136 – 174 MHz VHF Theory of Operation: VHF Frequency Generation Circuitry 8-5
8.4.2 PLL IC Prescaler and Comparator
The reference frequency from VCTCXO is divided to 6.25 kHz or 5 kHz by reference counter, R. The
RF signal input from the VCO is divided to by prescaler (1/64), divided by N and A counters in PLL IC
to determine frequency steps and then supplied to the comparator. The comparison frequency is
6.25/5 kHz. The internal phase comparator compares the phase difference between the reference
and VCO signal. When the phase of the reference frequency is leading, Pin15 (R) is the output.
When VCO frequency is leading, Pin16 (P) is the output. When P=R, small pulses are the output of
the phase detector.
8.4.3 Voltage Doubler and Charge Pump
The voltage doubler (U507) converts 5 V to 10 V and is applied to the charge pump circuitry. The
charge pump is used for charging output signals P, R supplied by PLL IC from 0 – 3.3 V to 0 – 10 V.
This voltage is used to drive the VCO.
8.4.4 Loop Filter
The loop filter contains C751 – C754, R726 – R728. It reduces the residual side-band noise to get
the best signal-to-noise ratio. The output signal from loop filter is applied to VCO.
8.4.5 Dual VCO
The dual VCO module contains a RX VCO and a TX VCO. They are configured as colpitts oscillators
and connected to power up through transistor switches. Only one VCO is selected at a time. A
steering line voltage between 0.35 V and 9.7 V at varactor CR701 tunes the full RX frequency range
from 181.1 MHz to 219.1 MHz, and varactor CR703 tunes the full TX frequency range from 136 MHz
to 174 MHz.
In Receiver mode, high signal of RX_EN from pin 71, U101 activates Q305. When Q305 is activated,
current flows through the base of Q304 and thus activates the Q705. The varactor CR701 sets the
resonance frequency. When there is a change in voltage supplied by loop filter, there is a change in
the resonance frequency. L703 is the resonating coil, which forms the tank circuit together with
variable cap C710.
In Transmit mode, high signal of TX_EN1 from pin 84, U101 enables current flows through collector
of Q503 and thus activates Q706. The varactor CR703 sets the resonance frequency. When there is
a change in voltage supplied by loop filter, there is a change in the resonance frequency. L706 is the
resonating tuning coil, which forms the tank circuit together with variable cap C722.
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