Pll Circuits - Icom ID-800H Service Manual

4-3 PLL CIRCUITS

4-3-1 GENERAL (MAIN UNIT)
A PLL circuit provides stable oscillation of the transmit fre-
quency and the receive local frequency. The PLL circuit
compares the phase of the divided VCO frequency to the
reference frequency. The PLL output frequency is controlled
by a crystal oscillator and the divided ratio of the program-
mable divider.
4-3-2 VHF LOOP (VCO BOARD)
The generated signal at the V-VCO (Q3, D1, D2) enters the
PLL IC (MAIN unit; IC1, pin 8) via the buffer amplifiers (Q6,
Q8) and VCO switch (D50).
The applied signal is divided at the prescaler and program-
mable counter section by the N-data ratio from the main CPU
(MAIN unit: IC505). The divided signal is detected on phase
at the phase detector using the reference frequency and out-
put from pin 15. The output signal is passed through the loop
fi lter and is then applied to the V-VCO circuit.
If the oscillated signal drifts, its phase changes from that of
the reference frequency, causing a lock voltage change to
compensate for the drift in the oscillated frequency.
• VHF PLL CIRCUIT
Loop
filter
Q2
X1
Buffer
15.2 MHz
Q4
VHF VCO
Buffer
Q3, D1, D2
Phase Programmable
15
detector counter
Programmable
divider
1
45.6 MHz 2nd LO signal
×3
to the FM IF IC (IC8, pin 2)
Q7
4-3-3 UHF LOOP (VCO BOARD)
The generated signal at the U-VCO (Q101, D100, D101)
enters the PLL IC (MAIN unit; IC2, pin 8) via the buffer
amplifiers (Q50, Q102) and VCO switch (D51).
The applied signal is divided at the prescaler and program-
mable counter section by the N-data ratio from the main CPU
(MAIN unit: IC505). The divided signal is detected on phase
at the phase detector using the reference frequency and out-
put from pin 5. The output signal is passed through the loop
fi lter and is then applied to the U-VCO circuit.
If the oscillated signal drifts, its phase changes from that of
the reference frequency, causing a lock voltage change to
compensate for the drift in the oscillated frequency.
4-3-4 VCO CIRCUIT (VCO BOARD )
The 1st LO circuit contains a separate V-VCO (Q3, D1, D2)
and U-VCO (Q101, D100, D101).
• 1ST LO CIRCUIT
Frequency range
118–174 MHz
430–440 MHz (EXP)
440–450 MHz (USA)
230–550 MHz
800–1000 MHz
LO switch
D5
Q5
Buffer
D6
VCO
switch
Buffer
Q50
D50
IC1 MB15A02PFV1
8
Prescaler
9
10
Shift register
11
4 - 7
1st
VCO
mixer
V-VCO Q23
U-VCO Q35
U-VCO IC12 pin 3
U-VCO IC12 pin 3
to the transmitter circuit
to the 1st mixer circuit
PLLCK
PLLDATA
PLLSTB1
Lo
switch
D6
D104
D104
D105