
eliminate
all
but
the
1.000
MHzsignals;
these
pass
through
to
b e
pro cessed
and
display
ed.
Figure
HARBLK12
here.
Figure
12-6.
Harmonic
Analysis
External
Source
Mode
In
external
source
mo
de,
the
analyzer
phase
lo
c
ks
its
receiv
er
to
an
external
signal
source.
This
source
m
ust
b e
CW
(not
sw
ept),
but
it
do es
not
need
to
b e
syn
thesized.
The
user
m
ust
en
ter
the
source
frequency
in
to
the
analyzer.
(The
analyzer's
in
ternal
source
output
is
not
used.)
T
o
accomplish
this,
the
phase
lo
c
k
lo
op
is
reconnected
so
that
the
tuning
v
oltage
from
the
A11
phase
lo
c
k
assem
bly
con
trols
the
V
CO
of
the
A14
fractional-N
assem
bly
and
not
the
A3
source.
See
Figure
12-7.
The
V
CO's
output
still
driv
es
the
1st
LO
of
the
samplers
and
do
wn-con
v
erts
the
RF
signal
supplied
b
y
the
external
source.
The
resulting
1st
IF
is
fed
bac
k
to
the
A11
phase
lo
c
k
assem
bly
,
compared
to
the
1.000
MHzreference,
and
used
to
generate
a
tuning
v
oltage
as
usual.
Ho
w
ev
er,
the
tuning
v
oltage
con
trols
the
V
CO
to
lo
c
k
on
to
the
external
source,
k
eeping
the
1st
IF
at
exactly
1.000
MHz.
The
analyzer
normally
go
es
through
a
pretune-acquire-trac
k
sequence
to
ac
hiev
e
phase
lo
c
k.
In
external
source
mo
de,
the
fractional-N
V
CO
pretunes
as
a
closed-lo
op
syn
thesizer
referenced
to
the
100
kHzsignal
from
the
A12
reference
assem
bly
.
Then,
to
acquire
or
track,
a
switch
causes
the
V
CO
to
b e
tuned
b
y
the
A11
phase
lo
c
k
assem
bly
instead.
(Refer
to
the
Ov
erall
Blo
c
k
Diagram
at
the
end
of
Chapter
4,
\Start
T
roublesho oting
Here.")
12-16
Theory
of
Operation
DRAFT
3/21/106
15:14
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