1-12 (No.RA028)2.4.3.3 APC CircuitThe automatic transmission power control (APC) circuit detects part of a final amplifier output with a coupler circuit and applies a volt-age to IC301.IC301 compares the APC control voltage (APCC) generated by the baseband IC (IC709) and DC amplifier (IC503) with the detectionoutput voltage.IC301 generates the voltage to control IC300 and stabilizes transmission output.The APC circuit is configured to protect over current of Q300 and IC300 due to fluctuations of the load at the antenna end and tostabilize transmission output at voltage and temperature variations.Fig.6 APC circuit and Power amplifier circuit2.4.4 PLL Frequency SynthesizerThe PLL circuit generates the first local oscillator signal for reception and the RF signal for transmission.2.4.4.1 PLL CircuitThe frequency step of the PLL is 2.5, 5.0, 6.25 or 7.5 kHz. A 16.8MHz reference signal is divided at IC2 by a fixed counter to producethe reference frequency.The voltage controlled oscillator (VCO) feedback output is divided by a programmable counter in IC2.The 2 signals are phase compared, filtered through a low pass filter and passed to VCO to control the oscillator frequency.2.4.4.2 VCO CircuitThe operating frequency is generated by Q7 in transmit mode and Q5 in receive mode.The oscillator frequency is controlled by applying the control voltage, which obtained from the phase comparator, to varactor diodes(D5, D6, D9, D10, D11 in transmit mode and D7, D8, D12, D13, D14 in receive mode).The TX/RX pin is set “High” in receive mode causing turn on Q9. And T/R pin is set “Low” in transmit mode causing turn on Q6.Fig.7 PLL and VCO circuit2.4.4.3 Unlock CircuitIf low level appears at the “LD” pin of IC2, then PLL an unlock condition occurs.It causes the voltage applied to the “PLL_UL” pin of the microprocessor to go low.When the microprocessor detects this condition, the transmitter is disabled by ignoring the push-to-talk switch input signal.IC300Q300DRIVE AMP RF POWERMODULEDCAMPIC503ANTSWD305,D306D309,D310LPFANTIC301APCcontrolCouplercircuitD16APCCIC709pin 32D5,D6D9,D10,D11D12,D13,D14ASTCIC2AssistvoltageQ7TX VCOQ11BUFFAMPD7,D8 Q5RX VCOQ6,Q9T/R SWChargepumpOPAMPLPFPhasecomparator1/M1/NREFOSC16.8MHzPLL_DATAPLL_ULMCUIC2: PLL ICQ2AMP
