Service manual

CIRCUIT DESCRIPTION
6-5
Month 2000
Part No. 001-2004-601
6.1.8 BUFFER
A cascode amplifier formed by Q410/Q411 pro-
vides amplification and isolation between the VCO
and Synthesizer. A cascode amplifier is used because
it provides high gain, high isolation and consumes
only a small amount of power. The input signal to this
amplifier is coupled from the VCO RF output on
pin 5. DC blocking and coupling to the VCO is pro-
vided by C455 and to the buffer by C456.
Bias for the amplifier is provided by R442, R445,
R446 and R277. Q411 is a common-emitter amplifier
and Q410 is a common-base with C458 and C457 pro
-
viding RF bypass. L405 provides some filtering of the
cascode output. R448 lowers the Q of L405. The out
-
put of the amplifier is coupled by C442/C441 to U401,
pin 11.
6.1.9 SYNTHESIZER
The inputs/outputs of synthesizer U401 are
shown in Figure
6-4. The output signal from the syn-
thesizer loop is the receiver first injection frequency.
This signal is produced by a VCO (voltage-controller
oscillator). The frequency of this oscillator is con
-
trolled by a DC voltage. This DC voltage is generated
by integrating the pulses from the phase detector in
synthesizer chip U401.
Frequencies are selected by programming
counters in U401 to divide by a certain number. This
programming is provided through J201, pins 12, 18
and 20. The frequency stability of the synthesizer is
established by the ±1.0 PPM stability of TCXO Y401.
The output of this oscillator is stable from
-30°C to +60°C (-22°F to +140°F).
Figure 6-4 SYNTHESIZER BLOCK DIAGRAM
(OPEN-DRAIN OUTPUT)
15
OUTPUT B
DATA OUT
24 BITS
A REGISTER
INPUT AMP
PRESCALER
64/65
in
f
in
f
10
11
LOGIC
MODULUS
CONTROL
N COUNTER
12-STAGE
A COUNTER
6-STAGE
CONTROL
INTERNAL
POR
LOGIC
STANDBY
8 BITS
C REGISTER
ENABLE
17
19
DATA IN
CLOCK
18
LOGIC
CONTROL
AND
REGISTER
SHIFT
16 BITS
R REGISTER
DOUBLE-BUFFERED
13-STAGE R COUNTER
DIVIDER
4-STAGE
OSC OR
out
REF
20
in
REF
1
OR (UP)
OV (DOWN)
16
OUTPUT A
LOGIC
SELECT
PORT
V
f
R
f
V
R
2LOCK DETECT
AND CONTROL
LD
f
f
V
R
f
f
PHASE/FREQUENCY
DETECTOR B
AND CONTROL
PHASE/FREQUENCY
AND CONTROL
DETECTOR A
V
R
f
f
3
4
8
6
Rx
PDout
TEST 2
TEST 1
13
9