Icom IC-208H Service Manual page 15

4-3-3 UHF LOOP (VCO BOARD AND MAIN UNIT)
The generated signal at the U-VCO (Q101, D100, D101)
enters the PLL IC (MAIN unit; IC2, pin 8) via buffer-ampli-
fiers (Q102, Q50) and VCO switching circuit (D51) and is
divided at the programmable divider section and is then
applied to the phase detector section.
The phase detector compares the input signal with a refer-
ence frequency, and then outputs the out-of-phase signal
(pulse-type signal) from pin 15 (MAIN unit).
The pulse-type signal is converted into DC voltage (lock
voltage) at the loop filter (MAIN unit; IC7, Q11, Q12, D5),
and then applied to the U-VCO to stabilise the oscillated fre-
quency.
4-3-4 1ST LO CIRCUIT
(VCO BOARD AND MAIN UNIT)
The 1st VCO circuit contains a separate V-VCO (Q3, D1,
D2) and U-VCO (Q101, D100, D101).
• WHILE RECEIVING VHF BAND
The oscillated signal at the V-VCO circuit is amplified at the
buffer amplifier (Q4), and is then applied to the VCO switch-
ing circuit (D6). The signal passes through the attenuator
(C34, R24–R26) and low-pass filter (L7, L8, C36–C38), and
is then applied to the LO switching circuit.
(1) RECEIVING 118MHz–174 MHz
The signal is applied to the LO switching circuit (MAIN unit;
D528) as "V_RXLO" signal, and is applied to the 1st mixer
circuit (MAIN unit; Q32), then mixed with the 1st IF signal.
(2) RECEIVING 230 MHz–550 MHz
The signal is applied to the LO switching circuit (MAIN unit;
D15, D529, D530) as "V_RXLO" signal, and is applied to the
1st mixer circuit (MAIN unit; Q35), then mixed with the 1st IF
signal.
A portion of the RF signal from the buffer amplifier (Q4) is
amplified at the buffer amplifier (Q50), and is then fed back
to the PLL IC (MAIN unit; IC1, pin 8) as the comparison sig-
nal.
• UHF PLL CIRCUIT
Loop
filter
X2
15.3 MHz
UHF VCO
Buffer
Q101, D100, D101
16
Phase Programmable
15
detector counter
Programmable
divider
1
• WHILE RECEIVING UHF BAND OR 800 MHz BAND
The oscillated signal at the U-VCO circuit is amplified at the
buffer amplifiers (Q102, Q103), and is then applied to the
VCO switching circuit (for UHF band; D104, for 800 MHz;
D105).
While receiving UHF band, the signal passes through the
attenuator (C125, R117, R119, R120) and low-pass filter
(Q105, L7, L8, C36–C38), and is then applied to the VCO
switching circuit (D106).
While receiving 800 MHz band, the signal is doubled at the
doubler amplifier (Q104), and passes through the bandpass
filter (L108–L110, C127, C129, C132, C133, C135) and
attenuator (R127, R128). The signal is applied to the VCO
switching circuit (D107).
(1) RECEIVING UHF BAND
The signal is applied to the LO switching circuit (MAIN unit;
D9) as "U_RXLO" signal, and is applied to the 1st mixer cir-
cuit (MAIN unit; Q35), then mixed with the 1st IF signal.
(2) RECEIVING 800 MHz BAND
The signal is applied to the LO switching circuit (MAIN unit;
D10) as "U_RXLO" signal, and is applied to the 1st mixer cir-
cuit (MAIN unit; IC12, pin 6), then mixed with the 1st IF sig-
nal.
A portion of the RF signal from the buffer amplifier (Q102) is
amplified at the buffer amplifier (Q50) via the VCO switching
circuit (D51), and is then fed back to the PLL IC (MAIN unit;
IC1, pin 8) as the comparison signal.
Q103
Buffer
Q102
VCO
switch
Buffer
Q50
D51
IC2 MB15A02PFV1
Prescaler
Shift register
4 - 7
VCO
switch
D103
to the transmitter circuit
D104
to the 1st mixer circuit
D105
to the 1st mixer circuit
2
Q104
8
9
PLLCK
10
PLLDATA
11
PLLSTB2