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

[Jens Elsner]

Just a quick answer - I hope I understood your problem correctly this
time.

On Fri, Aug 17, 2007 at 05:23:06PM -0600, Bahn William L Civ USAFA/DFCS wrote:
> 
> I'm asking the question for several reasons - some theoretical and others 
> practical. The basic driving force is as follows:
> 
> I have the following processing chain:
> 
> BBC -> FILE -> GR -> USRP -> RF -> USRP -> GR -> FILE -> BBC
> 

[file format of input and output should be equal]

> This raises the following questions:
> 
> 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. The fundamental concept here is that
sine and cosine are orthogonal, i.e., one can transmit a ("real") valued
signal modulated by a sine and, at the same time, transmit another
("real") signal modulated by a cosine. Just add them up 
and seperate them by demodulation (and filtering) in the receiver.

This should answer your question: if you're just concerned with
transmission of the data, you can divide your input stream
into 2 streams and transmit one over I and the other over the Q branch.
You might want to preprocess your signal and transmit it digitally,
though (to add channel coding, synch frame etc.).

If you're looking at a radar/sonar application or simple AM, you
probably just want one carrier. Leave the I or Q branch blank.

Regards,
Jens