Service manual

CIRCUIT DESCRIPTION
5-16
April 2001
Part No. 001-9800-203
Figure 5-5 Synthesizer Chip U804 Block Diagram
(1) CLOCK
(2) DATA
(3) STROBE
(4, 12) Vss
(5) RF
IN
(6) RF
IN
÷64/65
Prescaler
Main Dividers
Fractional
Accumulator
Prescaler
Modulus
Control
FMOD
Serial Input + Program Latches
V
DD (7, 15, 20)
(19) TEST
IN
(8) REF
Reference Divider
÷2 ÷2 ÷2
Main
Main
Phase
Detector
Reference
Select
IN
(10) AUX
Normal
Output
Charge
Pump
Output
Charge
Pump
Speed-Up
Output
Charge
Pump
Integral
V
SSA
V
DDA
LOCK (18)
PHA (11)
RA (9)
PHI (13)
PHP (14)
RN (16)
RF (17)
f
V
f
R
÷N
÷A
F INC
÷R
reference oscillator frequency is divided by 297 to
produce this signal. Fractional-N division with modulo
5 or 8 selection allows the loop frequency to be 5 or 8
times the channel spacing. Modulo 8 is used to allow
6.25 kHz (12.5 kHz) channel spacing.
The fV input is produced by dividing down the
VCO frequency applied to the RF IN input. The first
divider is a prescaler which is a special counter
capable of operating at relatively high frequencies.
This counter divides by 64 and 65 in this application.
This divides a signal in the 400 MHz range down to
approximately 6 MHz. For each main divider output
pulse, the prescaler divides by 65 for a certain number
of pulses and then 64 for an additional number of
pulses. The number counted in each mode is deter-
mined by the programming of the “N” and “A” divide
numbers. The basic operation is as follows:
The main divider begins counting down from the
“A” number. Then when zero is reached, it begins
counting down from the “N” number until zero is
reached. The cycle then repeats. While it is counting
down the “A” number, the prescaler divides by 65, and
while it is counting down the “N” number, it divides
by 64.
To illustrate the operation of these dividers, an
example will be used. Assume a transmit frequency of
450.750 MHz is selected. Since the VCO oscillates on
the transmit frequency in the transmit mode, this is the
frequency that must be produced by the VCO. To
produce this frequency, the “N” and “A” divide
numbers are programmed as follows:
N = 83 A = 55
UHF SYNTHESIZER DESCRIPTION