GL-T8541 PA
Glenayre Document Number: 9110.01002
THEORY OF OPERATION
09/10/99
Page: 6-2
Copyright © 1997 Glenayre
6.1.1 Adding The Optional Isolator
The 250 watt rated power amplifier output is measured at the output of the combiner board.
If an isolator is added, the RF power output available at the isolator is less than the rated
power output of the PA due to the isolator loss. Table 6-1 lists the three optional isolators,
their losses, their minimum isolation, and the output of each if the PA output power is set
at the rated 250 watt level at the combiner board.
The isolator directs reflected power from the antenna or its feedline into the isolator reject
load, therefore providing the power amplifier module with a 50 ohm load regardless of the
load at the antenna terminal. The degree of protection the isolator offers a PA from antenna
system VSWR is referred to as its isolation. Isolation is typically 30 dB per section. High
antenna system VSWR can cause several problems, two of which are:
1.
The excessive reflected power caused by high antenna system VSWR is dissipated in
the isolator reject load. This increases the isolator and reject load dissipation and oper-
ating temperature.
2.
High antenna system VSWR means less power will be radiated from the antenna,
resulting in lower effective radiated power (ERP).
6.2 Driver Board
Refer to Figure 6-2, left side, for a simplified block diagram of the driver board. Refer to
Figure 6-12 and Figure 6-13 for detailed circuit board assembly and schematic diagrams.
There are two RF amplifier stages on the driver pc assembly; both Q1 and Q2 are FETs.
The drain current is metered for the pair, not individually, and may be monitored locally or
remotely using a video display terminal. See the exciter VDT manual for monitoring
information.
The bias, idle current, gain, and output power of each stage is determined by its own gate
bias control.The parameter that is observed when adjusting the bias control for each stage
is its drain idle current. This is the current that flows when voltages are applied, the trans-
mitter is keyed, and no RF is supplied from the exciter.
Table 6-1 GL-T8541 PA Optional Isolator RF Power Output
Isolator Type
Insertion Loss
Output Power
(with 250 watts input)
Isolation
Single section
0.35 dB
230 watts
30 dB
Dual section
0.6 dB
220 watts
60 dB
Triple section
0.85 dB
210 watts
90 dB
