Icom IC-A210 Service Manual

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Icom IC-A210 Service Manual Manual pdf 21 page image

3 - 1SECTION 3 CIRCUIT DESCRIPTIONWhile transmitting, serial-connected PIN diodes are ON,thus the TX line is connected to the antenna, and the RXline is connected to the GND simultaneously to preventtransmit signal entering.The attenuator functions as a part of the AGC circuit. TheAGC volatge which is applied to the PIN diodes controlls RXsignal level to enter the RX circuits.The tuned-BPF is adjusted so that it responds to receivingfrequency and rejects all others, by the variable capacitorwhose capacitance is varied by applied voltage "T1" and"T2."The RF AMP amplifies RX signals to a level suited to the 1stmixer.3-1 RECEIVER CIRCUITSRF CIRCUITSRF circuits consist of RF filters, antenna switch (ANT SW),RF AMPlifier (RF AMP), etc., and extracts and amplifies thesignals of frequency which desired to receive.The received signals (RX signals) from the antenna arepassed through the LPF, ANT SW (as an LPF in RX),attenuator, and the two-staged tuned BPF.The filtered RX signals are amplified by the RF AMP, andpassed through another two-staged tuned BPF. The filteredRX signals are then applied to the 1st IF circuits.The ANT SW toggles RX line and TX line. While receiving,the TX line and the antenna is disconnected to prevent RXsignals entering. The RX line is disconnected from the GNDsimultaneously, and an LPF which guides received signalsto the RX circuits is composed.LPFANTSWATTBPFRFAMPBPFANTBUFFD1 D2 D3D502 D503 D504L521 L523C572 575 577 578 582D10 D11L2 L4IC69D13 D14L8 L10PA UNITMAIN UNITTUNE1IC52RF AGC1ST IF CIRCUITSThe 1st IF circuits consist of 1st mixer, 1st IF filter and 1stIF amplifier. And it converts the RX signals into the 1st IFsignal, then filters to remove unwanted signals and amplifies.The filtered RX signals are applied to the 1st mixer to beconverted into the 1st IF signal, by being mixed with the 1stLocal Oscillator (LO) signals from the RX VCO via buffer.The converted 1st IF signal is passed through the 1st IFfilter (FI4 for AM RX, FI3 for WX RX) via filter switches, tobe removed unwanted signals. The filtered 1st IF signal isamplified by the 1st IF AMP. The amplified 1st IF signal isthen applied to the 2nd IF circuits.2ND IF AND DEMODULATOR CIRCUITSThe 2nd IF circuits consist of 2nd mixer, 2nd IF filter, 2ndIF amplifier. And it converts the 1st IF signal into the 2ndIF signal, then filters to extract 2nd IF signal only andamplifies. And the demodulator circuit converts the 2nd IFsignal to AF signals.• AM signalsThe amplified 1st IF signal is applied to the FM IF IC, andconverted into the 2nd IF signal, by being mixed with the2nd LO from the X2, at internal 2nd mixer. The converted2nd IF signal is filtered by external 2nd IF filter (FI5), andamplified by three external 2nd IF AMPs. The amplified2nd IF signal is AM-demodulated by Q21. A PN junctionconstruction inside Q21 is used for AM detection to obtainlow output impedance. The demodulated AF signals areapplied to the RX AF circuits via the ANL (Automatic NoiseLimiter) circuit which reduces pulse-type noises, when theANL function is activated.BPFXTALIFAMPFilterSWFilterSWFilterSWFilterSWBPFXTALBUFFIC68321st MIXERD19FI 3 BW 20 kHzBW 9 kHzFI 4D2 7D2 8Q20<1st IF=38.85 MHz>FIL1FIL2D20AGC 2Q45• RF CIRCUITS• 1ST IF CIRCUITS