Icom IC-A14 Service Manual

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Icom IC-A14 Service Manual Manual pdf 10 page image

4 - 44-3 FREQUENCY SYNTHESIZER CIRCUITSVCOA VCO is an oscillator whose oscillating frequency isdetermined by the applied volatge. This transceiver has twoVCOs; RX VCO and TX VCO. The RX VCO generates the1st LO signals for the 1st IF signal produce, and TX VCOgenerates TX signal.• RX VCO (Q58, D38, 48)The RX VCO oscillates 1st LO signals for normal RX(174.350 to 183.325 MHz) and for WX channels (208.000–209.625 MHz). The generated 1st LO signals are applied tothe 1st mixer (Q12) via the buffer AMPs (Q60 and Q28), LOSW (D17) and the ATT.• TX VCO (Q59, D45, 46)The TX VCO oscillates 118.000–136.975 MHz transmitsignals. The generated TX signal is applied to the pre-drivervia buffers (Q60 and Q28), LO SW (D6) and ATT.A portion of each VCO output is applied to the PLL IC viathe buffer (Q29).PLL (Phase Locked Loop) CIRCUITThe PLL circuit provides stable oscillation for both of theTX and 1st LO frequencies (for RX). By comparing fedbackVCO output and reference frequency signals, the PLLcorrects the difference of these frequencies.A portion of RX/TX VCO output is applied to the PLL IC viabuffer (Q29). The applied VCO output is divided accordingto the serial data including divide ratio from the CPU, at theprescaler and programmable divider. In the same way, thereference frequency signal from the TCXO is applied to thePLL IC and divided so that these two applied signals arethe same frequency.The divided and frequency-matched signals (VCO outputand the reference frequency signals) are applied to thephase comparator and phase-compared. The resultedphase difference is detected as a phase-type signal, andlevel-adjusted at the charge pump then output. The outputpulse type signal is passed through the loop filter to beconverted into the DC voltage (=Lock Voltage).Applying the lock voltage to the variable capacitor whichcomposes a part of the resonator of RX/TX VCO, thecapacitance of variable capacitor changes correspondingto the applied lock voltage. This causes the change ofresonation frequency that determines the VCO oscillatingfrequency to keep the VCO frequency constant.When the oscillation frequency drifts, its phase changesfrom that of the reference frequency, causing a lock voltagechange to compensate for the drift in the VCO oscillatingfrequency.• FREQUENCY SYNTHESIZER CIRCUITSPLLICBUFFBUFFFILLOOPVCOSWBUFFCR-783X5SHIFTLO SW154.350-183.325 MHz118.000-136.975 MHzD6,Q28Q60Q29RX VCOTX VCOQ58D38,D48Q59Q57R63,335,336(15.300MHz)IC29RX-VCOTX-VCOPCK,PDATA,PSTB(serial data)ATTD17D453(45.900MHz)45.9MHz2nd LO1st LO signalTX signalQ84208.000-209.625 MHzR36-38ATT