Phase Lock Loop Synthesizer; Figure 2-3. Pll Synthesizer Block Diagram - Motorola CP185 Basic Service Manual

Theory Of Operation: Major Assemblies
2.2.3

Phase Lock Loop Synthesizer

The Phase Lock Loop (PLL) synthesizer subsystem consists of the reference oscillator (VCTCXO),
VCO, PLL IC, Charge pump and Loop filter.
FL701
Modulating
Signal
12.8MHz
Ref.OSC.
VCTCXO (Voltage Controlled Temperature Compensated crystal Oscillator) reference frequency
(12.8 MHz) provides reference to PLL IC, with stability of +/-2.5PPM at -30° to +60°C. This reference
frequency is divided to 6.25 kHz or 5 kHz by PLL IC. PLL IC outputs 2 Signals (P & R) depending on
phase difference. A charge pump is used to charge these output signals from 0 – 3.3 V up to
0 – 10 V which is required to control the VCO. A voltage doubler (U507) converts 5 V to 10 V to
supply the necessary voltage for a higher frequency resolution in VCO. The Loop filter is a Low Pass
filter (C751 – C754, R726 – R728) to reduce the residual side-band noise of VCO Reference
Frequency for the best signal-to-noise ratio. The VCO module contains both RX VCO and TX VCO,
configured as Collpits oscillators and connects to DC power through cascaded buffers. Q705 and
Q305 enable RX VCO when RX_EN is high. Q706 and Q503 enable TX VCO when TX_EN1 is high.
The input audio signal for TX VCO is from (U501-B) and applied to a varactor diodes (CR703) in
TX VCO to be modulated into TX RF signal.
Synthesizer Charge
U701 Pump
Voltage
PLL Data
Doubler
from μP

Figure 2-3. PLL Synthesizer Block Diagram

TX VCO
Circuit
TX Out
Loop
Filter
RX VCO
Circuit
2-5
To PA Driver
To Mixer
RX Out