E.F. Johnson Company ATLAS 4500, Техническое руководство

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A-4
ATLAS 4500 Multimode Station Technical Manual
May 2017
135-175 MHz Exciter Module Appendix A - VHF Circuit Descriptions
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 (15 dBm) is mixed with the 119.64 MHz modulated signal
by mixer X8 (O/P of ~ -24 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 first RF band pass filter then the first
MMIC amp then second RF band pass filter, then the second MMIC amp before
being output to the Power Amplifier module through CN1.
The135 - 175 MHz band pass filter function is supplied by selectable bands. 3
bands are fitted 135 – 147 MHz, 146 – 161 MHz and 161 – 175 MHz.
Each 1st band pass filter I/P is fed by switch IC20, then the filter O/P into switch
IC18 to make up a selector circuit that switches the frequency band programmed by
the Controller, followed by 1st MMIC amp IC2 and next a fixed attenuator.
Then each second filter I/P is fed by switch IC26, then the filter O/P into switch
IC19, for the frequency band of operation. Then a notch filter and the second RF
MMIC amp IC23 providing gain to give a ~ 13 dBm output level.
A.1.2.6 DPD Loop
The modulator includes a DPD (Digital Pre Distortion) 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.
The feedback signal from the Power Amplifier sample port comes into the exciter
module through CN6 at ~ 3 dBm. This is attenuated then down-converted with the
main VCO output (254.64 – 294.64 MHz) from buffer amp IC16 by mixer M1 to
provide an output of 119.64 MHz. This passes through a pad then a 150 MHz low
pass filter and is amplified by IC15.