[Discuss-gnuradio] Strange phase noise on N210

[Hanhijärvi Kalle]

Hi all!

I’ve been investigating the use of Ettus SDRs for frequency metrology.

I noticed a strange phase noise problem while testing the N210 with a BasicRX daughterboard. The experiment is a simple dual channel measurement, where 10 MHz signal from a DDS generator is split into both input channels. Phase difference of the two (identical) signals should reveal the “phase noise floor” of the N210.

If I use internal reference on the N210, I see considerable “jitter” or fluctuations in the phase difference. See linked Fig1 for the PSD (in time units). Using external reference split directly from the input signal removes the fluctuations (Fig2). However using an external reference not locked to the signal input (e.g. high quality OCXO) results in similar fluctuations as shown in Fig1.

Phase difference was calculated simply as the argument of ch1 * conjugate(ch2).

Measuring a signal detuned significantly from 10 MHz (e.g. 9.2 MHz) seems to remove the fluctuations.

Interestingly, repeating the same experiment with a B210 with internal reference gives results similar to Fig2 without any problems. Signal input was 100 MHz in this case, since B210 can’t measure 10 MHz directly.

Measurements were done with two recent versions of gnuradio. Repeating the experiment with uhd’s python interface gives similar results.

Any ideas what this might be related to?

Links to figures:

Fig1: https://drive.google.com/open?id=1ZwAyGtDUXseIkKFOIo0yrjccriSC9PGt

Fig2: https://drive.google.com/open?id=1wrqdBMuVT5lH0v8FJrLc0t7I0G2PGuBA

 

- Kalle Hanhijärvi