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Re: [Discuss-gnuradio] UHD I/O Data Types

From: Josh Blum
Subject: Re: [Discuss-gnuradio] UHD I/O Data Types
Date: Wed, 20 Apr 2011 16:50:03 -0700
User-agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv: Gecko/20110223 Lightning/1.0b2 Thunderbird/3.1.8

On 04/20/2011 04:31 PM, Arya Santini wrote:
> By a real signal, I mean a signal like a real valued sinusoid. Say in
> a grc graph, I have this:  signal generator (sine, float o/p) ---->
> float to short converter ----> usrp sink (short i/p). In this example,

Time out.

The USRP sink w/ short input is this hack-y thing in gnuradio that we
called interleaved shorts. Every other even sample is I and every other
odd sample is Q. In binary, the format its identical to a

So you are generating a real sinusoid and sending every other sample
into I and Q, which would make I and Q basically identical with a
sinusoid of double the rate.

> I'm inputting a stream of shorts representing a pure sine wave to the
> usrp module. I wanted to ask how I can do this exactly when I call the
> send() to the UHD inside a C++ program? (an array of float values
> represent the pure sinusoid, which is the waveform I wish to
> transmit).

You cannot represent a "pure" sinusoid with a real signal. Think e^(jw)
vs Re{e^(jw)} Maybe someone could think of a better way to say this.

All USRP devices input and output complex baseband. You need to
represent your signal in complex baseband. I suggest experimenting by
feeding your signal into the _complex_ fft plotter in grc and see how it
looks spectrally, because thats how your actual spectrum will look when
you transmit, but shifted up by fc Hz.


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