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Re: [Discuss-gnuradio] Recovering x(t) from IQ samples

From: Jonathan P Jacky
Subject: Re: [Discuss-gnuradio] Recovering x(t) from IQ samples
Date: Sat, 18 Aug 2007 11:06:45 -0700 (PDT)

On Sat, 18 Aug 2007, Jens Elsner wrote:

On Fri, Aug 17, 2007 at 05:23:06PM -0600, Bahn William L Civ USAFA/DFCS wrote:

Q1) One of the formats in which I can send data to the USRP is as IQ data. What 
does the USRP do with IQ data pairs? In the USRP documentation there is a block 
diagram of the Digital Down Converter, but there is no diagram of the Digital 
Up Converter.

It is exactly the opposite of the DDC. ...

I'm not sure if this is pertinent for your project, but there is code that does 
just this in the FPGA, in the MRFM project in GNU Radio.  This is not the usual 
FPGA code that runs in the USRP, but it can run in the USRP.  This code 
implements the block diagram at


in the slide labelled Heterodyne control.  The block labelled DUC does the 
upconversion, reconstructing x(t) from I and Q.

The DUC in this code is not the DUC used by the usual USRP FPGA code, which (as 
I understand it) uses a built-in DUC in the Analog Devices chip.   For our 
application, we need to synchronize the DDC and DUC exactly.  The only way to 
do that was to put the DUC in the same FPGA program as the DDC.

This FPGA code is in the respository at


The Verilog code in mrfm_proc.v describes the block diagram discussed above.

Matt Ettus wrote this code.

Jon Jacky

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