Frequency Synthesis; Reference Oscillator; Fractionai-N Synthesizer - Motorola CDM1550 LS+ Detailed Service Manual

2-28
Theory of Operation
programmed at several points across the frequency range of the transmitter to offset frequency
response variations of the transmitter's power detector circuits.
The PCIC provides a de output voltage at pin 4 (I NT) which sets the drain current of the first stage
(U3401) via current control opamp U3503-1. This adjusts the transmitter power output to the
intended value. Variations in forward transmitter power cause the de voltage at pin 1 to change, and
the PCIC adjusts the control voltage above or below its nominal value to raise or lower output power.
Capacitors C3501-3, in conjunction with resistors and integrators within the PCIC, control the
transmitter power-rise (key-up) and power-decay (de-key) characteristic to minimize splatter into
adjacent channels.
U3502 is a temperature-sensing device, which monitors the circuit board temperature in the vicinity
of the transmitter driver and final devices, and provides a de voltage to the PCIC (TEMP, pin 29)
proportional to temperature. If the de voltage produced exceeds the set threshold in the PCIC, the
transmitter output power is reduced so as to reduce the transmitter temperature.
The power control circuitry includes a safety switch (S3501) that consists of a conductive portion of
the rubber pad which pushes the RF final transistor 03440 against the chassis. When the chassis
cover and rubber pad are properly assembled, the conductive portion of the pad contacts a pattern
on the circuit board, electrically closing switch S3501. If the cover and pad are not in place, switch
S3501 is open-circuited and the power control circuit is disabled. This prevents thermal damage of
the RF final transistor which might occur if the transmitter is operated without adequate heatsinking .
2.9 Frequency Synthesis
The synthesizer subsystem consists of the reference oscillator, the low voltage fraction a I-N
synthesizer (LVFRAC-N), and the Voltage Controlled Oscillator VCO.
2.9.1 Reference Oscillator
The reference oscillator (Y3201) contains a temperature compensated crystal oscillator with a
frequency of 16.8 MHz. An Analog-to-Digital (AID) converter internal to U3201 (LVFRAC-N) and
controlled by the IJP via serial interface (SRL) sets the voltage at the warp output of U3201, pin 25 to
set the frequency of the oscillator. The output of the oscillator (pin 3 of Y3201) is applied to pin 23
(XTAL 1) of U3201 via an RC series combination.
2.9.2 Fractionai-N Synthesizer
The LVFRAC-N synthesizer IC (U3201) consists of a pre-scaler, a programmable loop divider,
control divider logic, a phase detector, a charge pump, an A/D converter for low frequency digital
modulation, a balanced attenuator to balance the high frequency analog modulation and low
frequency digital modulation, a 13V positive voltage multiplier, a serial interface for control, and
finally a super filter for the regulated 5 volts.
A voltage of 5V applied to the super filter input (U3201 pin 30) supplies an output voltage of 4.5 Vdc
(VSF) at pin 28. It supplies the VCO and the synthesizer charge pump resistor network. The
synthesizer supply voltage is provided by the 5V regulator U3202.
In order to generate a high voltage to supply the phase detector (charge pump) output stage at pin
VCP (U3201 pin 47), a voltage of 13 Vdc is being generated by the positive voltage multiplier
circuitry (D3201 and C3212-13). This voltage multiplier is basically a diode capacitor network driven
by two signals (1.05MHz) 180 degrees out of phase (U3201-14 and -15).
_)