Motorola PRO Series, Detailed Service Manual

Page: 535 / 686

UHF Band 2 (450–512/520 MHz) 25–40 W Bipolar Frequency Synthesis Section 10: 2-5
6881091C63-F
3.7 Harmonic Filter
Inductors L5491, L5492, and L4493 along with capacitors C5448, C5493, C5494, C5496, and
C5498 form a low-pass filter to attenuate harmonic energy from the transmitter. Resistor R5491
drains any electrostatic charges that might otherwise build up on the antenna. The harmonic filter
also prevents high level RF signals above the receiver passband from reaching the receiver circuits
to improve spurious response rejection.
3.8 Power Control
The transmitter uses the power control IC (PCIC, U5501) to control the power output of the radio. A
portion of the forward RF power from the transmitter is sampled by the bi-directional coupler and
rectified to provide a DC voltage to the RFIN port of the PCIC (pin 1) that is proportional to the
sampled RF power.
The PCIC has internal digital-to-analog converters (DACs) which provide a reference voltage to the
control loop. The reference voltage level is programmable through the SPI line of the PCIC and is
proportional to the desired power setting of the transmitter. Factory programming at several points
across the frequency range of the transmitter is used to offset frequency response variations of the
transmitter’s power detector circuits.
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 U5401. 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.
Capacitor C5502-4, 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.
U5502 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 to decrease the transmitter temperature.
4.0 UHF Band 2 (450–512/520 MHz) 25–40 W Bipolar Frequency
Synthesis
The synthesizer, shown in Figure 10-3, consists of a reference oscillator (Y5261 or Y5262), low
voltage LVFRAC-N synthesizer (U5201), and a voltage controlled oscillator (VCO).
4.1 Reference Oscillator
The reference oscillator (Y5262) contains a temperature compensated crystal oscillator with a
frequency of 16.8 MHz. An analog-to-digital (A/D) converter internal to U5201 (LVFRAC-N) and
controlled by the µP via serial interface (SRL) sets the voltage at the warp output of U5201, pin 25 to
set the frequency of the oscillator. The output of the oscillator (Y5262, pin 3) is applied to pin 23
(XTAL1) of U5201 via an RC series combination.