Re: [Discuss-gnuradio] Strange phase noise on N210

Ok, thanks for the idea, but that doesn't seem to be it. The problem persists, even if I disable automatic DC offset correction in the FE corrections tab of UHD source block.

Anyway, tuning slightly off the target frequency (say 100 Hz or couple of kHz) doesn't seem to fix the problem.

- Kalle

On 12/19/2018 04:11 AM, Hanhijärvi Kalle wrote:

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

My guess is the automatic DC offset removal, and since you're operating in the DC zone (your tuned frequency is the same as your incoming signal), you're probably looking at that. This helps to explain why when you're tuned away from the carrier you don't see this effect.

From:	Hanhijärvi Kalle
Subject:	Re: [Discuss-gnuradio] Strange phase noise on N210
Date:	Fri, 21 Dec 2018 12:33:06 +0000