Re: [Discuss-gnuradio] Frequency Offsets in RFX 2400

[LRK]

On Wed, Jan 09, 2008 at 11:15:37AM -0800, Eric Blossom wrote:
> On Wed, Jan 09, 2008 at 11:02:47AM -0800, Shyamnath wrote:
> > 
> > I am trying to run a few experiments to transmit a data  stream from a
> > transmitter to receiver using a RFX 2400.  However, I am observing that the
> > frequency offset is extremely high at 40-80kHz. In addition, it also changes
> > significantly, nearly 2-8kHz, every time I restart the usrps.  I have used
> > various ways of estimating the frequency offset and all of them converge on
> > the above observations.
> 
> I'm not sure of the tolerance of the oscillators Matt's currently
> using on the 2400's, but 40k/2.4G is about 17 ppm.  I wouldn't call
> that "extremely high".  In general any receiver has to be able to
> track out any frequency offset and symbol timing variation.  Expensive
> test equipment has better specs (and costs more) because they use
> things like ovenized crystal oscillators.

I have found similar problems trying to use usrp_fft.py at 2.2 GHz. Mine
is off about -33 kHz with a drift of about 3 kHz from idle to stable when
the program is started. I'm using the DBSRX fed from a pre-amp.

While the 'right' fix would be to supply a stable frequency source to
the USRP, it would be difficult.

Next best would be an 'offset' or 'calibration' value in the environment
which would be included in the frequency calculations. This should be an
indication of the error at 64 MHz and scaled. Thus '+914.125 Hz' at 64 MHz
would cause a 914.125 x 35.15625 = +32137 Hz correction at 2250 MHz
(which is what I need at this moment :).

And best in the future would be a source on the USRP which could just
lock itself to a 1PPS signal from a GPS at 0-5 volt levels.


-- 
LRK
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