Re: [Discuss-gnuradio] Decimation and Interpolation of USRP2

[Eric Blossom]

On Thu, May 28, 2009 at 08:53:17AM +0200, Yc Park wrote:
> Hello,
> 
> I asked this question before, but no one answered yet -- maybe it was
> not recognized out of several questions.
> Anyway, I have two related questions with usrp2.
> (gnuradio 3.2svn + dbrx2400 db'd)
> 
> <Decimation>
> I see the minimum decimation rate of usrp2 is still 4 with GRC, and
> wonder if there is any way to further reduce it such as 3 or 2?

Not without going to smaller samples then 16-bit I & Q.  
Gigabit ethernet is 1e9 bit/s, not 1.1e9 bit/s ;-)

> <Interpolation>
> Also, I found that the observed spectrum of my usrp2 is very choppy with
> the interpolation lower than 10, which means my baseband sampling rate
> can be only 10Msps.  So, is this the limitation with usrp2?

No.

> or are there any ways to lower this interpolation rate down to
> around 4, at least to be compatible with the digitized data with
> decimation of 4?

Are you using the latest firmware and fpga image?  What kind of a
computer are you running this on?  To get to interp 4 reliably, you'll
probably need a machine on the order of a 3 GHz quadcore machine, and
you may need to use a kernel with the CONFIG_PREEMPT configuration
option.

Eric