User Manual
CIRCUIT DESCRIPTION
4-4
June 1997
Part No. 001-3492-001
capacitance that is multiplied. If a noise pulse or other
voltage change appears on the collector, the base volt-
age does not change significantly because of C842.
Therefore, base current does not change and transistor
current remains constant. CR840 decreases the charge
time of C842 when power is turned on. This shortens
the start-up time of the VCO. C840 and C841 are RF
decoupling capacitors.
4.2.6 VCO FREQUENCY SHIFT
The VCO must be capable of producing frequen-
cies from 840-960 MHz to produce the required
receive injection and transmit frequencies. If this
large of a shift was achieved by varying the VCO con-
trol voltage, the VCO gain would be undesirably high.
Therefore, capacitance is switched in and out of the
tank circuit to provide a coarse shift in frequency.
The 928-960 MHz band is divided into two seg-
ments, 928-944 MHz and 944-960 MHz. The band
selection is controlled by shift register U840, digital
transistors Q843/Q844 and pin diode CR820 on the
VCO board.
A frequency shift of 87.85 MHz is required to go
from transmit to receive mode and visa versa. Trans-
mit to receive frequency shift is accomplished by pro-
gramming shift register U840 which drives the digital
transistors Q841/Q842. In Transmit mode, Q841/
Q842 forward bias pin diode CR821 which switches
in an inductive transmission line in parallel with the
VCO resonator causing the VCO frequency to
increase. In Receive mode Q841/Q842 reverse bias
CR821 which switches out the inductive transmission
line and lowers the VCO frequency for the mixer
injection.
Figure 4-2 U800 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