Icom IC-R2 Service Manual

Also see for IC-R2: Instruction manual Instruction manual

Contents

SECTION 4 CIRCUIT DESCRIPTION4 - 14-1 RECEIVER CIRCUITS4-1-1 ANTENNA SWITCHING CIRCUIT (RF UNIT)The RF signals from the antenna connector pass throughthe limitter (D68) and an attenuator (D69). The signals arethen applied to the RF circuit the antenna switching circuit(D13, D73, D75) which suppress out-of-band signals.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.(1) 0.495 MHz–29.999 MHzRF signals (0.495–29.999MHz) from an antenna switchingcircuit (D73) pass through a low-pass filter (C511–C515,L81, L82). The filtered signals are amplified at an RF ampli-fier (Q505) passing through each band-pass filter dependingon the receiving frequency. The amplified signals are thenapplied to the 1st mixer circuit (IC1) after being amplified atanother RF amplifier (IC11) via the band switchingdiode(D71).The signals below 1.9 MHz pass through a low-pass filter(C534, C535, C657, C658, L88, L89) via the band switchingdiode (D66), and are then applied to the RF amplifier circuit(Q505) via the band switching diode (D67).The 1.9 MHz–14.995 MHz signals pass through the bandswitching diode (D65) and band-pass filter (C522–C531,L85–L87, L91), and are then applied to the RF amplifier cir-cuit (Q505) via the band switching diode (D70).The 15 MHz–29.995 MHz signals pass through the bandswitching diode (D63) and high-pass filter (C516–C520,L83, L84) and are then applied to the RF amplifier circuit(Q505) via the band switching diode (D64).(2) 118 MHz–174.995 MHz, 330 MHz–832.995 MHzRF signals (118 MHz–174.995 MHz, 330 MHz–832.995MHz) from an antenna switching diode (D75) are passedthrough each bandpass filter and RF amplifier, and are thenapplied to the 1st mixer circuit (IC1) via the band switchingdiode (D71) and RF amplifier (IC11).The 118 MHz–174.995 MHz signals pass through the bandswitching diode (D74) and low-pass filter (C8–C13, C67,C416, L14, L57–L59, L70), and are then amplified at RFamplifier (Q14). The amplified signal passes through thetunable band-pass filters (D1, D2) and band switching diode(D25).The 330 MHz–469.995 MHz signals are amplified at RFamplifier (Q35) via the band switching diode (D3) and band-pass filter (C19–C23, C216, L2–L5). The amplified signalpasses through the tunable band-pass filters (D3, D4) andband switching diode (D29).The 470 MHz–832.995 MHz signals are amplified at RFamplifier (Q24) via the band-pass filter (C32, C33,C35–C37, C39, C144, C145), between the band switchingdiode (D11, D32).(3) 30–117.995 MHz, 175–329.995 MHzThe 30 MHz–117.95 MHz, 175 MHz–329.995 MHz signalspass through the low-pass filter (C40–C43, C665, C666, L9,L10, L92), and are then applied to the RF amplifier (Q36).The amplified signals are amplified at the RF amplifier (IC11)via band switching diodes (D34, D71). The amplified signalsare applied to the 1st mixer circuit (IC1).(4) 833 MHz–1309.995 MHzThe 833 MHz–1309.995 MHz signals pass through thebandpass filter (C5, C45–51, L11–L13, L43), and are thenapplied to the RF amplifier (Q26). The amplified signals areamplified at the RF amplifier (IC11) via band switchingdiodes (D36). The amplified signals are applied to 1st mixercircuit (IC1).4-1-3 1ST MIXER CIRCUIT (RF UNIT)The 1st mixer circuit converts the received RF signals to afixed frequency of the 1st IF signal with a PLL output fre-quency. By changing the PLL frequency, only the desiredfrequency will pass through the bandpass filters at the nextstage of the 1st mixer.The filtered RF signals are mixed with 1st LO signals at the1st mixer circuit (IC1) to produce a 266.7 MHz 1st IF signal.The 1st IF signal is output from pin 6, and passed throughthe bandpass filter (FI1) to suppress unwanted harmoniccomponents. The filtered 1st IF signal is applied to the 2ndmixer circuit.The 1st LO signals are generated at the 1st VCO (Q28, Q30,D45) and are applied to the 1st mixer (IC1, pin 3) directly orpassing through the doubler circuit (Q31) after being ampli-fied at the buffer amplifiers (IC4, Q40).4-1-4 1ST IF AND 2ND MIXER CIRCUITS (RF UNIT)The 2nd mixer circuit converts the 1st IF signal to a 2nd IFsignal.The filtered 266.7 MHz 1st IF signal from the bandpass filteris mixed with the 2nd LO signal at the 2nd mixer circuit(IC10, pin 1) to produce a 19.65 MHz 2nd IF signal. The 2ndIF signal pass through (except WFM mode) or bypass (WFMmode) the bandpass filter (FI3), and is then amplified at the2nd IF amplifier (Q5). The amplified signal is applied to thedemodulator circuit.4-1-5 DEMODULATOR CIRCUITS (RF UNIT)The demodulator circuit converts the 2nd IF signal into AFsignals.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, pin16) and is then mixed with the 3rd LO signal for conversioninto a 450 kHz 3rd IF signal.