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Re: [Discuss-gnuradio] USRP problems (interpolation, decimation, samplin

From: Steven Clark
Subject: Re: [Discuss-gnuradio] USRP problems (interpolation, decimation, sampling rate, frequency)
Date: Wed, 17 Nov 2010 09:04:47 -0500

On Wed, Nov 17, 2010 at 3:41 AM, Songsong Gee <address@hidden> wrote:
I use two USRPs. One sends a sinusoidal signal, and the other receives it.

A flow graph and a result are here
flow graph: http://lh5.ggpht.com/_byQV8nmc5FA/TOOTX9ReSYI/AAAAAAAAAQk/hWJ9Kf5GcAA/s800/%EC%8A%A4%ED%81%AC%EB%A6%B0%EC%83%B7-1.png
result: http://lh5.ggpht.com/_byQV8nmc5FA/TOOTYGnBCUI/AAAAAAAAAQo/sQmJlvlJJVU/s800/%EC%8A%A4%ED%81%AC%EB%A6%B0%EC%83%B7-2.png

It seems fine, but notice that the receive signal goes 10 times faster than the original one. (see time-axes in both plots)

And lots of uO (usrp overrun). Don't think about uO because I run it on Virtual Machine, then what the hell is going on this flow graph?!

And I really wanto know the ratio of an interpolation and a decimation
I thought that interpolation:decimation = 2:1, because USRP sink processes 128Msamples/interpolation and USRP source processes 64Msamples/decimation
In order to match up both in same value, it has to be 2:1

In short, I want to know about 2 things.
1. RX goes 10 times faster than TX
2. ratio between interpolation and decimation

Your scope sinks aren't using the same sample rate. One is using 500, one is using 5k.
2:1 is correct for the ratio I believe.
Not sure what is causing your overruns.
Keep in mind that your true sample rate is being determined by your USRPs, not by your samp_rate variable.


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