Glenayre Document Number: 9110.01002
GL-T8541 PA
09/10/99
THEORY OF OPERATION
t8541
p6.
fdk
Copyright © 1997 Glenayre
Page: 6-3
6.2.1 Q1 Bias and Idle Current Adjustment
The bias for Q1 is determined by a resistive voltage divider which consists of dropping
resistor R5 and potentiometer R6. Drain current for Q1 flows as long as dc power is
supplied to the power amplifier. Bias for Q1 is set correctly when its idle current is 20mA.
This current level is below the threshold of the VDT. It is measured by connecting a milli-
ampere meter between test points TP1 and TP2 with JW6 removed.
Figure 6-2 Block Diagram Driver and 1 to 3 Splitter Boards
6.2.2 Q2 Bias and Idle Current Adjustment
Bias for Q2 is supplied by a preamp key signal generated by the metering board. When
unkeyed, the preamp key signal value is 0Vdc. When keyed, the preamp key signal has a
value of +5.0Vdc. The Q2 bias adjustment is R16.
Depending on the bias setting, the driver will put out up to 40 watts, but it is usually set
much lower due to the lower PA drive requirements.
The VDT driver current display indicates the combined current of both Q1 and Q2. The Q2
idle current is correctly set when the VDT driver current display indicates 1.0A with the Q1
gate bias set correctly, the transmitter keyed, and an RF output level of zero watts.
When the transmitter is keyed and producing normal RF output, the driver current will be
approximately 2.5A.
The driver board bias adjustment procedure is given in Section 7.7.3.
RF Input
From
DSP
Exciter
Driver Board
Preamp
+28 Vdc
Key
1 to 3 Splitter Board
RF
Output
to the
Three
PA
Boards
Q1
Q2
CR1
CR2
R16
R6
5.1V
20K
20K
R5
49.9K
R15
100K
Hy1
R1
Hy2
R2
50
Ω
100
Ω
0Vdc = unkeyed
+5.8 Vdc = keyed
