E.F. Johnson Company ATLAS 4500, Техническое руководство

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ATLAS 4500 Multimode Station Technical Manual
May 2017
A-5
135-175 MHz Exciter Module Appendix A - VHF Circuit Descriptions
After a pad the 119.64 MHz IF is down-converted with the 120.45 MHz local
signal from buffer amp IC21 by mixer M2 to result in an 810 KHz IF signal.
The local oscillator for this second down converter is provided by the 120.45 MHz
Synthesizer IC24 and associated components. Programming for this is provided by
the FPGA IC32.
The 810 KHz IF signal is band pass filtered and then amplified by a 20 dB gain
differential amplifier IC14, then further amplified by the 16 dB gain differential
amplifier front end of IC31, a high performance Analogue to Digital converter.
This IF signal is sampled at a 20.16 MSps rate, then passed through an 810 kHz
digital FIR band pass filter inside IC31, BW is ~ 100 kHz and decimated by 8 for a
2.5 MSps O/P rate.
The 16 bit digital samples from IC31 are read by IC32, the main FPGA/ARM for
processing by internal software. An area for pre-distortion correction tables is
provided by IC27 RAM. The FPGA DPD filter data is stored in the serial Flash
ROM IC35.
Inside the FPGA IC the 2.52 MSps 810 kHz IF is decimated by 7 to effectively give
a 1/9 converted 360 MSps 90 kHz signal plus other unwanted signals. This is then
passed though a narrow 90 kHz IF band pass FIR filter to removing other products
and noise.
The correction software in the FPGA/ARM has a short initial delay acquisition
followed by I/Q correction. This allows corrections to reduce overall distortion
products at the PA O/P.
A.1.2.7 Exciter DC Supplies
The exciter is supplied a nominal 13.8 V through CN3-2. This input supply is then
distributed to IC1, IC4, IC9 and IC11.
IC1 is configured as a 1 MHz buck DC-DC converter and provides the main
reg5.2 V DC output. A shielded inductor is used to reduce fields.
IC9 is configured as a boost DC-DC converter and pr25 V DC output for
the main PLL.
IC3 is a linear regulator regulating the +5.2 V supply down to +5 V DC for linear
circuits, A/D, RF amps etc.
NOTE
The sample port CN2 provides a -6 dB sample of one side of the final differential analog O/P of
the 810 kHz IF at ~ -10 dBm level.