Icom IC-F50 Service Manual

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Icom IC-F50 Service Manual Manual pdf 7 page image

4 - 1SECTION 4 CIRCUIT DESCRIPTION4-1 RECEIVER CIRCUITS4-1-1 ANTENNA SWITCHING CIRCUIT(MAIN UNIT)The antenna switching circuit functions as a low-pass filterwhile receiving and a resonator circuit while transmitting.This circuit does not allow transmit signals to enter thereceiver circuits.Received signals enter the antenna connector (CHASSIS;J1) and pass through the low-pass filter (L1, L2, C1–C5).The filtered signals are passed through the λ⁄4 type antennaswitching circuit (D5, D6, L5, L6) and then applied to the RFcircuit.4-1-2 RF CIRCUIT (MAIN UNIT)The RF circuit amplifies signals within the range of frequen-cy coverage and filters out-of-band signals.The signals from the antenna switching circuit pass throughthe two-stage tunable bandpass filters (D4, D8, L7, L8). Thefiltered signals are amplified at the RF amplifier (Q2) andthen passed through the another two-stage tunable band-pass filters (D9, D10, L9, L11) to suppress unwanted sig-nals. The filtered signals are applied to the 1st mixer circuit.D4, D8–D10 employ varactor diodes, that are controlled bythe CPU via the D/A converter (IC6), to track the bandpassfilter. These varactor diodes tune the center frequency of anRF pass band for wide bandwidth receiving and good imageresponse rejection.4-1-3 1ST MIXER AND 1ST IF CIRCUITS(MAIN UNIT)The 1st mixer circuit converts the received signal into fixedfrequency of the 1st IF signal with the PLL output frequency.By changing the PLL frequency, only the desired frequencypasses through a crystal filter at the next stage of the 1stmixer.The RF signals from the bandpass filter are mixed with the1st LO signals, where come from the RX VCO circuit via theattenuator (R26–R28), at the 1st mixer circuit (Q3) to pro-duce a 46.35 MHz 1st IF signal. The 1st IF signal is passedthrough a monolithic filter (FI1) in order to obtain selectioncapability and to pass only the desired signals. The filteredsignal is applied to the 2nd IF circuit after being amplified atthe 1st IF amplifier (Q4).4-1-4 2ND IF AND DEMODULATOR CIRCUITS(MAIN UNIT)The 2nd mixer circuit converts the 1st IF signal into a 2nd IFsignal. The double-conversion superheterodyne system(which convert receive signals twice) improves the imagerejection ratio and obtains stable receiver gain.The 1st IF signal from the IF amplifier (Q4) is applied to the2nd mixer section of the FM IF IC (IC1, pin 16), and is mixedwith the 2nd LO signal to be converted into a 450 kHz 2ndIF signal.The FM IF IC (IC1) contains the 2nd mixer, 2nd local oscil-lator, limiter amplifier, quadrature detector, active filter andnoise amplifier circuits. A 2nd LO signal (45.9 MHz) is pro-duced at the PLL circuit by tripling it’s reference frequency(15.3 MHz).The 2nd IF signal from the 2nd mixer (IC1, pin 3) passesthrough the ceramic filter (FI2) to remove unwanted hetero-dyned frequencies. It is then amplified at the limiter amplifi-er section (IC1, pin 5) and applied to the quadrature detec-tor section (IC1, pins 10, 11) to demodulate the 2nd IF sig-nal into AF signals.The demodulated AF signals are output from pin 9 (IC1) andapplied to the AF circuit via the receiver mute circuit.Mixer16Limiteramp.2nd IF filter450 kHz X215.3 MHz45.9 MHzIC1 TA31136FN12 1st IF from the IF amplifier (Q4)"RSSI" signal to the CPU (FRONT unit; IC401, pin 50)111098 7 5AF signal "DET""SQIN" signal from theD/A converter IC(IC6, pin 2)R5VX22ActivefilterNoisedetectorFMdetector13"NOIS" signal to the CPU (FRONT unit; IC401, pin 41)RSSINoisecomp.×3 PLL ICIC4Q1912FI23• 2ND IF AND DEMODULATOR CIRCUITS