User Manual

CIRCUIT DESCRIPTION
4-3
June 1997
Part No. 001-3412-002
Figure 4-2 U801 SYNTHESIZER BLOCK DIAGRAM
CLOCK
DATA
STROBE
Vss
RF
IN
RF
IN
64/65/72
PRESCALER
MAIN DIVIDERS
EM
FB
2 2 12 8
PR
NM1
NM3
NM2
FRACTIONAL
ACCUMULATOR
PRESCALER
MODULUS
CONTROL
3
FMOD
NF
FB
SERIAL INPUT + PROGRAM LATCHES V
DD
TEST
IN
REF
EM+EA
REFERENCE DIVIDER +2 +2 +2
MAIN
MAIN
PHASE
DETECTOR
REFERENCE
SELECT
2
SM
EM
REFERENCE
SELECT
2
SA
SECONDARY
PHASE
DETECTOR
EA
AUXILIARY
IN
EA
AUX
12
NR
12
NAPA
AUXILIARY DIVIDER
PRESCALER
1/4
NORMAL
OUTPUT
CHARGE
PUMP
OUTPUT
CHARGE
PUMP
SPEED-UP
OUTPUT
CHARGE
PUMP
INTEGRAL
OUTPUT
CHARGE
PUMP
AUXILIARY
CN
8
CL
2
CK
4
2
V
SSA
V
DDA
LOCK
PHA
RA
PHI
PHP
RN
RF
Frequency Control and Modulation
The VCO frequency is controlled in part by DC
voltage across varactor diodes CR854, CR855, CR856
and CR851. As voltage across a reverse-biased varac-
tor diode increases, its capacitance decreases. There-
fore, VCO frequency increases as the control voltage
increases. CR854/CR855 and CR856/CR851 are par-
alleled varactors to divide the capacitance and
improve linearity. The varactors are biased at -2.0V
so the control line can operate closer to ground. The
control line is isolated from tank circuit RF by choke
L851/L854 and decoupling capacitor C854. The
amount of frequency change produced by CR854/
CR855/CR856/CR851 is controlled by series capaci-
tor C853.
4.2.1 VOLTAGE-CONTROLLED OSCILLATOR
Oscillator (Q850)
The VCO is formed by Q850, several capacitors
and varactor diodes, and ceramic resonator Z850. It
oscillates at the transmit frequency in transmit mode
and first injection frequency in the receive mode (380-
512 MHz in transmit and 432.950-564.950 MHz in
receive).
Biasing of Q850 is provided by R862, R867,
RC=867 and R868. An AC voltage divider formed by
C859, C861 and C862 initiates and maintains oscilla-
tion and also matches Q850 to the tank circuit.
Ceramic resonator Z850 is grounded at one end to pro-
vide shunt inductance to the tank circuit.