[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
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 Jördens 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.
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