Icom IC-R20 Service Manual - Page 12

(3) AM 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 applied to the IF amplifier (Q531). The amplified signal is demodulated at the AM detector (Q4). The AF signals pass through the ANL circuit (D114), and are then applied to the AF switch (IC12, pin 4) via the "A_DETAM" line, and are then applied to the AF circuit (LOGIC unit) as "A_DET" signal. (4) SSB/CW MODES The 3rd IF signal is output from the FM IF IC (IC2, pin 3), and passes through the mode switch (D110) and ceramic bandpass filter (FI6). The filtered signal passes through the mode switch (D113), and is then applied to the IF amplifier (Q531). The amplified signal is mixed with BFO signal from the LOGIC unit at the mixer (D115 and D116) to demodulate to AF signals. The AF signals are applied to the SSB AF amplifier (Q532), and are then applied to the AF switch (IC12, pin 1) via the "A_DETSSB" line, and are then applied to the AF circuit (LOGIC unit) as "A_DET" signal. • B-BAND CIRCUIT The 19.65 MHz 2nd IF signal from the 2nd IF amplifier (Q542) is applied to the 3rd mixer section of the FM IF IC (IC17, pin 16) and is then mixed with the 3rd LO signal for conversion into a 450 kHz 3rd IF signal. IC17 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 (IC17, pin 3), and passes through the mode switch (D134) and ceramic bandpass filter (FI8). The filtered signal passes through the mode switch (D136), and is then fed back and amplified at the limiter amplifier section (pin 5). The signal is demodulated AF signals at the quadrature detector section (pins 10, 11) and detector coil (L105). The demodulated AF signals are output from pin 9 and are applied to the AF switch (IC21, pin 1) via the "DET_OUT" line, and are then applied to the AF circuit (LOGIC unit) as "B_DET" signal. (2) WFM MODE The 3rd IF signal is output from the FM IF IC (IC17, pin 3), and passes through the mode switch (D135) and low-pass filter (L106, C781 and C782). The filtered signal passes through the mode switch (D137), 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 (L105). The AF signals are output from pin 9 and are applied to the AF switch (IC21, pin 1) via the "DET_OUT" line, and are then applied to the AF circuit (LOGIC unit) as "B_DET" signal. By connecting R436 to R437 in parallel, the output characteristics of pin 12, "RSSI", change gradually. Therefore, the FM IF IC can detect WFM components. • A-BAND 3RD MIXER AND DEMODULATOR CIRCUITS "BFO" signal from the LOGIC unit Q530 "A_DETSSB" signal to the AF switch AF IF Q532 D115 Q531 D116 MUTE SW Q524, Q526, Q528 Q523 DET Q521, Q525 NB AGC Q522 SSB/CW FI6 "A_AGC" signal to RF amplifiers AGC AM DET IF AM/NFM FI2 Q4 Q4 Q1 C98 C99 R59 8 R60 7 5 R61 Active filter C96 FM detector Noise detector Limiter amp. RSSI 9 10 11 "A_DETFM" signal to the AF switch R56 C95 C101 R54 L21 C93 WFM LPF 3 Q540 LO 2 REF OSC REF CONT. Q556 X1 D171 19.2 MHz 3rd Noise Mixer comp. IC2 TA31136F 12 13 16 2nd IF (19.65 MHz) from Q5 "A_NOISE" signal to the CPU (LOGIC unit; IC3, pin 5) C94 R55 "A_RSSI" signal to the CPU "A_WFMC" (LOGIC unit; IC3, pin 91) Q41 C242 4 - 4