Table of Contents
Advertisement
through
pin
diodes CR17 and CR18,
voltage-to-cur-
rent converter
Ql controls
the
level of the
640-MHz
drive
(and
thus
the
level
of
the
10-kHz to
249.9999-
MHz
output).
The input of Ql
is
driven by the 0.01 to
1
AGC
signal
from the
Wide band Amplifier
(A17
A2)
.
In
addition
to the
desired
difference frequency,
other
sianals
are
contained in the
output of
mixer
U2. These
si;nals
include
640-MHz
and
fc:Hz
feedthrough, the
s
um
frequency,
and
higher-order
mixer
products.
Microstrip
traps,
which are tuned
by
C4 and
C5,
attenuate
640-MHz
and
fcHz
feedthrough
.
A
low-pass
filter (Ll
, L2,
C7
and C8)
attenuates
the sum
fre-
quency
and
higher-order mixer
products, as
well
as
640-MHz and fc:H
1
•
feedth
rough.
R49
and C6
improve
gain-flatness-with-frequency
by
providing
fr~quency
compensation.
After filtering and compensatiOn, the
10-kHz
to
249.9999-MHz signal
is
amplified
by
U3,
switched
through
relay
Kl,
and amplified
by
U4.
11
.2.3.2.2
Bands B and C
The 50 to
349.9999-MHz and 350
to 499.999-MHz
bands
are
provided
by
filtering
harmonics
from
the
GHz
Loop
board's
250 to
500-MHz
output.
For
oper-
ation
from
250
to
349.9999
MHz,
pin
diodes
CR12 and
CR13
steer
the
250
to
500-MHz
input
through a
350-
MHz
low-pass
filter
(C38,
C39,
C40,
Lll
and
Ll2).
For
operation
from 350
to
499.9999
MHz,
pin
diodes
CRlO
and
CR11 steer the 250
to
500-MHz input
through a
500-MHz
\ow-pass
filter
(C34,
C35, C36,
L8,
and
L9)
.
For
either
band,
pin diode
CR15 steers
the
low-pass
fi
lter's
output to amplifier U4.
For operation in
other
bands. pin
diode
CR14
isolates
the
250
to
500-MHz
input
from
the SYNTH
RF
output.
11.2.3.2.3
Band 0
The
500 to
1000-MHz
band
is
provided
by
fre-
quency-compensating
the GHz
Loop
board's
500
to
1000-MHz
output.
Pin
diodes
CR5,
CR7,
and CR9
steer
that
input
through
frequency-compensation
cir-
cuits
R31,
C54,
R33,
and
C30 and then to amplifier
U4.
For operation in
other
bands,
pin
diodes
CR6
and CR8
isolate the
500
to 1000-MHz
input
from the SYNTH
RF
output.
11
.2.4
60.5-MHz
LOOP BOARD (A9A4)
11
.2.4.1
General
The
60.5-MHz
Loop
board
provides
an
output
fre-
quency
(variable
in
50-Hz
steps
over
a range of 60.5
±
0.25
MHz)
to
the GHz Loop
board.
In
addition,
the
60.5
MHz
Loop
board
provides the
modulation
and
sweep
capability
for the
Synthesizer.
.
A block diagram
of
the
60.5-MHz Loop
board
IS
shown
at
the
end
of the section
in Figure
11
-12,
a
sche-
matic
in
Figure
11
-13,
and
the printed
wiring
board
assembly and
parts
list
in
Figure 11
-14.
11 .2.4.2 Theory
of
Operation
The
60.5-MHz Loop board
consists of
three
sections:
1.
A translation
loop,
comprising
a phase-locked-loop
(PLL)
IC
(U9),
a loop filter
(U1,
U10,
Ull),
a
VS:O
(Q3,
Q4),
a buffer amplifier
(Q5,
Q6)
,
a
translatiOn
mixer
(U14)
,
a
filter
amplifier
(Q?), and
a
two-
modulus
pre-scaler (Q8, U12,
Ul3).
2. A
modulation control circuit
(U3).
3.
An
offset
loop,
consisting of a
PLL IC
(\.!6),
a loop
filter (U?),
a
VCO
(Q1)
,
a
buffer
amphfier (Q2),
and a
pre-scaler
(U8).
The
PLL
ICs
(U6. U9)
provide digital
dividers, con-
trol functions
and the phase
detector
on one
IC.
The
refe
rence
inp~t
is
divided
by
1000
and applied to .the
on-chip
phase
detector,
where
it
is
compared agamst
the
reference
divider's
output.
The phase
detector's
output
in
each.
loop
is
appli:d
to t he
loop
filter. which
sets
the
bandw1dth
and
stabil-
ity
of the
loop
and attenuates
reference-frequency
components
coming
from the
phase
detector.
Modulation and
spurious requirements
cause the
translation
loop filter
to be considerably
more
com-
plex than the
offset
loop
filter.
II_J addition
to
t~e
stan-
dard
filter
amplifier,
the
translatiOn loop filter
mcludes
bandwidth switching
and
an
elliptic
low-pass filter.
Bandwidth
switching
prevents
the
translation
loop
from tracking-out sweep
modulation.
In
each
loop,
the loop
filter's
output
tunes the VCO
output,
putting the
phase
detector's inputs
i~
phase.
The proper
frequency
for
the offset loop
IS
deter-
mined
by the values
programmed into
the
N
and
A
registers
of
U6
-
foFF
=
1.05
kHz (Norr 64
+
Aon·).
The
output frequency
of
the
translation loop
is deter-
mined
by
the programmed
values
of
N
and A
in the
registers
of
U9
and the
frequency
of
the
offset
loop
-
fT
=
1kHz
(N-rl
28
+
AT)
-
fon
··
Mixer
U14
sums
the output frequencies of
both
loops.
The
output
of
Ul4
is
filtered
by
the
filter amplifier (Q?
)
and is
then applied
to
Q8,
U12 and
Ul3,
which
form a
divide-by-28/129,
two-modulus
divider.
For modulation
control,
the
switch
outputs
of U6
and U9
control
an
analog
multiplexer
(U3).
R13
con-
trols
the
modulation
sensitivity.
To adjust R1
3,
refer
to
Section
3
on
a
lignment.
11
-3
11
.2.5
GHz
LOOP BOARD (A9A5)
11 .2.5.1
General
T he
GHz
Loop
board provides
RF
outputs
of 500 to
1000-MHz
and
250
to
500
MHz.
It
obtains t hese
out-
puts by using
the
output of
the
60.5-MHz
~oop
board
(A9A4)
as
a
reference frequency
and
by
usmg
the
out-
put of the
310
to
440-MHz Loop
board
(A9A2) as a
translation
frequency.
The
reference,
f
60 ,
can
be
pro-
grammed in
the
range
of
60.5
± 0.125 MHz
in
50-Hz
Advertisement
Table of Contents
Troubleshooting
