Technical Manual

760-870 MHz Power Amplifier
Appendix A - 700/800 MHz Circuit Descriptions
A-10
ATLAS 4500 Multimode Station Technical Manual July 2021
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.
A.3.2 Schematic Description
A.3.2.1 General
The RF from the Exciter passes via a coaxial cable to the input of the PA Module,
CN1 at ≥12 dBm enters attenuator IC1, this provides attenuation steps of 0.25 dB.
This is used by the main Controller PCB for power control via CN2 pins 3, 4, 5
lines. The control lines are protected from ESD and RF. The RF signal then enters a
Driver Module. From the Driver Module it then matched by a broadband network to
drive the low input impedance associated with the final transmit (FET) power
amplifier transistor. The transistor's low Drain impedance is then also matched back
to 50 ohms by two broadband PCB matching network covering a very wide
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.