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PLL OPERATION
The UHF/VHF tuner local oscillator signal is input to the programmable divider
of the MIX-OSC/PLL IC (CXA3135AN).
The programmable divider has two stages, a prescaler stage (1/8) and a
programmable counter stage. The prescaler stage divides the local oscillator
frequency down to the operating range of the programmable counter. The
programmable counter is composed of a 9-bit main counter and a 5-bit
swallow counter which further divides the local oscillator frequency according
to the CPU control pulses input to pins 1 and 2. The possible frequency
division ratio of the programmable divider is from 1616 (CH 02) to 13552 (CH
69) and of course will be different for each channel selected. When the
division is complete, the output is coupled to the phase comparator.
The 4 MHz crystal controlled oscillator outputs is divided by 512 to provide the
reference frequency of 7.8125 KHz. This reference frequency from the divider
is input to the phase comparator. The divided local oscillator frequency is
phase-compared with the divided reference oscillator frequency by the phase
comparator. Any phase error will generate a correction voltage which is
amplified and applied to the tuner local oscillator.
SUMMARY
The PLL always phase-compares the local oscillator frequency with the
reference frequency. If the local oscillator frequency deviates even slightly
from the normal value, a correction signal corresponding to that deviation is
immediately generated by the phase comparator. This correction signal is
then amplified and applied to the tuning terminal, thus returning the local
oscillator frequency to the correct value.
MAIN
SWALLOW
CH 02 PLL data (Binary)
000110010
10000
(2)
(2)
Programmable Divider Bits
Phase Locked Loop Operation
TUNER
1/8
CH 02 Local frequency 101MHz
12625 KHz
Programmable
Divider (1/N)
7.8125 KHz
1/N = 1/1616
locked
N = M8 x 2
13
+ . . . + S2 x 2
2
+ S1 x 2 + S0
= 1 x 2
10
+ 1 x 2
9
+ 1 x 2
6
+ 1 x 2
4
= 1024 + 512 + 64 + 16
= 1616
Lock Frequency
F
OSC
= Fr x 8 x (32M + S)
= 7.8125 x 8 x (32 x 50 + 16)
= 101,000 [KHz]
= 101 [MHz]
(M) = 50
(S) = 16
Note: 4 MHz (Crystal)
Fr = 7.8125 KHz
Step F = 62.5 KHz
F
OSC
: Lock Frequency
Fr
: Reference Frequency
M : MAIN COUNTER
(32 M 511)
S
: SWALLOW COUNTER ( 0 M 31)
