Copyright 2005 ISC Technologies
3.2 Features
The 9T97A250 power amplifier has the features listed below.
•
Adjustable power output.
• Continuous
operation.
•
Mounts on a 19-inch standard EIA rack panel.
•
Self-resetting protection circuitry.
• Modular
construction.
•
Status and power output display.
•
Easy access to amplifiers and protection circuitry for adjustment.
•
Independent fusing of each final module and the driver with open fuse
indicator on the rear panel.
3.3 Physical Description
The 9T97A250 UHF power amplifier is rack panel mountable (19-inch). Front and rear
covers are removable to gain access to all controls without removing the 9T97A250 from
the rack.
The front panel display consists of a red LED bar-graph display showing output power. Two
red and two green status LEDs are also located on the front panel.
The rear panel consists of an RF input connector, an RF output connector, a DB-15
connector, and a two-terminal barrier strip for dc power connections.
4 P. A. CIRCUIT AND LOGIC CIRCUIT DESCRIPTIONS
4.1 Power Amplifier Circuit Description
4.1.1 General
The RF input, which is sampled by the alarm logic, is amplified by the RF power driver. The
driver’s net RF power output is varied by adjusting the dc voltage supply. The driver’s
power output is split into four parts which become the input power to each final. Each final
amplifies this input power and the output from the finals are combined and filtered. The
alarm logic samples both the output forward power and reflected power.
4.1.2 Driver
The driver module has two functions. Its primary function is to provide a variable power
input to the final power amplifier module, so that the net output power of the final amplifier
can be adjusted. The second function is to provide a sampling point for the alarm logic at
the input power point to the power amplifier.
The driver uses a MOS-FET transistor (Q1) with two dies in one package, operated in
push-pull configuration. R2 and R3 provide zero bias at the gates of Q1 for class C
operation. T1 and T2 are 1:1 balun transformers. Capacitor C4 at the input of the driver is
adjusted to fine tune the input match to 50 ohms, thereby minimizing input standing wave
reflection. Capacitor C16 couples a small amount of input power through diodes D1 and
D2. This dc voltage, which is proportional to the RF input, is fed to the alarm logic. The
rectified RF at the anode of D1 is smoothed by capacitor C17 and loaded by resistor R1.
The main part of the RF input goes through T1, C2, C3, Z1 and Z2 and capacitors C1, C4
and C5 match the transistor’s input impedance across the frequency band. Power (3-5 Vdc)
is fed via a “fast-on” tab to the drains of transistor Q1.
Matching of the transistor’s output impedance across the frequency band is provided by Z3,
Z4, C6. C7 and C10. Capacitors C5 and C9 are for dc blocking.