GL-T8541 PA
Glenayre Document Number: 9110.01002
THEORY OF OPERATION
09/10/99
Page: 6-6
Copyright © 1997 Glenayre
6.5 3 to 1 Combiner Board
Refer to Figure 6-4, for a block diagram of the 3 to 1 combiner board. Refer to Figure 6-18
and Figure 6-19 for detailed circuit board assembly and schematic diagrams. This board
contains the 3 to 1 PA combiner circuitry, the low pass filter, the forward and reflected
directional couplers, and also functions as the interface between the metering board and the
driver and PA boards.
6.5.1 The 3 to 1 Combiner
A 3-way Wilkinson Hybrid is used as the 3 to 1 combiner on the combiner board. This is a
microstrip combiner circuit that combines the three, in phase outputs of the three power
amplifier boards into one signal.
If one PA fails, the remaining power amplifier boards will continue to see an acceptable
load impedance and function normally. The AGC circuit will increase the output of the
remaining power amplifiers in an attempt to keep the output power at the desired level. The
currents of the remaining amplifiers will be higher than normal, while the defective
amplifier current will be lower than normal or zero.
R1 on the combiner board is a balanced, 3-resistor array arranged in a star formation. Each
resistive leg of the star has a value of 50 ohms. Each leg or this resistor network is
connected to one input of the combiner. Under normal operating conditions, the resistor
dissipates no RF power because the same, in-phase RF voltage level is present at each leg
or the resistor array.
6.5.2 Low-Pass Filter
Refer to Figure 6-4, for a simplified diagram of the low pass filter. RF output from the
combiner is routed to the input of the low-pass filter which attenuates harmonics and other
components above its cutoff frequency (which is approximately 560 MHz).
6.5.3 Directional Coupler
A microstrip directional coupler is etched into the output circuit of the combiner board. The
forward and reflected RF signals are rectified by CR1 and CR2 respectively and filtered to
produce forward and reflected dc samples. These samples are sent to the metering board.
Both signals are multiplexed and sent to the DSP exciter. The forward sample is used in the
metering board to develop the PA AGC voltage. The reflected sample is monitored for high
VSWR.
