Re: [Discuss-gnuradio] Interpolation in usrp_siggen

[shyamnath gollakota]

Hi Eric,

The OS I am using is Linux 2.6.17-1.2174_FC5
The hardware I am using is Memory of 512MB, Intel(R) Celeron(R) CPU
2.53GHz.

As u predicted, I do observe a number of underruns on stderr when the
interpolation rate is set to low values. This might explain the results
for the case when interpolation is set to 16  but I dont understand
still why the receiver is getting almost nothing when the interpolation
at the sender is 4, 8,12. The decimation rate is set to constant at the
receiver at 64.

Your mail has raised a number of questions with me (kindly bear with
me).

1) What is the behavior (output) of the DUC and interpolator when it
doesnot get samples at the required rate (say it gets a burst of them
with a period of silence followed by another burst)?

2) How do I bypass the bandpass filter on the RFX 2400, it would be
great if I could move to some other frequency range which doesn't have
802.11 interference.

3) What happens when I quit a program which is sending a sinosoidal
using ctrl-c (like the usrp_siggen.py). I mean what is the reaction of
the usrp. I am asking this because of yet another observation, when I
see the signal at the receiver in this case. The end of the samples for
the sinusoidal I am observing again ends with a low frequency component
and then a low amplitude component, I have attached the plots on the
site.

http://nms.csail.mit.edu/~gshyam/sineStop2.jpg
The x axis is the samples and the y axis the I component of the signal.
As u can see at the sampl point 4000 the low frequency sin(7.45k)
starts. 

http://nms.csail.mit.edu/~gshyam/sineStop1.jpg

This figure shows how the low frequency component ends.

Kindly pls answer my questions.

Thanks a ton for ur time
Shyam 

On Tue, 2006-12-12 at 08:48 -0800, Eric Blossom wrote:
> 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
>