Icom IC-R20 Service Manual - Page 11

• B-BAND CIRCUIT The RF signals from the band switch (D32, D36, D143 and D147) are amplified at the RF amplifier (IC18, pins 1, 4), and are mixed with 1st LO signals at the 1st mixer circuit (IC15, pin 1) to produce a 266.7 MHz 1st IF signal. The 1st IF signal is output from pin 6, and passed through the bandpass filter (FI1) to suppress unwanted harmonic components. The filtered 1st IF signal is applied to the 2nd mixer circuit. The 1st LO signals are generated at the 1st B_VCO circuit and are applied to the 1st mixer (IC15, pin 3) directly. 4-1-5 2ND MIXER AND 2ND IF CIRCUITS (RF UNIT) The 2nd mixer circuit converts the 1st IF signal to a 2nd IF signal. • A-BAND CIRCUIT The filtered 429.1 MHz 1st IF signal from the bandpass filter (FI4) is mixed with the 2nd LO signal at the 2nd mixer circuit (IC10, pin 1, 6) to produce a 19.65 MHz 2nd IF signal. In case of AM/CW/SSB/Narrow FM mode, the 2nd IF signal passes through the mode switch (D58), crystal bandpass filter (FI3) and another mode switch (D60). In case of WFM mode, the 2nd IF signal passes through the mode switch (D59), low-pass filter (C202, C745, R207 and R536) and another mode switch (D61). The filtered 2nd IF signal is amplified at the 2nd IF amplifier (Q5), and is then applied to the demodulator circuit. • B-BAND CIRCUIT The filtered 266.7 MHz 1st IF signal from the bandpass filter (FI1) is mixed with the 2nd LO signal at the 2nd mixer circuit (IC16, pin 1, 6) to produce a 19.65 MHz 2nd IF signal. In case of AM/CW/SSB/Narrow FM mode, the 2nd IF signal passes through the mode switch (D130), crystal bandpass filter (FI7) and another mode switch (D132). In case of WFM mode, the 2nd IF signal passes through the mode switch (D131), low-pass filter (C755, C756, R428, R429 and R537) and another mode switch (D133). The filtered 2nd IF signal is amplified at the 2nd IF amplifier (Q542), and is then applied to the demodulator circuit. 4-1-6 3RD MIXER AND DEMODULATOR CIRCUITS (RF UNIT) The 3rd mixer circuit converts the 2nd IF signal to a 3rd IF signal. The demodulator circuit converts the 3rd IF signal into AF signals. • A-BAND CIRCUIT The 19.65 MHz 2nd IF signal from the 2nd IF amplifier (Q5) is applied to the 3rd mixer section of the FM IF IC (IC2, pin 16) and is then mixed with the 3rd LO signal for conversion into a 450 kHz 3rd IF signal. IC2 contains the 3rd mixer, limiter amplifier, quadrature detector and S-meter detector, etc. A frequency from the PLL reference oscillator is used for the 3rd LO signal (19.20 MHz). (1) FM MODE The 3rd IF signal is output from the FM IF IC (IC2, pin 3), and passes through the mode switch (D109) and ceramic bandpass filter (FI2). The filtered signal passes through the mode switch (D112), and is then fed back and amplified at the limiter amplifier section (pin 5). The signal is demodulated to AF signals at the quadrature detector section (pins 10, 11) and detector coil (L21). The demodulated AF signals are output from pin 9 and are applied to the AF switch (IC12, pin 11) via the "A_DETFM" line, and are then applied to the AF circuit (LOGIC unit) as "A_DET" signal. (2) WFM MODE The 3rd IF signal is output from the FM IF IC (IC2, pin 3), and passes through the mode switch (D108) and low-pass filter (L99, C686 and C687). The filtered signal passes through the mode switch (D111), and is then fed back to the limiter amplifier section (pin 5). The amplified signal is demodulated at the quadrature detector section (pins 10 and 11) and detector coil (L21). The AF signals are output from pin 9 and are applied to the AF switch (IC12, pin 11) via the "A_DETFM" line, and are then applied to the AF circuit (LOGIC unit) as "A_DET" signal. By connecting R55 to R54 in parallel, the output characteristics of pin 12, "RSSI", change gradually. Therefore, the FM IF IC can detect WFM components. • 1ST, 2ND MIXER AND 2ND IF CIRCUITS 1st LO signal from 2nd LO signal from the 1STAVCO board the 2NDCVCO board RF signals from the A-BAND circuit 1st mixer IC1 BAND BPF SW 2nd mixer IC10 1st LO signal from 2nd LO signal from the 1STBVCO board the 2NDDVCO board RF signals from the B-BAND circuit 1st mixer IC15 BAND BPF SW 2nd mixer IC16 Xtal BPF FI3 LPF BAND SW Q5 RF A-BAND circuit 2nd IF signal Xtal BPF FI7 LPF BAND SW Q542 RF 2nd IF signal B-BAND circuit 4 - 3