Icom IC-R20 Service Manual

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Icom IC-R20 Service Manual Manual pdf 9 page image

4 - 14-1 RECEIVER CIRCUITS4-1-1 BAND SWITCHING CIRCUIT (RF UNIT)The RF signals from the antenna connector pass throughthe limiter (D68) and an attenuator* (D69). The signals arethen applied to the antenna switching circuit (D3, D11, D13,D65, D66 and D73–D75 ).*Above 2 GHz RF signals do not pass through the attenua-tor.4-1-2 RF CIRCUIT (RF UNIT)The RF circuit amplifies the received signals within therange of frequency coverage and filters out-of-band signals.• A-BAND CIRCUIT(1) MF (above 0.15 MHz, below 1.9 MHz) signalsRF signals (0.15–1.9 MHz) from an attenuator (D69) passthrough the low-pass filter (L123–L125 and C851–C853),band switch (D66) and another low-pass filter (L88, L89,C533–C535, C657, C658). The filtered signals pass throughanother band switch (D67), and are then amplified at an RFamplifier (Q505). The amplified signals are applied to thenext band switch (D72).(2) HF-L (above 1.9 MHz, below 15 MHz) signalsRF signals (1.9–15 MHz) from an attenuator (D69) passthrough the low-pass filter (L123–L125 and C851–C853),band switch (D65) and bandpass filter (L85–L87, L91,C522–C531 and C891). The filtered signals pass throughanother band switch (D70), and are then amplified at an RFamplifier (Q505). The amplified signals are applied to thenext band switch (D72).SECTION 4 CIRCUIT DESCRIPTIONRF RFBPFBPFBPFRFBPFBPFPreAmpPreAmpHF-L (1.9 15 MHz)MF (15 30 MHz)A-VHF (118 175 MHz)B-VHF (118 175 MHz)30 300M (30 300 MHz except A-VHF range)A-UHF (300 470 MHz)800M (470 833 MHz)1G (833 1305 MHz)B-UHF (300 470 MHz)MF (0.15 1.9 MHz)from thebar antennaHPFHPFLPFLPFLPFLIMITERBANDSWBANDSWBANDSWBANDSWBANDSWBANDSWBANDSWANTENNAATTANTSWto 2nd mixercircuitRF-A circuitRF-B circuit1st mixer1st mixerto 2nd mixercircuitLO (1001 or 2002MHz)RFIC2IC3mixer1305 3305 MHzRFRFBPFRFBPF RFBPFRFRFRFBPFBPFHPFQ505Q36Q550 Q14Q549Q551Q35Q548Q24Q25IC18IC15IC1IC19• RF CIRCUITS