E.F. Johnson Company ATLAS 4500, Техническое руководство

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A-10
ATLAS 4500 Multimode Station Technical Manual
May 2017
135-175 MHz Power Amplifier Appendix A - VHF Circuit Descriptions
A.3.2 Schematic Description
A.3.2.1 General
The RF input is ~ 12dBm into CN1. The PA provides 1-100 W output across 135 –
175 MHz band at the main N type connector. The Main supply is 48 V at < 9 A at
CN3. The secondary supply is 24 V 450 mA at CN2 and 5V at 10 mA at CN2.
A.3.2.2 Attenuator
CN1 at ~12 dBm enters a 4 dB pad and then the attenuator IC1 which 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
~24 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 heatsink.
The input matching uses a broadband 9:1 impedance transformer / balun loaded
with ferrite to prevent oscillations. This feeds a stabilization network of series
resistors/ gate loads and a parallel resistor to prevent instability. This then finally
feeds the ~ 4
Ω Zin of the MOSFET.
The output match from MOSFET ~ 8 Ω Zout consists of a 1:4 transformer using 2
x 25 Ω transmission lines then a matching network to a 50 Ω transmission line
balun to give 50 Ω feed. The PA operates in a class AB linear mode with plenty of
headroom for digital modulation.
Extensive decoupling of power supply prevents RF instability. Supply current at
CN3 - 2 is < 7 A at 100W.
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.