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 Oscillating freq. (MHz) 1BVCO 2BVCO Q1, D1 and D2 Q2 and D3 863.3-1099.6999 683.3-866.6999 3BVCO 4BVCO Q3 and D4 Q4, Q5 and D5 566.3-686.6999 384.7-441.6999 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. 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 Loop filter A_R3V Q4 Ripple filter OSC Buffer (19.2 MHz) Q3 VCO Q1, D1 2ND VCO 51 8 PLL IC IC1 to the 2nd mixer (IC10, pin 3) • B-BAND PLL CIRCUIT Q6, Loop Q7 filter RF UNIT to the FM IF IC REF. CONT. X1 19.2 MHz REF. OSC. 1BVCO LO switch Q1, D1, D2 D7 2BVCO Q2, D3 D8 3BVCO Q3, D4 D9 4BVCO Buffer IC2 Buffer Q9 to the 1st mixer circuit Buffer Q8 Q4, Q5, D5 D10 IC3 (PLL IC) 5 Phase detector Programmable counter Prescaler 11 Programmable divider Shift register 16 17 18 PLLBSTB PLLDATA PLLCK 19 4 - 8