Icom IC-U82 Service Manual

Also see for IC-U82: Instruction manual Addendum Specifications Instruction manual Instruction manual

Contents

4 - 1SECTION 4 CIRCUIT DESCRIPTION• 2ND IF AND DEMODULATOR CIRCUITSMixer16LimiterAMP45.9 MHz 2nd LO signalIC2 TA31136FN1st IF signalfrom the IF amplifier (Q14)111098 7 5AF "DET" signalto the AF circuits"SQLIN" signalfrom the D/A converter (IC10, pin 23)15.3 MHzQ8122 1ActivefilterNoisedetectorFMdetector13"NOIS" signal to the CPU (IC8, pin 26)PLLIC(IC19)R5“DIGI_W” signal from the CPU (IC8, pin 81)×3 X1NoiseAMPNoisecomparatorX3Q65FI234-1 RECEIVER CIRCUITS4-1-1 ANTENNA SWITCHING CIRCUITThe antenna switching circuit toggles receive line and transmitline. This circuit does not allow transmit signals to enter thereceiver circuits.Received signals from the antenna connector (J1: CHASSISUNIT) are passed through the low-pass filter (LPF: L1, L2,C1–C5) and applied to the /4 type antenna switching circuit(D2, D8).While receiving, no voltage is applied to D2 and D8. Thus,the receive line and the ground are disconnected and L15,L58, C52, C76 and C516 function as a two-stage LPF whichleads received signals to the RF circuits via the limiter (D50).4-1-2 RF CIRCUITSThe RF circuits amplify received signals within the range offrequency coverage and filters off out-of-band signals.The signals from the antenna switching circuit are amplifiedat the RF amplifier (Q12).The amplified signals are passed through the two-stageBPF (D11, D12, D65, L19, L57, C92, C94, C96, C97, C502–C505, C507) to suppress unwanted signals. The filtered sig-nals are then applied to the 1st mixer circuit.4-1-3 1st MIXER AND 1st IF AMPLIFIER CIRCUITSThe 1st mixer circuit converts received signals into the 1stintermediate frequency (IF) signal by mixing with the localoscillator (LO) signal. The converted 1st IF signal is filteredat the 1st IF filter, then amplified at the 1st IF amplifier.The signals from the two-stage BPF are converted into the46.35 MHz 1st IF signal at the 1st mixer (Q13) by beingmixed with the 1st LO signals generated at the VCO (Q76,D59, D60).The 1st IF signal from the 1st mixer is passed through thecrystal filter (FI1) to suppress unwanted signals, and thelimiter (D63) and then applied to the 1st IF amplifier (Q14).The amplified 1st IF signal is applied to the FM IF IC (IC2,pin 16).4-1-4 2nd IF AND DEMODULATOR CIRCUITSThe 1st IF signal is converted into the 2nd IF signal anddemodulated by the FM IF IC. The FM IF IC contains 2ndmixer, limiter amplifier, quadrature detector, etc. in its package.The 1st IF signal from the 1st IF amplifier is applied to theFM IF IC (IC2, pin 16), and mixed with the 45.9 MHz 2nd LOsignal from the tripler (Q81) at the mixer section, to convertinto the 450 kHz 2nd IF signal. The 2nd IF signal is outputfrom pin 3 and is filtered by the ceramic filter (FI2) to sup-press the heterodyne noise. The filtered signal is applied toIC2 (pin 5) again and amplified at the limiter amplifier sectionand demodulated by the quadrature detector.The quadrature detector is a detection method which uses aceramic discriminator (X3). Q65 is a mode switch and togglesthe detecting mode from wide and narrow, controlled by the"DIGI_W" signal from the CPU (IC8, pin 81).The demodulated AF signals are output from pin 9.4-1-5 AF CIRCUITSThe AF signals from FM IF IC (IC2, pin 9) are passedthrough the AN switch (IC14, pins 10, 11), HPF (IC3, pins6, 7), analog switch (IC4, pins 1, 2), the LPF (IC3, pins13, 14) and analog switch (IC4, pins 10, 11). The signalsfrom the analog switch are applied to the D/A converter(IC10, pins 11, 12) to adjust the AF output level, and thenpassed through the emphasis switch (Q73) whitch togglesthe AF response characteristic according to the selectedoperating mode (analog/FM or digital). The AF signals fromthe emphasis switch are applied to the AF amplifier (IC5)to obtain 300 mW of AF output power. The amplified AFsignals are applied to the internal speaker (SP1) via [SP]jack (J3).