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Loop board (A9A2) and
the
60.5-MHz Loop board
(A9A4).
The 1-MHz
signal
is further divided to
50
kHz,
and
then is mixed
with
1 MHz
to
provide the
1.05-MHz
reference
for the
24-MHz loop on
A9A4.
The +33V filter,
shown
in Figure 11-3a, provides
isolation
between the
system's
+
33V
line and
the sub-
modules
in
the
RF
Synthesizer
where
+33V
is
used.
Figure ll-3b
shows
the level
translator (U9)
and the
shift register (U10). The level translator converts three
serial-data inputs,
plus the clock
input
and
the
latch
input
(all of
which
come
from
A9,
the
Processor
Inter-
face
board),
as
well
as one of
the
Synthesizer's control
bits (5V,
300
to
350
clamp)
from
a
logic ofO
to
+
5V
to
a logic of
0 to
+ 8V.
These
0 to
+
8V
logic levels
are
required
by
the 310 to
440-MHz Loop board
(A9A2),
the 60.5-MHz
Loop board
(A9A4), and
the GHz Loop
board
(A9A5).
Serial-to-parallel
conversion
of
eight
bits
of
Synthe-
sizer
control
data is
done by
shift register
U10.
The
serial
input
comes from the Processor
Interface board
(All).
The
eight output
bits
are
used
by
the
RF
Syn-
thesizer
to produce
the desired
output
frequency.
11 .2.2 310 TO 440-MHz
LOOP BOARD
(A9A2)
11
.2.2.1 General
The
310 to 440-MHz
Loop board provides RF fre-
quencies
between 310 and
440
MHz
in
125-kHz
steps.
This
output
is
used
as
the translation
frequency in the
GHz
Loop board
(A9A5).
Also,
the
310 to
440-MHz
Loop
board contains cir-
cuitry
which
selects
Synthesizer
modulation and mod-
ulation
sense.
The selected modulation
goes
to
the
60.5-MHz
Loop board
(A9A4).
Frequency programming
and
modulation
selection
are
controlled by
data,
clock,
and
latch lines
which
come from the
Reference
and
Control
board
(A9A1).
A
block diagram
of the 310 to 440-MHz
Loop board
is
sh
own at the end of the section
in Figure
11-6,
a
schematic
in Figure
11 -7, and the printed wiring board
assembly
and
parts
list in
Figure
11-8.
11 .2.2.2 Theory
of
Operation
The 310 to
440-MHz Loop board consists
of two
main
sections:
the 310 to
440-MHz
loop and the cir-
cuitry
for
selecting
modulation. The
310 to
440-MHz
loop
consists
of the
reference
divider and
the
phase
detector
(U1),
the loop
filter (U2)
,
the voltage-con-
trolled
oscillator
(VCO)
(Q1),
the
RF
amplifier
(Q2),
the
two-modulus
divider
(U3
and U4)
,
the
divide-by-
N
and
divide-by-A
counters
and the counter
control
logic
(Ul).
The reference divider
divides
the
1-MHz reference
frequency
by
eight.
The resulting
125-kHz
signal
is
compared by
the
phase
detector
to the
divided-down
VCO
output
coming
from the di
vide-by-N.
The phase
11-2
detector
then generates an output that
locks
the
phase
of the
VCO
output to t he phase of the
125-kHz refer-
ence.
That phase-detector
output
drives the
loop fil
-
ter.
The loop
filter
sets
the
bandwidth
and stability of
the loop and
attenuates
the reference-frequency
com-
ponents
coming
from
the
phase
detector.
The
output
of the
loop
filter tunes the
VCO
frequency to
the
required
value
to
maintain phase-Jock. Following the
VCO
is
an
RF
amplifier which
provides
a
nominal
power
of
+ 7
dBm
to the
RF
output.
A
small portion
of
this
output
is
used
to
drive
the two-modulus
divider.
When the
modulus control line is low,
the modulus is
41
;
when
high,
the
modulus
is
40. The
output of the
two-modulus
divider
feeds
the
divide-by-A and divide-
by-
N
counters.
The
8V,
310 to 440
DATA line
pro-
grams the values of N and
A, depending upon
the
desired
output frequency
-
f:1w 1u~4o
=125
kHz
(N 40+A)
.
The counter control
logic
coordinates
the
operation of
the
divide-by-A
counter, the divide-by-N
counter,
and
the
two-modulus divider
to achieve the
proper
divi-
sion
of the
VCO
frequency.
11
.2.3
OUTPUT
BOARD
(A9A3)
11 .2
.3.1
General
The
Output
board
provides
a combination of
switching, filtering and
mixing
to generate
the
Syn-
thesizer's
0.01 to 1000-MHz
output.
This
board
also
controls
the
output level for frequencies below
1 MHz.
A
block
diagram
of
the Output
board
is shown at
the
end of
the
section in
Figure 11-9,
a schematic
in Figure
11-10,
and the
printed
wiring
board
assembly
and
parts
list
in Figure 11 -11.
11 .2.3.2 Theory of Operation
T he
Output
board
provides its
10-kHz to 1-GHz
frequency range
in four
bands:
Band
A-10
kHz
to
249 .9999
MHz;
Band
B-
250 to 349.9999
MHz;
Band
C-350
to
499.9999
MHz;
and
Band D-500 to
1000
MHz. Comparators
U6 and U7 control pin diodes,
which, in
turn,
control band
switching.
11.2.3.2.1 Band A
The
10-kHz to 249.9999-MHz
band
is
provided
by
mixing
the 640-MHz Loop board's output with the
GHz Loop
board's
output
of 500 to 1000
MHz
(fc:Hz>·
For a
particular
output
frequency,
f.,
the
GHz
loop
is
programmed to
fc
H
z =
f.,
+
640 MHz. The
resulting
difference
frequency
at the output of
mixer
(U2) is
then
equal
to
f.,.
Pin
diode CR4
switches
the 500 to
1000-MHz input
to
the input of amplifier
U5. The output
of
U5 drives
the
L.O.
port
of mixer
U2. The 640-MHz input drives
the RF
port
of
mixer
U2. By controlling
the
current
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