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Re: [Discuss-gnuradio] Negative values sent to usrp.sink_c ?


From: Matt Ettus
Subject: Re: [Discuss-gnuradio] Negative values sent to usrp.sink_c ?
Date: Mon, 31 Jul 2006 13:17:46 -0700
User-agent: Mozilla Thunderbird 1.0.8-1.1.fc4 (X11/20060501)

Lee Patton wrote:

I generated a data file in which each complex sample is stored as a
32-bit value, with real and imaginary values ranging from 32000 to
-32000.
If I connect a file source directly to a file sink, then I "receive" the
transmit data just fine.  However, if I put a USRP in between them, then
it appears that the negative values in my data are being converted to
positive before being transmitted.

I've attached a plot of my transmit data (transmit.jpg), and my received
data (received.jpg).  As you can see, the fourth pulse shows up as
positive instead of negative.  Does anyone have any ideas why?  It seems
like I'm making some simple mistake or an incorrect assumption, but I
can't figure it out.  I've attached my python script, and data file for
examination.
Are you doing any carrier tracking? Have you modified your RFX board to be fully coherent? I.e. disabled the onboard clocks as in

http://comsec.com/wiki?USRPClockingNotes


Ignore the text where it says " On the back of the MOTHERBOARD,"

Matt





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