Technical Manual

380-520 MHz Power Amplifier
Appendix A - UHF Circuit Descriptions
A-10
ATLAS 4500 Multimode Station Technical Manual July 2021
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.
NOTE
The Controller uses the PA calibration data stored in IC11 during setting of the PA output power.
The Calibration data is derived from, FWD VOLTS from the power detector, TX frequency and O/P
power settings set from the DSP to control the attenuator values dynamically.
bandwidth.
Prior to transmission, the PA O/P first feeds an RF coupler providing ~ 4 dBm O/P
for a 100W output. This port is used for the DPD loopback, feeding the loopback I/P
of the Exciter. It then passes through a low loss element elliptical low pass filter,
which filters out the unwanted harmonics.
A dual directional coupler consists of coupled microstrip transmission lines
fabricated on the PCB artwork, each with a low pass filter and detector circuit. The
sampled RF energy is rectified to provide a proportional DC voltage output.
The PTT signal enables the amplifier circuit by providing bias to the transistors. The
PTT line from CN – 8 controls the switch TR2. This switches 26 V and allows PA
MOSFET bias to operate. PA Bias is normally set for ~4 V at CN2 – 7 and pot
adjusted for ~2.5 A. When the PTT line is enabled, gate bias is provided by a dual
diode network for thermal compensation thermal stability to keep bias current
constant with temperature.
A thermistor TS1, physically located on the PA heatsink monitors the heatsink’s
temperature and is monitored by the Micro Controller.
The PA provides 1-100 W output across allocated frequency band at the main N
type connector. The Main supply is 48 V at < 8.5 A at CN3-2.
The secondary supply is 26V I/P at 450 mA at CN2-2 and 5V I/P at 10 mA at CN2-
14, GND is CN2-1.
The +5V I/P from CN2-14 feeds the temperature sensor TS1 with Temp Data O/P
to CN2 – 13 and a +3.3V regulator IC7 providing 3.3V for attenuator IC 1 and
serial flash IC11 using I2C bus on CN2- 9, 10.
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.