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

Bob Camp kb8tq at n1k.org
Fri Jul 29 20:51:44 EDT 2016


Keep in mind that if you apply pre-filtering, an ADEV plot is lying to you ….


> On Jul 29, 2016, at 6:58 PM, Kevin Rosenberg <kevin at rosenberg.net> wrote:
> Jeff,
> Thanks for your very useful paper Oscillator Metrology with SDRs[1]. I
> created a C++ program and checked residuals using a 10 MHz clock split
> to the A and B channels of a LFRX and BasicRX boards and sampled at 1
> Mhz. Using boxcar averaging of 1000 samples at 1 kHz, I was impressed
> by the low noise floor approaching that of my Timepod which was
> several times the cost. I included the Allan Deviation without
> averaging showing the sqrt(1000) increase in noise floor without the
> averaging[2].
> I had a question about your experience. You mentioned using a input
> signal near the maximum of the USRP’s ADC to get the best SNR. I
> reviewed the schematics and application notes. I found a maximum Vpp
> mentioned of 3.3V. I was wondering what voltage you were using to
> drive the USRPs. When I go above 1.5-2 Vpp, I start getting signal
> distortions and not much increase in the amplitude.
> Many thanks for publishing your work in this area.
> Kevin
> [1]
> https://arxiv.org/abs/1605.03505
> [2]
> <usrp-pn.png>_______________________________________________
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