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

[shyamnath gollakota]

Hi Eric,

Thanks a ton for the reply. My main concern is not that the frequency
offset is high enough but that it changes pretty often (every time I run
a new run) which makes it difficult for repeatability. 

So is there a way in software  where I can make sure that the frequency
offset is constant either by finely controlling the center frequency at
the tranmitter every time I run or is there some other way around this
problem?

Thanks
Shyam  


On Wed, 2008-01-09 at 11:15 -0800, Eric Blossom wrote:
> On Wed, Jan 09, 2008 at 11:02:47AM -0800, Shyamnath wrote:
> > 
> > Hi,
> > 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.
> > 
> > So I have a couple of questions
> > 1) Is this high a frequency offset in the specs? 
> > 2) Is this variation in the frequency offset  in the specs of these radio?
> >  (I have been working on other costume made hardware and the frequency
> > offset has been almost constant for the past couple of months)
> > 3) Is there are way to prevent the frequency offset variation after every
> > reboot? 
> > 
> > Thanks in advance for any replies
> > Shyam
> 
> 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.
> 
> All xtal oscillators move around with temperature.  The question is
> how much, and how much money is being spent on compensation and
> environmental control.
> 
> Eric