Re: [Discuss-gnuradio] Interpolation in usrp_siggen

[Eric Blossom]

On Tue, Dec 12, 2006 at 01:03:08AM -0500, Shyam wrote:
> hi Eric,
> The daughterboard I use is RFX2400 so I set -f 2.45G ( btw my gnuradios
> have been properly set up since benchmark_tx and receive  work fine for
> gmsk)  . I have tried the below stated experiment using two antenna
> types to see if it is due to the antennas but it is consistent across
> antennas.
> 2400-2480 MHz ISM Band PCB Quad Patch, and
> 
> 2400-2480 MHz ISM Band Vertical Antenna.
> 
> Also for the above configuration can u kindly also let me know is my
> observation that 7KHz is the frequency offset due to the oscillators is
> true?
> 
> Thanks in advance for ur reply
> Shyam

Are you seeing underruns ("uU") on stderr?  If so, then for your low
interpolation rate cases your system is failing to be able to drive
the USRP at full speed (32MB/s).  What kind of h/w are you running?

7KHz offset between two 2.4GHz boards is well within spec.
That's only 3 parts per million.

There's a known problem with the first few samples being hosed on
either or both of receive and transmit.  Larry Doolittle fixed this
in his UXO sweeper gadget, but we have yet to sort out the fix and
integrate it into our USRP firmware.

Eric

> Eric Blossom wrote:
> > On Mon, Dec 11, 2006 at 08:30:25PM -0800, Shyamnath wrote:
> >   
> >> hi,
> >> I am new to gnuradio architecture and am trying to learn about it. I was
> >> using the usrp_siggen,py sample code provided with different interpolation
> >> rates (4, 8,12,16,64)  since it has to be a multiple of 4.  the 
> >> observations
> >> are as follows (which  cannot explain)
> >>
> >> 1) For interpolation rates set to 4,8,12, at the sender, what I observe on
> >> the receiver (directly connecting the usrp_source to a file_sink), is just
> >> noise.
> >>
> >> 2) For a interpolation rate set to 64, I almost get the required 
> >> sinuosoidal
> >> at the required frequence(100khz), with the difference that the frequency 
> >> of
> >> the signal at the receiver for nearly 200 samples at the beginning  is
> >> nearly 7KHZs. To understand this better, I just send a constant signal on
> >> the sender (using usrp_siggen.py) and received a sinousoidal at the 
> >> receiver
> >> with a frequency of 7Khz (which I presume is because of the frequency
> >> offset). Although I think the above two observations are related I could 
> >> not
> >> explain them.
> >>
> >> 3) Another interesting case is when the interpolation rate is set to 16. 
> >> the
> >> output at the reciver is a signal which has a sinousoidal with the required
> >> frequency(100khz) spaced with sinousoidals with a frequency of 7khzs.  This
> >> observation is something I cannot understand.
> >>
> >>  Kindly help me out with this.
> >>
> >> Thanks
> >> Shyam
> >>     
> >
> > You didn't mention which daughterboard you are using, but assuming
> > it's a Basic Tx, be sure to set the -f <freq> option to something like
> > 10M.  Neither the Basic Tx nor Basic Rx will pass signals under about 
> > 100kHz.
> >
> > Eric