Re: [Discuss-gnuradio] help generating frequency offset

[George Nychis]

Matt Ettus wrote:
> George Nychis wrote:
> > Hi all,
> >
> > I'm attempting to generate frequency offset in a script that is adding 
> > some channel features:
> >   http://cyprus.cmcl.cs.cmu.edu/matched_filter/gen_noise_samples.py
> >
> > The frequency offset is specified, for which the original signal is 
> > mixed and then the offset signal is added with the noise.
> >
> > But I'm not seeing the appropriate frequency offset.  If I specify a 
> > frequency offset of 100Hz, I'm only seeing a single point shift, all 
> > other points are the same as if there is no offset:
> >   http://cyprus.cmcl.cs.cmu.edu/matched_filter/graphs/freq_offset.jpg
>
> I'm not sure what you have plotted in that picture.  Is that just the 
> raw samples?  Can you write out to a file both the 2 inputs to the mixer 
> and the 1 output?  Then do an FFT on all 3 in octave to make sure the 
> frequency has really moved as you expected.
>
> Matt

Let me step all the way back and just try to be a little clearer with 
what I'm trying to do.

I'm trying to simulate the frequency offset added by the crystal 
oscillator in the USRP.  64MHz*20ppm/1e6=1280Hz max, +/- ?

So, I'm trying to add this frequency offset to a modulated GMSK signal.

I believe the proper solution, which Brian suggested to me, is too 
upconvert and then downconvert to add the frequency offset.

But, is it possible to just use gr_rotator to apply the frequency offset 
instead by setting the phase equal to offset/sampling_rate*2*pi ?  If 
I'm simulating the USRP at 1Msps with 2 samples per symbol, I think I 
would set it to 1280Hz/500KHz*2*pi.

http://gnuradio.org/trac/browser/gnuradio/trunk/gnuradio-core/src/lib/filter/gr_rotator.h

But, I'm not entirely sure this is accomplishing what I want to.

- George