Re: [Discuss-gnuradio] How to retune a daughterboard/bring up the sample rate on USRP?

[Marcus D. Leech]

On 06/28/2011 07:48 PM, Minhoo Kim wrote:
> Hello, I'm new to this mailing list and gnuradio.
> sorry if I fail follow certain etiquettes..
> what I'm trying to do now is to have a transmitter that hops around under
> 1ms, which needs to have 1MHz bandwidth and hop around in 10MHz range total
> (at least).
> the problem I'm having is that I can't get the USRP motherboard to generate
> that high of frequency.
> I need the motherboard to generate at least up to 10MHz range because the
> signal needs to be hopped around pretty fast..
> and has to be passed on to 2.4GHz range using RFX2400.
> This is the error message I get:
>
>    File "multiplytest.py", line 86, in<module>
>      test_signal = my_top_block()
>    File "multiplytest.py", line 46, in __init__
>      fmtx = blks2.nbfm_tx (self.sample_rate/10, self.sample_rate, tau=75e-6,
> max_dev=1e5)
>    File "/usr/lib/python2.6/dist-packages/gnuradio/blks2impl/nbfm_tx.py",
> line 69, in __init__
>      40)              # stopband atten dB
>    File "/usr/lib/python2.6/dist-packages/gnuradio/optfir.py", line 44, in
> low_pass
>      taps = gr.remez (n + nextra_taps, fo, ao, w, "bandpass")
>    File
> "/usr/lib/python2.6/dist-packages/gnuradio/gr/gnuradio_swig_py_general.py",
> line 352, in remez
>      return _gnuradio_swig_py_general.remez(*args, **kwargs)
> RuntimeError: gr_remez: insufficient extremals -- cannot continue
>
> i looked up gr_remez, and I haven't really found a solution to get USRP to
> have 10MHz bandwidth.
> I did find that the frequency has to do with self.sample_rate, and the
> highest rate I got was 2 Mega, which in turn can generate up to 1MHz because
> of the nyquist rate.
> I know the interpolation rate has to be in multiples of 4 ( 4, 8, 12, ...)
> and the USRP rate hast to be 128Mega to match the USB data flow rate (I
> don't know if that's the right term).
> I have my code attached.
> Another way I'm looking at is to change the center frequency of the
> daughterboard, but I haven't been able to figure out to make that happen
> fast enough.
>
> so the questions are,
> How to get the sample rate up to the maximum (which should be 32MHz in
> theory)?
>
> How to retune the daughterboard while it's running?
>
>
> Any help/suggestions would be great!
>
> I am using USRP1 and RFX2400.
>
> http://old.nabble.com/file/p31947513/multiplytest.py multiplytest.py
>
> Thanks for reading this.
The maximum sample rate across the USB bus is 8Msps. The samples travel 
across as complex (I and Q) with a default of
  16-bits I and 16-bits Q.  That's 32Mbyte/second, which is close to 
the limit of what USB-2.0 is capable of.  Alternatively, if you're
  willing to use 8-bits 'I' and 8-bits 'Q', you can theoretically move 
16Msps across the bus. But your host machine has to be capable to
  supporting that data rate in the face of whatever heavy computations 
it's also doing.

Also, getting re-tuning to happen in under 1ms is going to be a 
challenge.  It's unlikely that you'll get the PLL synthesizer on the 
RFX2400 to
  re-lock in under 1ms.


--
Marcus Leech
Principal Investigator
Shirleys Bay Radio Astronomy Consortium
http://www.sbrac.org