Icom IC-A14 Service Manual

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Icom IC-A14 Service Manual Manual pdf 7 page image

4 - 1SECTION 4 CIRCUIT DESCRIPTION4-1 RECEIVER CIRCUITSRF CIRCUITSRF circuits consist of RF filters, antenna switch (ANT SW),RF amplifi er (RF AMP), etc., and extracts and amplifi es thesignals of frequency which desired to receive.The received signals (RX signals) from the antenna areentered to the ANT UNIT and passed through the LPF. Thefiltered RX signals are entered to the PA UNIT, and passedthrough the LPF and ANT SW (an LPF in RX), then enteredto the MAIN UNIT.The RX signals from the PA UNIT are passed through theattenuator (ATT), BEF and 2-staged tuned BPF, then appliedto the RF AMP (Q11). The amplifi ed RX signals are passedthrough another 2-staged tuned BPF, and applied to the 1stmixer (Q12).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.While 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 ATT functions as a part of the AGC circuit. The AGCvoltage which is applied to the PIN diodes of the ATTcontrols RX signal level to enter the RX circuits, to keep thedemodulated AF signal level stable.The tuned-BPF is adjusted so that it responds to receivingfrequency and rejects all others, by the variable capacitorwhose capacitance is varied by applied tuning voltages; "T1–T4."The RF AMP amplifies RX signals to a level suited to the 1stmixer.1ST 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 46.35 MHz 1st IF signal, by being mixedwith the 1st Local Oscillator (LO) signals from the RX VCOvia buffers (Q60 and Q28), LO SW (D17) and ATT (R63,335, 336).The converted 1st IF signal is passed through the 1st IF filter(FI3) to be removed unwanted signals, and amplified by the1st IF AMP (Q13). The amplified 1st IF signal is then appliedto the 2nd IF circuit.• RF AND 1ST IF CIRCUITS• 2ND IF CIRCUIT2ND IF AND DEMODULATOR CIRCUITSThe 2nd IF circuit consists of 2nd mixer, 2nd IF filter, 2ndIF amplifier. And it converts the 1st IF signal into the 2nd IFsignal, then filters and amplifies 2nd IF signal only. And thedemodulator circuit converts the 2nd IF signal to AF signals.IC1 is an IF IC which contains the whole of the 2nd IFand demodulator circuits for the both of AM and FMdemodulation.The amplified 1st IF signal from the 1st IF AMP is appliedto the IF IC, and converted into the 2nd IF signal, by beingmixed with the 45.9 MHz 2nd LO signal at internal 2ndmixer.The 2nd LO signal is generated by PLL IC (IC29) and X5,and tripled by Q84 before being applied to the 2nd mixer.• AM signalsThe converted 2nd IF signal is filtered by external 2nd IFfilter (FI2), and amplified by the internal 2nd IF AMP. Theamplified 2nd IF signal is AM-demodulated by internal AMdetector, then output from pin 14.The demodulated AF signals are applied to the RX AFcircuits via the ANL (Automatic Noise Limiter) circuit whichreduces pulse-type noises, if the ANL function is activated.• FM (Weather Alert) signalsThe converted 2nd IF signal is filtered by external 2ndIF filter (FI2), and amplified by internal 2nd IF AMP. Theamplified 2nd IF signal is FM-demodulated by quadraturedetector, then output from pin 11.The demodulated AF signals are applied to the RX AFcircuits.ANTENNALPFANTSWBPFRFAMPBPFBPFXTALIFAMPAGCAM PAGCAM PATTT4T3T1T246.350MHz D1,D8D75BEFL52,C235Q80D13,50Q10Q11D14,D15L21,22C67,71,73,326Q121st IF FILTER 1st MIXER1st IF AMPQ13IC17DACSCK,SDATAD A2STBFI3RF AGCRF AGC from IF IC1st LO signalPA UNITMAIN UNITLPFL1,45C210-212L3C5-7L18,69,C60,327,780,781ANT UNITBPFCERAMICFMDETAMDETX4 (Discriminator)NOISE450kHz2nd IF2nd IF FILTER2nd MIXERIC1IF IC FI2DET AM DETAGC feedback20116171011121314 18 5 7 3FM DETRSSIRF AGC 45.9MHz 2nd LO(AM)(FM)2ndIF2ndIFOP.AMPRSSI