Icom IC-V8 Service Manual

Also see for IC-V8: Instruction manual Instruction manual Service manual Instruction manual

Contents

SECTION 4 CIRCUIT DESCRIPTION4 - 14-1 RECEIVER CIRCUITS4-1-1 ANTENNA SWITCHING CIRCUITReceived signals passed through the low-pass filter (L1, L2,C1–C5). The filtered signals are applied to the 1/4 λ typeantenna switching circuit (D1, D2, D8, L15, C76).The antenna swtiching circuit functions as a low-pass filterwhile transmitting. However, its impedance becomes veryhigh while D2 and D8 are turn ON. Thus transmit signals areblocked from entering the receiver circuits. The antennaswitching circuit employs a 1/4 λ type diode swtiching sys-tem. The passed signals are then applied to the RF amplifi-er circuit.4-1-2 RF CIRCUITThe RF circuit amplifies signals within the range of frequen-cy coverage and filters out-of-band signals.The signals from the antenna switching circuit are applied tothe limitter (D50), and are then passed through the band-pass filter (D9, L16, C80). The filtered signals are amplifiedat the RF amplifier (Q12), then applied to the 1st mixer cir-cuit after out-of-band signals are suppressed at the band-pass filter (D11, D12, L19, C91–C97).D9, D11, D12 employ varactor diodes that track the band-pass filters and are controlled by the T1–T3 signals from theD/A convertor (IC10, pins 2, 3, 10). These diodes tune thecenter frequency of an RF passband for wide bandwidthreceiving and good image response rejection.4-1-3 1ST MIXER AND 1ST IF CIRCUITSThe 1st mixer circuit converts the received signal to a fixedfrequency of the 1st IF signal with a PLL output frequency.By changing the PLL frequency, only the desired frequencywill pass through two crystal filters at the next stage of the1st mixer.The signals from the RF circuit are mixed at the 1st mixer(Q13) with a 1st LO signal coming from the VCO circuit toproduce a 21.70 MHz 1st IF signal.The 1st IF signal is applied to two crystal filters (FI1 and FI3)to suppress out-of-band signals. The filtered 1st IF signal isapplied to the IF amplifier (Q14), then applied to the 2ndmixer circuit (IC1, pin 16).4-1-4 2ND IF AND DEMODULATOR CIRCUITSThe 2nd mixer circuit converts the 1st IF signal to a 2nd IFsignal. A double conversion superheterodyne system (whichconverts receive signal twice) improves the image rejectionratio and obtain stable receiver gain.The 1st IF signal from the IF amplifier is applied to the 2ndmixer section of the FM IF IC (IC2, pin 16), and is mixed withthe 2nd LO signal to be converted to a 450 kHz 2nd IF sig-nal.The FM IF IC contains the 2nd mixer, limiter amplifier, quad-rature detector and active filter circuits. A 21.25 MHz 2nd LOsignal is produced at the PLL circuit.The 2nd IF signal from the 2nd mixer (IC2, pin 3) passesthrough a ceramic filter (FI2) to remove unwanted hetero-dyned frequencies. It is then amplified at the limiter amplifi-er (IC2, pin 5) and applied to the quadrature detector (IC2,pins 10, 11) to demodulate the 2nd IF signal into AF signals.4-1-5 AF CIRCUITThe AF amplifier circuit amplifies the demodulated AF sig-nals to drive a speaker.AF signals from the FM IF IC (IC2, pin 9) are applied to theanalog swtich (IC4, pin 1) via the AF filter circuit (IC3b, pins6, 7). The output signals from pin 2, 3 are passed throughthe low-pass filter (IC3d, pins 13, 14), and are then appliedto the analog swtich (IC4, pin 9, 10) again. The signals fromthe IC4, pin 11 are applied to the AF power amplifier (IC5,pin 4) after passing through the D/A convertor (IC10, pins12, 11).Mixer16Limiteramp.2nd IF filter450 kHz PLL ICIC1X121.25 MHzIC2 TA31136F12 1st IF from the IF amplifier (Q14)"SD" signal to the CPU pin 98111098 7 5 3AF signal "DET"R5X3R86C122 C121R88R87R83"SQLIN" signal from theD/A convertor (IC10, pin 23)R82C112 C113C116216 1ActivefilterFI2NoisedetectorFMdetector13"NOIS" signal to the CPU pin 19RSSINoisecomp.R84LPF• 2ND IF AND DEMODULATOR CIRCUITS