Motorola GM328 Service Manual page 422

2-6
The PCIC provides a dc output voltage at pin 4 (INT) and applied as CNTLVLTG to the power-adjust
input pin of the first transmitter stage U4401. This adjusts the transmitter power output to the
intended value. Variations in forward or reflected transmitter power cause the dc 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 C4502-4, 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.
U4502 is a temperature-sensing device, which monitors the circuit board temperature in the vicinity
of the transmitter driver and final devices, and provides a dc voltage to the PCIC (TEMP, pin 29)
proportional to temperature. If the dc voltage produced exceeds the set threshold in the PCIC, the
transmitter output power is reduced so as to reduce the transmitter temperature.
4.0 Frequency Synthesis
The synthesizer subsystem consists of the reference oscillator (Y4261 or Y4262), the low voltage
fractional-N synthesizer (LVFRAC-N, U4201), and the Voltage Controlled Oscillator VCO.
4.1 Reference Oscillator
The reference oscillator (Y4262) contains a temperature compensated crystal oscillator with a
frequency of 16.8 MHz. An Analog-to-Digital (A/D) converter internal to U4201 (LVFRAC-N) and
controlled by the µP via serial interface (SRL) sets the voltage at the warp output of U4201, pin 25 to
set the frequency of the oscillator. The output of the oscillator (pin 3 of Y4262) is applied to pin 23
(XTAL1) of U4201 via an RC series combination.
In applications where less frequency stability is required the oscillator inside U4201 is used along
with an external crystal Y4261, varactor diode D4261, C4261, C4262 and R4262. In this case,
Y4262, R4263, C4235 and C4251 are not used. When Y4262 is used, Y4261, D4261, C4261,
C4262 and R4262 are not used, and C4263 is increased to 0.1 uF.
Frequency Synthesis