[time-nuts] Commercial software defined radio for clock metrology

Sherman, Jeffrey A. (Fed) jeff.sherman at nist.gov
Wed Jun 1 12:00:39 EDT 2016


Atilla Kinali:
Yes, the spec'ed SNR of the ADS62P44 is 74dBFS (typ) while the LTC2216
is spec'ed with 81dBFS (typ). Additionally, the input amplitudes in
Sherman and J├Ârdens experiments were kept around half scale, which is
another -6dB in SNR. There is another ~15dB in difference, but I currently
don't see where it comes from.

I'm not sure exact which ~15dB you're contemplating, but I'll hazard a guess. We observe some non-idealities in the SDR (or our noise environment), the effect of which scales with decimation factor. In principle, reducing the bandwidth through low-pass decimation---suppose by a factor of 100---should increase the signal-to-noise by 20 dB. This result relies on a completely white-noise spectrum, ideal filters, and no round-off or quantization noise. For a decimation factor of 100, we observed an SNR improvement of about 11 dB instead of 20 dB. This deficit in "process gain" was another 3 dB worse at a decimation factor of 500. Some additional details are in Appendix B of the paper.

Best wishes,
-js


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