3-2-5 IF AMPLIFIER AND MIXER CIRCUITS(MAIN AND RF UNITS)The modulated 3rd IF signal from the DSP board (DTIF: 36kHz) is applied to the 3rd mixer circuit (MAIN unit; IC221).The applied 3rd IF signal is mixed with the 3rd LO signalfrom the DDS circuit (PLL unit; IC701) to produce a 455 kHz2nd IF signal.The 2nd IF signal is output from pin 6 and amplified at the IFamplifier (MAIN unit; Q241). The amplified signal is passedthrough the ceramic bandpass filter (MAIN unit; FI131:FM/AM modes, FI133: other modes) for unwanted signalsare suppressed. The filtered 2nd IF signal is ampllified at IFamplifier (MAIN unit; Q261) and applied to the 2nd mixer circuiton the RF unit via J101.The 2nd IF signal is mixed with the 64 MHz 2nd LO signal,coming from the PLL unit, at the 2nd mixer circuit (RF unit;D741) to obtain a 64.455 MHz 1st IF signal. The 1st IF signalis passed through the MCF (RF unit; FI711) to cut-off theundesired signals then amplified at the IF amplifier (RF unit;Q751) via the T/R switch (RF unit; D711). The amplified 1stIF signal is applied to the 1st IF mixer circuit (RF unit; D771).The operating (transmitting) frequency is produced at the 1stIF mixer circuit (RF unit; D771) by mixing the 1st IF and 1stLO signals. The mixed signal is then applied to the RF circuit.3-2-6 RF CIRCUIT (RF AND PA UNITS)The RF circuit amplifies operating (transmitting) frequencyto obtain 100 W of RF output.The signal from the 1st IF mixer is passed through the lowpassfilter (RF unit; L961, L962, C961–C966) and amplifiedat the RF amplifier (RF unit; IC961). The amplified signal isagain amplified at the wide-band YGR amplifier (RF unit,IC151) after passing through one of 11 bandpass (Refer topage 3-1 for bandpass filters used) and high-pass filters,and is then applied to the PA unit via J151.The signal applied from the RF unit is amplified at the predrive(Q1), drive (Q2, Q3) and power amplifiers (Q4, Q5) insequence to obtain a stable 100 W of RF output power. Theamplified signal is applied to one of 8 low-pass filters in theFILTER unit.3-2-7 LOW-PASS FILTER CIRCUIT (FILTER UNIT)The low-pass filter circuit contains 8 Chebyschev low-passfilters to suppress the higher harmonic components.The signal from the power amplifiers in the PA unit is appliedto one of the low-pass filters, which is selected by the I/Oexpander (IC11) in the CTRL unit via the buffer-amplifier(CTRL unit; IC12).The filtered signal is then applied to one of 2 antenna connectorsvia the CTRL only/and TUNER unit/s.3-2-8 ALC CIRCUIT (MAIN UNIT)The ALC (Automatic Level Control) circuit controls the gainof IF amplifiers in order for the transceiver to output a constantRF power set by the [RF POWER] control even whenthe supplied voltage shifts, etc.The RF power level is detected at one of the APC detectorcircuits (CTRL unit; D2) to be converted into DC voltage andapplied to the MAIN unit as the FORV signal.The FORV signal from the CTRL unit is applied to the comparator(IC551b, pin 6). The POCV signal, controlled by the[RF POWER] control via the I/O expander (IC3751, pin 5), isalso applied to the other input (pin 5) for reference. Thecompared signal is output from pin 7 and applied to the IFamplifiers in the MAIN (Q261) and RF (Q751) units to controlamplifying gain.When the FORV signal exceeds the POCV voltage, ALCbias voltage from the comparator controls the IF amplifiers.This adjusts the output power to a specified level from the[RF POWER] control until the FORV and POCV voltages areequalized.In AM mode, the comparator operates as an averaging ALCamplifier. Q502 turns ON and the POCV voltage is shiftedfor 40 W AM output power (maximum) through R510.