[USRP-users] Short-term accuracy and stability of the GPSDO

Sivan Toledo stoledo at tau.ac.il
Tue Apr 23 00:31:03 EDT 2013


I tried to measure the short term accuracy and stability of the GPSDO. My
application requires high stability and the GPSDO seems to not perform very
well in that respect. I would like to check if others had similar
experiences.

What I did was to send a carrier from one USRP using tx_waveform at
434.6Mhz (using a WBX) and to receive 10s chunks in a second USRP, both of
which had a locked GPSDO. The FFTs of these samples were analyzed. I did
this for 10 minutes.

I discovered 3 interesting things. First, the signal has a bandwidth of
about 20Hz, which I think means that the entire TX and RX chains had
significant phase noise, at least when multiplied up to 434MHz.

Second, the signal was about 45Hz off from where it should have been, which
I think mean that the two GPSDOs where off by around 1hz (given that the
10MHz reference is multiplied by about 43).

Third, the signal wandered over about 40Hz within short time spans of about
a minute or less, again indicating frequency instability of about 0.5Hz per
GPSDO.

This is less than a 1e-7 error, which is huge over the long term average,
but perhaps not so large for the short term (say a few seconds or a minute).

Are these consistent with what others have seen (or what Ettus has measured
with these GPSDO units)?

Thanks, Sivan
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