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

Attila Kinali attila at kinali.ch
Thu Jun 2 04:15:20 EDT 2016

On Wed, 1 Jun 2016 16:00:39 +0000
"Sherman, Jeffrey A. (Fed)" <jeff.sherman at nist.gov> wrote:

> I'm not sure exact which ~15dB you're contemplating, but I'll hazard
> a guess. 

You measured an ADC noise floor of -140dBc/Hz. The TimePod has a spec'ed
noise floor of -170dBc/Hz(typ). That's a difference of ~30dB.

The different specs of the ADCs and the signal level account for a difference 
of about ~14dB. There is still a ~16dB difference that I cannot explain.

> 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.

I would have guessed, that the TimePod is plagued by the same effects.
But maybe it has less spurs and thus can achieve a higher SNR gain
during decimation.

			Attila Kinali

Reading can seriously damage your ignorance.
		-- unknown

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