Icom IC-R20 Service Manual page 16

A portion of the signal from the buffer amplifier (IC2) is
amplified at the buffer amplifier (Q8) and is then fed back to
the PLL IC (IC3, pin 11) as the comparison signal.
• B-BAND CIRCUIT (1STBVCO BOARD)
The oscillated signal at 1st B-VCO circuits is applied to the
buffer amplifiers (IC2) via one of the LO switches (D7–D10).
The amplified signal is applied to the 1st mixer circuit (RF
unit; IC15, pin 3) via the LO amplifier (Q9). Components and
osillating frequences of each circuits are as follow list.
1st B-VCO Components
1BVCO Q1, D1 and D2
2BVCO Q2 and D3
3BVCO Q3 and D4
4BVCO Q4, Q5 and D5
A portion of the signal from the buffer amplifier (IC2) is
amplified at the buffer amplifier (Q8) and is then fed back to
the PLL IC (IC3, pin 11) as the comparison signal.
4-2-4 2ND VCO CIRCUITS
(2NDCVCO AND 2NDDVCO BOARDS)
The 2nd VCO circuit generates the 2nd LO frequencies, and
the signals are applied to the 2nd mixer circuit.
• A-BAND CIRCUIT (2NDCVCO BOARD)
The 2nd C-VCO circuit (Q1 and D1) generates 409.4500
MHz and 448.7500 MHz. The oscillated signal is applied to
the 2nd mixer (RF unit; IC10, pin 3), and is then mixed with
the 1st IF signal.
• B-BAND PLL CIRCUIT
RF UNIT
REF.
CONT.
X1
19.2 MHz
Oscillating freq. (MHz)
863.3–1099.6999
683.3–866.6999
566.3–686.6999
384.7–441.6999
1BVCO
Q1, D1, D2
2BVCO
3BVCO
4BVCO
Q4, Q5, D5
Q6, Loop
Q7 filter
5
to the FM IF IC
19
REF.
OSC.
A part of oscillated signal from the 2nd C-VCO circuit is
applied to the PLL IC (IC1, pin 8) as the comparison signal,
and is then output from pin 5. The signal passes through the
loop filter (R14, R15 and R22), and is then applied to the 2nd
C-VCO circuit again as PLL lock voltage.
• 2ND C VCO CIRCUIT
Q4
A_R3V
Ripple
filter
Buffer
OSC
(19.2 MHz)
Q3
LO
switch
D7
D8
Q2, D3
D9
Q3, D4
D10
Phase Programmable
detector counter
Programmable
Shift register
divider
4 - 8
Loop
filter
VCO
2ND
VCO
Q1, D1
to the 2nd mixer (IC10, pin 3)
Buffer
to the 1st mixer circuit
Q9
Buffer
IC2
Buffer
Q8
IC3 (PLL IC)
11
Prescaler
16
PLLBSTB
17
PLLDATA
18
PLLCK
5 1
8
PLL IC
IC1