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

2-4
The APC (Auto Power Control) keeps the current supplied to Final PA (Q403) constant. Resistor,
R417 is used for current sensing. The voltage difference ratio of R423 to R417 is amplified through
U401 and passed to Q404 and Q405 to produce constant power output to the antenna. Do not
exceed the maximum allowed bias voltage of the device.
Note: Retune the TX Power if Final PA (Q403) is replaced. Refer Chapter 5.5: Transmitter
Alignment Options on page 5-3.
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.
PLL DATA
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 variable capacitor diode
(CR703) in TX VCO to be modulated into TX RF signal.
To Transmit
U701
PLL IC
FL701
VCTCXO

Figure 2-3. PLL Synthesizer Block Diagram

5V
VCO
Loop
filter
Charge
10V
U507
Pump
3.3V
U506
REG. 3.3V
AUDIO
[From filter (U501)]
Theory Of Operation: Major Assemblies
REG. 5V
U505
Voltage
Doubler