4.2.2 High Frequency Power Amplifier Circuit:
When the mode turns to be in a state of transmitting by turning the PTT switch of the microphone, frequency of PLL is set up to the transmitting frequency .The range of oscillation frequency of VCO is shifted by this transmitting/receiving shift circuit (Q31). This high frequency signal of VCO undergoes the amplification in the buffer amplifier Q17 and driving amplifiers Q4, Q3. After being
4.2.3 APC Circuit:
The output power is detected as a direct current signal by APC detecting circuit (D3). APC control circuit composed of Q1, Q2 and IC2 controls the transmitting output power, together with 25W/1W switching circuit (Q5).
4.2.4 Antenna Monitoring Circuit:
After passing through LPE which depresses harmonic waves, the amplified transmitting output power is outputted as an antenna output power via a split line on the PC board pattern and an antenna monitoring circuit composed of D311 and D312. After being detected at D311 and D312, the direct current signals are amplified at SWR REF AMP (IC30B) and SWR FWD AMP (IC30A) respectively. Then they are converted to digital signals by the A/D converter in CPU. The antenna monitoring circuit always watches the condition of the antenna by observing these digital signals.
If the antenna is normal: | VSWR - R < VSWR - F |
If the antenna is open or short: | VSWR - R ≥ VSWR - F |
4.3. PLL CIRCUIT OPERATION
The oscillation circuit of PLL IC (IC9) oscillates 12.8MHz frequency by the crystal oscillator (XTAL1) attached to it. This 12.8MHz frequency is divided into 1/512 by the divider inside the IC to make 25KHz frequency, which is the reference frequency of PLL. Frequencies are set up at the time of transmitting and receiving respectively by data from CPU (DATA, CLK, STB). Error signals from PLL pass through the loop filter (LPF). Frequencies of these error signals are controlled by variable capacity diode (DIG) of VCO (Q15).
27