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

Sivan Toledo stoledo at tau.ac.il
Wed Apr 24 04:02:03 EDT 2013

Thanks Matt. When you write that "You will also need to have self-surveyed
the position", is that some active procedure that we need to do, or just to
let the GPSDO run for a while and it will survey it's position?

On Tue, Apr 23, 2013 at 8:50 AM, Matt Ettus <matt at ettus.com> wrote:

> Sivan,
> The first thing to make sure is that both GPSDOs are locked, and that the
> USRPs are locked to the GPSDOs.  Also, the GPSDOs will take some time to
> fully lock when first powered up.  In general, these should be left on long
> term for best results.  You will also need to have self-surveyed the
> position.
> Matt
> On Mon, Apr 22, 2013 at 9:31 PM, Sivan Toledo <stoledo at tau.ac.il> wrote:
>> 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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