System
Components
The block diagram of this configuration is also shown below:
Resistive
Pad
Exciter
Input
S
p
litt
e
r
C
o
m
b
in
er
Low Pass
Filter
RF
Output
RF Output Gain
Control And Protection
Circuit
RF Input
Power Sensor
Forward And Reflected
Power Sensors
RF Final
4 Stages
Thermal
Sensor
Pre
Amp
Module
Figure 8 – Power Amplifier Block Diagram
The input from the exciter (approximately 750 milli-watts and 1.0 Watts) is fed
directly into the Pre-Amp module. The 6dB pad is a resistive pi-attenuator,
which lowers the input power to meet the preamp input requirement as well
as provide optimal input impedance for the amplifier. The pre-amp module is
a high gain amplifier with positive gain control through the use of its bias
voltage. This bias voltage is provided by the PA Control board and is
manually adjustable from the front of the transmitter as well as being
controlled by the PA Control board main processor for VSWR and thermal
shutdown. This information is utilized for a variety of purposes such as fan
control and VSWR validation.
The driver stage is a hybrid module used to provide the required input power
levels to the final stages of the amplifier. It is a self-contained module and is
the final stage of the driver board. Its output is connected to a network of
three hybrid splitters used to provide one to four split. Each output of the
splitter drives the input to a final stage amplifier.
There are four final stage amplifiers used in the amplifier. All four amplifiers
are of equal gain, phase and output power levels. The purpose of these final
stages is again to provide additional gain to the amplifier. The output of each
final stage amplifier is fed to an output combiner, which is used to re-combine
all four signals into one higher power signal.
Each RF. final stage consists of a transistor with a board/copper heat
spreader assembly. This circuit board material is thermally bonded to a
heavy-duty copper heat spreader for added thermal capacity. The heat sink
is instrumented to allow the microprocessor on the Power Amplifier Control
board to continually monitor the thermal performance of the unit.
Version 3
3-12
07/18/01
