Motorola MTR2000 T5544 Instruction / Field Service Manual page 49

Description
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which is derived from power control signals (from the SCM) and high VSWR/thermal pro-
tection circuitry on the PA output board.
A combination of hardware and software controls are used to regulate the power output level.
To set the power and current limits, the SCM provides software control through a D/A con-
verter connected to the SPI bus. This control relies on various monitored PA signals which
are fed back to the SCM via an A/D converter (also connected to the SPI bus).
The directional coupler is essentially a calibrated wattmeter which feeds a dc voltage propor-
tional to the output power to the power control circuitry to serve as the feedback signal in the
power control loop. Under normal operating conditions, the power control circuitry compares
this dc voltage from the directional coupler to a reference voltage from the D/A converter
which represents the desired output power. Based on the comparison, a power control voltage
is generated to control the output power from the PA Module.
The modulated rf signal is amplified by the Driver/Final Module and is output to the site trans-
mit antenna via a circulator and a harmonic filter/coupler. During excessive output VSWR,
the ratio of the forward and reflected voltages from the directional coupler may be used to re-
duce, or turn off, the transmitter power. Additional circuitry is also provided to reduce output
power during excessive current drain and high temperature conditions, and to control the fan
used in high power PA Modules.
68P81096E36-H
06/28/05