Motorola MTR2000 Installation And Operation Manual page 18

Description
6
between the Pre-driver stage and the Final Module. The gain of the Pre-driver stage
is controlled by a power control voltage which is derived from power control signals
(from the SCM) and high VSWR/thermal protection circuitry on the PA output
board.
A combination of hardware and software controls are used to regulate the power out-
put level. To set the power and current limits, the SCM provides software control
through a D/A converter 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 con-
nected to the SPI bus).
The directional coupler is essentially a calibrated wattmeter which feeds a dc voltage
proportional to the output power to the power control circuitry to serve as the feed-
back signal in the power control loop. Under normal operating conditions, the pow-
er control circuitry compares this dc voltage from the directional coupler to a
reference voltage from the D/A converter which represents the desired output pow-
er. Based on the comparison, a power control voltage is generated to control the out-
put power from the PA Module.
The modulated rf signal is amplified by the Driver/Final Module and is output to the
site transmit 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 reduce, or turn off, the transmitter power. Additional circuit-
ry 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