Technical Manual

380-520 MHz Exciter Module
Appendix A - UHF Circuit Descriptions
A-4
ATLAS 4500 Multimode Station Technical Manual July 2021
A.1.2.4 RF Carrier Generation & Modulation
RF carrier modulation type Phase 1 FM or LSM and Phase 2 H-DQPSK is
generated within IC31 FPGA and IC32 DDS from a serial digital stream on the I2S
bus lines coming from the external digital signal processor (DSP) and fed into Pins
5, 7 and 8 of CN3. The initial setup for the FPGA internal Arm controller is done
through the I2C bus on Pins 9 and 12 of CN3. The ARM controller inside the
FPGA IC has a 20 MHz crystal clock and internal VCO.
The FPGA provides raised cosine filtering of the I/Q data and is sent in digital form
to the DDS digital I/Q modulator. The DDS IC generates a differential 119.64 MHz
I/Q modulated signal. This is filtered by the SAW filter F5 into the high level mixer
X8 input.
The VCO output signal is mixed with the 119.64 MHz modulated signal by mixer
X8 (O/P of approximately -23 dBm) at the desired transmitter output frequency.
A.1.2.5 RF Filter Amp Section
The RF output of the mixer X8 is fed through the first RF band pass filter, then the
first MMIC amp, then the second RF band pass filter, and finally the second MMIC
amp before being output to the Power Amplifier module through CN1.
The 380 - 520 MHz band pass filter function is supplied by selectable bands. 4
bands are fitted as standard 380 – 415 MHz, 415 – 450 MHz, 450 - 485 MHz and
485 – 520 MHz.
Each first band pass filter I/P is fed by Switch IC20, then the filter output into
Switch IC18 to make up a selector circuit that switches the frequency band
programmed by the controller, followed by first MMIC amp IC2, and then a fixed
attenuator.
Each second filter I/P is fed by Switch IC26, then the filter output into switch IC19,
for the frequency band of operation. Then a notch filter and the second RF MMIC
amp IC23 providing gain to give an approximately 13 dBm output level.
A.1.2.6 DPD Loop
The modulator includes a Digital Pre Distortion (DPD) feedback system that
corrects for distortion in the PA and allows for small adjustments to be made to the
modulated RF signal output. This enables excellent overall linearity and good close
in ACP performance for the transmitter.