Technical Manual

A-10 ATLAS 4500 Multimode Station Technical Manual December 2016
380-520 MHz Power Amplifier Appendix A - UHF Circuit Descriptions
A.3.2.2 Attenuator
CN1 at approximately12 dBm enters attenuator IC1. This provides attenuation
steps of 0.25 dB from a 2 dB minimum. This is used by the Controller for power
control through CN2 pins 3, 4, 5 lines.
A.3.2.3 Driver Module
The attenuator IC feeds an amplifier module IC2 to provide drive for the main PA
amp.
This module is a high linearity low intermodulation 75-Ω power driver with
approximately24 dB gain, 28 dBm RF power. It operates at 24 V at 450 ma.
A.3.2.4 Output Power Amp
The PA output power amp stage uses a high performance power FET in a Gemini
package. This is mounted on a copper heat spreader plate, on a fan-cooled
aluminium heat sink.
The input matching uses a 90° section 17-Ω transmission line balun loaded with
ferrite to prevent oscillations. Then, a balanced matching section gives
approximately 0.5-Ω Zin FET.
Input series resistors and a parallel resistor are used to prevent instability.
The output match from approximately 6 Ω Zout consists of a 2-section match, then
to a 90° section 17-Ω transmission line balun to give 50-Ω feed.
Due to use of open long balun there is high Q resonance for second harmonics at
one frequency area. A power absorber network for approximately900 MHz reduces
amplitude of second harmonics on FET drains and gives better intermodulation of
the PA.
A.3.2.5 Power Coupler
The PA feeds an output RF coupler port, providing about 3 dBm at 100 W output.
This is for DPD use, and is fed to the Exciter.
NOTE
The Controller uses the PA calibration data stored in IC11 during PA calibration, FWD VOLTS
from the power detector, TX frequency and O/P power settings set from the DSP to control the
attenuator values dynamically.