RadioProcessor
Receiving Section
The receiving section consists of a fast, 14-bit A/D converter intended for undersampling applications,
followed by digital quadrature detectors, filters, and an autonomous signal averager. To evaluate the
performance of the receiving section of the RadioProcessor, the most important thing is to examine the
performance of the A/D converter. Figure A1.3 shows a 5 MHz signal that was directly captured by the A/D
without being passed through the detectors or any of the internal digital filters. Figure A1.4 (next page) shows
a 70 MHz under-sampled signal (which has been folded back to 5 MHz) captured with the same setup. A
comparison of the two spectra indicates that the noise introduced by the undersampling process is relatively
small. Therefore, the RadioProcessor can be used with input frequencies higher than Nyquist (37.5MHz) with
performance comparable to regularly sampled input frequencies below Nyquist.
The input signals for these tests were generated by a PTS 250 Frequency Synthesizer (running un-locked
to the RadioProcessor's clock), and the spectra were generated by reading the captured data into MATLAB
and performing a complex FFT. The negative frequencies are the result of having only one (the real)
component sampled, with the imaginary values all set to zero.
2020-10-07
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Figure A1.3:
5 MHz signal directly captured.
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