Icom IC-F4021T Service Manual

Also see for IC-F4021S: Instruction manual Instruction manual Instruction manual

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Icom IC-F4021T Service Manual Manual pdf 8 page image

SECTION 5 CIRCUIT DESCRIPTION5 - 15-1 RECEIVER CIRCUITS5-1-1 ANTENNA SWITCHING CIRCUIT (PA UNIT)The antenna switching circuit toggles the receive (RX) line andtransmit (TX) line.The received signals from the antenna are passed throughthe low-pass filter (ANT UNIT; L801, L802, C802–C806) andantenna switch (D701, D704).While transmitting, the voltage on the T5V line is appliedto D701 and D704, and these are ON. Thus the TX line isconnected to the antenna. Simultaneously, the RX line isconnected to the ground (GND) to prevent transmit signalentering.While receiving, no voltage is applied to the D701 andD704, and these are OFF. Thus the TX line and the antennaare disconnected to prevent received signals entering.Simultaneously, the RX line is disconnected from the GND andthe received signals are passed through the low-pass filter(L712, C750, C751). The fi ltered signals are applied to the RFcircuits.5-1-2 RF CIRCUITS (MAIN UNIT)RF circuits filter and amplify the received signals within thefrequency coverage.The received signals from the antenna switching circuit arepassed though the two-staged bandpass filter (BPF; D19,D25, L7, L8, C21–C23, C25, C27–C29) to fi lter-out unwantedsignals, and the fi ltered signals are applied to the RF amplifi er(Q5). The amplifi ed received signals are then applied to the 1stmixer (Q6) via another two-staged BPF (D14, D15, L11, C39–C41, C44, C45).5-1-3 1st IF CIRCUITS (MAIN UNIT)The received signals are converted into the 1st IF signal,fi ltered and amplifi ed at the 1st IF circuits.The received signals from the two-staged BPF (D14, D15,L11, C39–C41, C44, C45) are applied to the 1st mixer (Q6)and converted into the 46.35 MHz 1st IF signal by being mixedwith the local oscillator (LO) signal from the RX VCO (Q17, D9,D11).The converted 1st IF signal is passed through the 1st IF fi lter(FI1) to filter-out adjacent signals, then applied to the 1st IFamplifi er (Q7). The amplifi ed 1st IF signal is then applied to theFM IF IC (IC9, pin 16).5-1-4 2nd IF AND DEMODULATOR CIRCUITS(MAIN UNIT)The 1st IF signal is converted into the 2nd IF signal, anddemodulated.The 1st IF signal from the 1st IF amplifi er is applied to the 2ndmixer in the FM IF IC (IC9, pin 16), and converted into the450 kHz 2nd IF signal by being mixed with the 45.9 MHz 2ndLO signal from the reference frequency oscillator (X2) via thetripler (Q22) and BPF (L33, C163, C164, C166). The converted2nd IF signal is output from pin 3, and passed through the 2ndIF fi lters to remove sideband noise.The 2nd IF signal is passed through the 2nd IF filter (FI2)and applied to the FM IF IC (IC9, pin 5) again via the resistor(R56).The filtered 2nd IF signal is amplified at the limiter amplifier,and FM-demodulated by the quadrature detector (IC9, pins 10,11, X1). The demodulated AF signals are output from pin 9,then applied to the AF amplifi er circuits.5-1-5 AF AMPLIFIER CIRCUITS (MAIN UNIT)The demodulated AF signals from the FM IF IC are amplifi edand fi ltered at AF circuits.The demodulated AF signals from the FM IF IC (IC9, pin 9) arepassed through high-pass fi lter (HPF; IC5, pins 2, 1) to removetone signals. The filtered AF signals are passed through thede-emphasis circuit (R142, C249) to obtain the 6 dB/oct offrequency characteristic. The de-emphasized AF signals arepassed through the RX mute switch (Q32, Q33), AF switch(Q36, Q37), HPF (IC5, pins 13, 14), analog switch (IC3, pins 1,2), AF mixer (IC5, pins 6, 7) and analog switch (IC3, pins 10,11) in sequence.D/A converter(IC12)MixerRSSIQuadraturedetector24231st IF signal from the 1st IF amplifier (Q7)16NoisedetectorR5VX11110FM IF IC (IC9)Filteramp. Noiseamp.Limiteramp.Demodulated signalsto the AF circuits9“NOIS” signal to the CPU (IC22: pin 75)“RSSI” signal to the CPU (IC22: pin 50)1312Q22X215.3 MHz45.9 MHz BPF238 7 35FI2R56TCXO• 2nd IF AND DEMODULATOR CIRCUITS