Icom IC-R20 Service Manual - Page 17

• B-BAND CIRCUIT (2NDDVCO BOARD) The 2nd D-VCO circuit (Q1 and D1) generates 247.0500 MHz and 286.3500 MHz. The oscillated signal is applied to the 2nd mixer (RF unit; IC16, pin 3), and is then mixed with the 1st IF signal. • 2ND D VCO CIRCUIT Loop filter Q4 A_R3V Ripple filter OSC Buffer (19.2 MHz) Q3 VCO Q1, D1 2ND VCO 51 8 PLL IC IC1 • B-BAND CIRCUIT The 19.2 MHz reference signal is amplified at the LO amplifier (Q555), and is then applied to the FM IF IC (IC17, pin 2) as the 3rd LO signal. The signal is mixed with 2nd IF signal to convert into 3rd IF signal. The 3rd IF signal is applied to the each demodulator (FM/WFM/AM) circuits. • 3RD LO CIRCUIT (B-BAND) D171 X1 REF CONT. Q556 REF OSC Q555 19.2 MHz 3rd LO signal LO FM IF IC (IC17) to the 2nd mixer (IC16, pin 3) to the demodulator circuits 3rd mixer 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 D-VCO circuit again as PLL lock voltage. 4-2-5 3RD LO CIRCUIT (RF UNIT) The 3rd LO circuit uses 19.2 MHz reference signal which is generates at the X1 and Q556. • A-BAND CIRCUIT The 19.2 MHz reference signal is amplified at the LO amplifier (Q540), and is then applied to the FM IF IC (IC2, pin 2) as the 3rd LO signal. The signal is mixed with 2nd IF signal to convert into 3rd IF signal. The 3rd IF signal is applied to one of demodulator (FM/WFM/AM/SSB) circuits. • 3RD LO CIRCUIT (A-BAND) D171 X1 REF CONT. Q556 REF OSC Q540 19.2 MHz 3rd LO signal LO FM IF IC (IC2) to the demodulator circuits 3rd mixer 2nd IF signal • DOWN CONVERTER VCO CIRCUIT 2nd IF signal 4-2-6 DOWN CONVERTER VCO CIRCUIT (DOWNCONV BOARD) The DOWN CONVERTER circuit uses 19.2 MHz reference signal which is generates at the X1 and Q556 on the RF unit. The DOWN CONVERTER VCO circuit (Q1 and D3) generates 1001 MHz or 2002 MHz. The oscillated signal is amplified at the buffer amplifier (Q4), and passes through band switch. When receiving 1305-2305.999 MHz RF signals, 1001 MHz LO signal is selected. The RF signals pass through band switch (D6), and then pass through another band switch (D8). When receiving 2306-3304.999 MHz, 2002 MHz LO signal is selectd. The RF signals pass through the band switch (D5), and then pass through the high-pass filter (L9, L10 and C28-C32). The signals passes through another band switch (D7). The 1001 MHz or 2002 MHz LO signal is applied to the mixer circuit (IC3, pin 3). A part of oscillated signal from the DOWN CONVERTER VCO circuit is amplified at the buffer amplifier (Q2), and is then applied to the PLL IC (IC4, pin 8) as the comparison signal, and is then output from pin 5. The signal passes through the loop filter (L6, C21, C22 and C40), and is then applied to the DOWN CONVERTER VCO circuit again as PLL lock voltage. D171 REF CONT. X1 Q556 REF OSC RF UNIT Q6 19.2 MHz Buff PLL IC IC4 LPF DOWNCONV BOARD Q2 Buff VCO Q1, D3 Q4 Buff 1001 MHz or 2002 MHz LO signal to the mixer (IC3) 4 - 9