Icom IC-F70DT Service Manual

Also see for IC-F70DS: Operating guide Instruction manual Instruction manual Instruction manual

Contents

Icom IC-F70DT Service Manual Manual pdf 9 page image

4 - 34-2 TRANSMITTER CIRCUITS4-2-1 MICROPHONE AMPLIFIER CIRCUITThe microphone amplifier circuit amplifies the audio signalsfrom microphone within +6 dB/oct pre-emphasis characteris-tic. The microphone signals are processed in the base bandIC which contains microphone amplifier, compressor, scram-bler, limiter, splatter filter, etc. in its package.The audio signals from the microphone (FRONT UNIT;MC201) are passed through the microphone mute switch(FRONT UNIT; IC204). The switched signals are amplified atthe microphone amplifiers (FRONT UNIT; IC203, pins 1, 2,13, 14) to obtain within +6 dB/oct pre-emphasis characteris-tics. The amplified signals are applied to the MAIN UNIT viaJ1 (pin 2).The amplified MIC signals from the FRONT UNIT areapplied to the base band IC (IC301, pin 3). The appliedMIC signals are amplified at the amplifier section, and leveladjusted at the volume control section. The level adjustedMIC signals are applied or bypassed the compressor sec-tion, pre-emphasis section, TX/RX HPF, de-scrambler, lim-iter, splatter, in sequence, then applied to another volumecontroller.The compressor compresses the MIC signals to providehigh S/N ratio for receive side, and the pre-emphasis obtains+6 dB/oct audio characteristics. The TX/RX HPF filters out250 Hz and lower audio signals, the limiter limits its leveland the splatter filters out 3 kHz and higher audio signals.The filtered MIC signals are level adjusted at another vol-ume control section and amplified at the amplifier section,and then output from pin 7 via smoothing section (SMF).4-2-2 MODULATION CIRCUITThe modulation circuit modulates the VCO oscillating signalwith the audio signals from the microphone.MIC signals from the base band IC (IC301) are passedthrough the MIC switch (IC302, pins 5, 4), PM filter (C338,R327), FM/PM switch (IC302, pins 1, 15), and then appliedto the AF mixer (IC12, pin 2) to be mixed with CTCSS/DTCSsignals.The mixed MIC signals are applied to the D/A converter(IC303, pin 4) to be adjusted its level. The level adjusted AFsignals are output from pin 3 and applied to the modulationcircuit (D8) to modulate the VCO oscillating signal by chang-ing the reactance of D8 at the TX VCO (Q5, D6, D7, D25).The CTCSS/DTCS signals are generated by the CPU (IC307)and output from pins 89–91 (“CENC0,” “CENC1,” ”CENC2”).The CTCSS/DTCS signals are passed through 3 regis-ters (R374–R376) to change its wave form. The wave formchanged CTCSS/DTCS signals are then passed through theLPF (IC12, pins 8, 10) and applied to the converter (IC303,pin 9) to be adjusted its level, and output from pin 10.The level adjusted CTCSS/DTCS signals are applied tothe AF mixer (IC12, pin 2) to be mixed with MIC signals.The mixed CTCSS/DTCS signals are output from pin 1 andapplied to the D/A converter (IC303, pin 4) to be adjusted itslevel again, then output from pin 3. The CTCSS/DTCS sig-nals from the D/A converter are applied to the both of refer-ence frequency oscillator (X1) and modulation circuit (D8) tomodulate the reference frequency signal and VCO oscillatingsignal.The modulated VCO output signal is amplified at the bufferamplifiers (Q6, Q10) and is then applied to the YGR ampli-fier (Q11) via the TX/RX switch (D10).4-2-3 TRANSMIT AMPLIFIERSThe VCO output signal is amplified to transmit output powerlevel by the transmit amplifiers .The buffer-amplified signal from the TX/RX switch is appliedto the YGR (Q11), the driver (Q12), and power (Q13)amplifiers, to be amplified to the transmit output power level.The power amplified transmit signal is passed through thepower detector (D11), antenna switch (D12), and two-stageLPFs (L22, L23, C204–C207, C209), and then applied to theantenna connector (CHASSIS UNIT; J1).4-2-4 ALC CIRCUITThe ALC (Automatic Level Control) circuit stabilizes transmitoutput power and controls transmit output power High or Low.The power detector circuit (D11) detects the transmit outputsignal and converts it into DC voltage.The detected voltage is applied to the ALC amplifier (IC5,pin 3). The “T2” signal from the D/A converter (IC310, pin2), controlled by the CPU (IC307), is applied to the anotherinput (pin 1) for reference, and the "T2" signal also controlstransmit output power (5 W or 1 W).The output voltage from the ALC amplifier controls the biasof the YGR (Q11), driver (Q12) and power amplifier (Q13) toreduce the output power by comparing the detected voltageand the reference voltage. Thus the ALC circuit maintains aconstant transmit output power.Poweramp.ALCamp.Driveamp.+–VCC• ALC CIRCUITto anntena switch(D12, D22)T2TMUTfrom TX/RX switch(D9, D10)T5VQ12YGRamp.Q11IC5Q13LPF LPFD11Power detecter