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

 From: Songsong Gee Subject: Re: [Discuss-gnuradio] USRP problems (interpolation, decimation, sampling rate, frequency) Date: Wed, 17 Nov 2010 23:31:09 +0900

Thank you, it works.
(I have done what you told me to, but at that time it didn't work. MYSTERIOUS....)

Anyway I've got a new problem

RX seems similar with TX, but it... JITTERS in an amplitude or a frequency

In other words, RX keeps a shape of TX, loosely.
Observing for a fine duration, however, an amplitude an a frequency is moving around the value of them from TX.
Therefore, it seems to be stretched or shrunk in vertical and horizontal.

Yes, it doesn't matter for a human to know that RX keeps a shape of TX
but I'm worried that USRP, GRC, PC may not recognize that.

2010/11/17 Steven Clark
On Wed, Nov 17, 2010 at 3:41 AM, Songsong Gee wrote:
I use two USRPs. One sends a sinusoidal signal, and the other receives it.

A flow graph and a result are here

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.

-Steven

--
Seokseong Jeon a.k.a., Gee Songsong
combined course in ITCE (IT Convergence Engineering), POSTECH WCU program