[Discuss-gnuradio] Re: Decimation rate

[Patrick Strasser]

Eric Blossom wrote am 2008-09-04 05:23:
> On Thu, Sep 04, 2008 at 12:56:13AM +0000, WeiXiaowei wrote:

> > Does anyone know whether it is possible to set demication rate to be
> > 2 to double the bandwidth when running usrp_rx_cfile.py to record
> > the incoming signal?
>
> No, decimation of 2 does not work with the standard fpga image.  
> With sufficient effort, it's probably possible to get 4-bit I & Q
> working, which could allow 32MHz bandwidth.

The limiting factor is USB bandwidth. USB hast 480 Megabit/sec gross 
data rate, which translates to a effective maximum of 32 Megabyte/sec to 
transfer between the host and the USRP. The USRP samples at a rate of 64 
Msamples/sec with 12 bit/sample complex, which are sent as 2x16 
bit/sample = 4 bytes/sample.
(32 Mbyte/sec)/(4 byte/sample)=8 Msamples/sec. This is exactly what a 
decimation factor of 8 yields.
With 8 bit/sample real, that is 16 bit/sec complex, you can double you 
sampling rate, but loose 4 bit of resolution.

Patrick
--
Engineers motto: cheap, good, fast: choose any two
Patrick Strasser <patrick dot strasser at student dot tugraz dot at>
Student of Telematik, Techn. University Graz, Austria