PHASE-LOCKED LOOP FREQUENCY SYNTHESIZER
The transmitter output frequency and the receiver operating frequency are controlled by the phase-locked loop frequency
In the receive mode, a voltage controlled oscillator (QV02, VCO) is oscillating at the receiver first local frequency, and
the signal is amplified by QV03, QP71, and QP81 to be used as first local injection. The signal is also used to obtain a
control signal for the VCO through the logic circuit.
First the signal is mixed down with the receive local frequency (43.63 MHz, QP34) and pre-scaled by a divider (QP06),
then fed to a multi-function integrated circuit (QPO1) to be further counted down to 6.25 kHz when locked on signals.
FIGURE 3. QP01 FUNTIONAL DIAGRAM
The front panel rotary switch controls the programmable counter to give the right division number to the integrated
circuit. The reference oscillator and the divider are also located in QPO1, which obtain a 6.25 kHz reference signal by
dividing 12.80 MHz.
After these two signals are compared in the phase Comparator in the IC, the output (control voltage) is fed back to the
VCO through a low pass filter to keep the oscillator frequency locked on.
The basic function of the PLL synthesizer for transmit operation is similar to receive operation, with two exceptions. The
local oscillator is switched to TX (50.76 MHz) and the programmable counter programming is changed by an added
circuit (QC02 and QCO1), thus the VCO is generating the transmit frequency. In the TX mode, the VCO signal is
modulated by the audio signal from the microphone amplifier (direct FM).