Copyright 2005 ISC Technologies
4.1.7 DC Distribution Board
The distribution board supplies 28 Vdc to each final module and 3 to 25 Vdc to the driver
module. Each final and the driver is fused through 10 A fuses F2 to FS, current monitoring
resistors R1 to R4 and EMI filters for short circuit protection and shielding purposes. For
added protection, the current to each final may be monitored. Capacitors and the low pass
EM! filters are used on each of these lines to prevent low and high frequency noise from
penetrating or leaving the shielded amplifier housing. Fuse Fl is for short circuit protection
on the logic board and driver module.
The adjustable dc supply to the driver module and the direct current signals representing
the RF input, RF output, and reflected RF are also filtered on this board. The power am-
plifier’s dc input terminals TB2 and test set connector JI are mounted on this board. Con-
nections to the logic board are also made through P2.
4.2 Logic Circuit Description
4.2.1 General
The logic board is a control board mounted behind the front panel of the 97 series RF
power amplifiers. It gives visual indication of the RF power amplifier’s operating condition,
and indication of the variable power supply to the driver of the RF power amplifier. The
logic board also contains the RF detection circuit, power meter, temperature controller, 2
minute timer, fan driver, and optional fixed bias supply. The logic board has three
connectors; a 1 x 2-pin connector for the dc fan power, a 6-pin connector for the regulator,
and a 2 x 8-pin connector for the power control signals.
4.2.2 Power Supplies
4.2.2.1 Introduction
The three power supplies internal to the PA are: the adjustable driver supply with the ability
to switch between high and low power, the 8 Vdc regulator, and the fixed bias power
supply.
4.2.2.2 Adjustable Driver Supply
An adjustable voltage (produced by regulator U5) buffered by a transistor external to the
logic board produces a stable, high power variable power supply even under a dynamic
load.
The output is adjusted by resistors R52 and R53, and can be switched to high power (de-
fault), low power (high at P10-10 to switch Q6, or a high at P10-6 when JR27 is installed),
or power off (high at P10-6 to switch Q3 when JR26 is installed). When Q3 is turned ON,
the adjustment pin of U5 is shorted to ground. This will cause the output of U5 to drop to a
minimum voltage (about 0.9 Vdc at P10-2, 3). Filtering capacitors C33, C30, C52, C51,
C54, C53, C55, C56, and C57 are used at the input and output of U5 and the external tran-
sistor, in order to reduce the ripple voltage which may be caused by the ac line voltage, dc
fan, or possibly the RF signal. Resistors R55 and R57 provide minimum load current for U5
and the external transistor while D22 and D24 protect U5 from potential reverse bias
conditions.
4.2.2.3 Eight Volt Supply
The 8 Vdc regulation and power to the logic circuit is handled by U6 which is protected by
diode D23.
