Icom IC-R20 Service Manual - Page 15

4-2 PLL CIRCUITS 4-2-1 PLL CIRCUIT A PLL circuit provides stable oscillation of the 1st LO frequencies. The PLL circuit compares the phase of the divided VCO frequency to the reference frequency. The PLL output frequency is controlled by the divided ratio (N-data) of a programmable divider. • A-BAND CIRCUIT (1STAVCO BOARD) An oscillated signal from one of the 1st VCO circuits (Q1-Q3, D1-D4) passes through the LO switch (D7-D19) and buffer amplifiers (IC2, Q8). The amplified signal is applied to the PLL IC (IC3, pin 11) and is prescaled in the PLL IC based on the divided ratio (N-data). The PLL IC detects the out-of-step phase using the reference frequency and outputs it from pin 5. The output signal is passed through the loop filter (Q6, Q7), and is then applied to the one of the 1st VCO circuits as the lock voltage. • B-BAND CIRCUIT (1STBVCO BOARD) An oscillated signal from one of the 1st VCO circuits (Q1-Q5, D1-D5) passes through the LO switch (D7-D10) and buffer amplifiers (IC2, Q8). The amplified signal is applied to the PLL IC (IC3, pin 11) and is prescaled in the PLL IC based on the divided ratio (N-data). The PLL IC detects the out-of-step phase using the reference frequency and outputs it from pin 5. The output signal is passed through the loop filter (Q6, Q7), and is then applied to the one of the 1st VCO circuits as the lock voltage. • DOWN CONVERTER PLL CIRCUIT (DOWNCONV BOARD) An oscillated signal from the VCO circuit (Q1, D3) is amplified at the buffer amplifier (Q2). The amplified signal is applied to the PLL IC (IC4, pin 8) and is prescaled in the PLL IC based on the divided ratio (N-data). The PLL IC detects the out-of-step phase using the reference frequency and outputs it from pin 5. The output signal is passed through the low-pass filter (L6, C21, C22 and C40), and is then applied to the one of the 1st VCO circuits as the lock voltage. 4-2-2 REFERENCE OSCILLATOR CIRCUIT (RF UNIT) The reference oscillator circuit (X1, Q556, D171) generates a 19.2 MHz reference frequency which is stabilized within the temperature range -10˚C (+14˚F) to +60˚C (+140˚F). The reference frequency is applied to the PLL ICs (refer to the list below in details) and the FM IF ICs (IC2, pin 2 for ABAND circuit: IC17, pin 2 for B-BAND circuit) via the LO amplifiers (Q540 for A-BAND circuit; Q555 for B-BAND circuit). BOARD NAME 1STAVCO 1STBVCO 2NDCVCO 2NDDVCO DOWNCONV PLL IC IC3 IC3 IC1 IC1 IC4 INPUT pin 19 pin 19 pin 1 pin 1 pin 1 OUTPUT pin 5 pin 5 pin 6 pin 6 pin 5 4-2-3 1ST VCO CIRCUITS (1STAVCO AND 1STBVCO BOARDS) • A-BAND CIRCUIT (1STAVCO BOARD) The oscillated signal at 1st A-VCO circuits is applied to the buffer amplifiers (IC2) via one of the LO switches (D7-D9). The amplified signal is applied to the 1st mixer circuit (RF unit; IC1, pin 3) via the LO amplifier (Q9). The 1st A-VCO circuit is composed of 1AVCO, 2AVCO and 3AVCO. Components and osillating frequences of each circuits are as follow list. 1st A-VCO 1AVCO 2AVCO 3AVCO Components Q1, D1 and D2 Q2 and D3 Q3 and D4 Oscillating freq. (MHz) 849.1-899.0999 604.1-849.0999 429.25-604.0999 • A-BAND PLL CIRCUIT Q6, Loop Q7 filter RF UNIT to the FM IF IC REF. CONT. X1 19.2 MHz REF. OSC. 1AVCO Q1, D1, D2 2AVCO Q2, D3 3AVCO LO switch D7 D8 Buffer IC2 Buffer Q9 to the 1st mixer circuit Buffer Q8 Q3, D4 D9 IC3 (PLL IC) 5 Phase detector Programmable counter Prescaler 11 Programmable divider Shift register 16 17 18 PLLBSTB PLLDATA PLLCK 19 4 - 7