4 - 3
4-2-5 APC CIRCUIT (MAIN UNIT)
The APC circuit stabilizes transmit output power.
The RF output signal from the power amplifier (IC4) is
detected at the power detector circuit (D21, D22, L31) and
is then applied to one of the differential amplifier inputs
(Q21, pin 3) via the High/Low control circuit (Q22, R135).
The applied voltage controls the differential amplifier output
(Q21, pin 1) and the bias voltage control (Q23). Thus the
APC circuit maintains a constant output power.
4-3 PLL CIRCUITS
4-3-1 GENERAL (MAIN UNIT)
The PLL circuit provides stable oscillation of the transmit fre-
quency and receive 1st LO frequency. The PLL circuit com-
pares the phase of the divided VCO frequency to the refer-
ence frequency. The PLL output frequency is controlled by a
crystal oscillator and the divided ratio of the programmable
divider.
IC8 on the MAIN unit is a dual PLL IC which controls both
VCO circuits for Tx and Rx, and contains a prescaler, pro-
grammable counter, programmable divider, phase detector,
charge pump, etc.
The PLL circuit , using a one chip PLL IC (MAIN unit; IC8),
directly generates the transmit frequency and receive 1st IF
frequency with VCOs. The PLL sets the divided ratio based
on serial data from the CPU on the LOGIC BOARD and
compares the phases of VCO signals with the reference
oscillator frequency. The PLL IC detects the out-of-step
phase and output from pins 8 for Tx and Rx. The reference
frequency (21.25 MHz) is oscillated at X2 (MAIN unit).
4-3-2 TX AND RX LOOP (MAIN UNIT)
The generated signal at the VCO (Q13, Q14, D11–D14)
enters the PLL IC (IC8, pin 2) and is divided at the pro-
grammable divider section and is then applied to the phase
detector section.
The phase detector compares the input signal with a refer-
ence frequency, and then outputs the out-of-phase signal
(pulse-type signal) from pin 8.
The pulse-type signal is converted into DC voltage (lock
voltage) at the loop filter (R211–R213, C252, C254), and is
then applied to varactor diodes (D13, D14) of the VCO to
stabilize the oscillated frequency.
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