Forum: GNU Radio Decimation and Interpolation of USRP2

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Yc P. (Guest)
on 2009-05-28 10:53
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?

<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? 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?

Your help will very much appreciated..

Best regards,
yc park
Eric B. (Guest)
on 2009-05-28 20:17
(Received via mailing list)
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
Matt E. (Guest)
on 2009-05-28 21:06
(Received via mailing list)
Yc Park wrote:
> Hello,
>
>
> <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? 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?

Have you made sure that you are enabling real time scheduling?  Even if
you don't use CONFIG_PREEMPT_RT like Eric said, this will help
enormously.

Matt
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