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

[Kevin Rosenberg]

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]