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Re: [Discuss-gnuradio] Problem with sinc pulse transmission using RFX240

From: Matt Ettus
Subject: Re: [Discuss-gnuradio] Problem with sinc pulse transmission using RFX2400
Date: Wed, 29 Aug 2007 23:37:03 -0700
User-agent: Thunderbird (X11/20070326)

S. Faisal A. Shah wrote:
> Hi,
> I am using a USRP with RFX2400 to transmit a sinc pulse using
> usrp_siggen.py routine and my own sinc pulse generator that is a
> modified version of gr_sig_source_c.cc. I am using a separate USRP
> with RFX2400 to receive the pulse using usrp_rx_cfile.py. The
> transmitter and receiver are one meter apart.
> First, I tried to put the sinc pulse on the "I" branch (real) and
> zeros on the "Q" branch. But the amplitude of the received pulse train
> is time varying. In fact, the received pulse train seems to be
> amplitude modulated by a low frequency signal (in KHz).

If you put a signal on I and nothing on Q, then you SHOULD see a
time-varying amplitude in the output.  Do the math in octave and you'll see.

>  Second I tried
> to put the same sinc pulse on both I and Q and had the same problem.

Again, do the math.  The same signal sent to both I and Q will always
result in what you are seeing.

> For curiosity, I repeated the experiment by putting few cycles of sine
> wave on both I and Q and encountered the same problem of amplitude
> variation. Finally, I tried using a truncated sine wave on I and
> cosine on Q and it works perfectly fine.

Exactly.  Sine on I and cosine on Q (or vice versa) will give you a
single frequency tone at the output, because it is what is known as a
complex sinusoid.

>  I am having difficulty in
> understanding the problem. Does this mean that I can only put sine and
> cosine on I and Q, respectively? Is it possible to transmit only
> single component not quadrature using RFX 2400? I am thinking to set
> MUX to disable the Q branch at the transmitter and receiver. Any help
> will be highly appreciated.

You need to generate analytic signals, which means they have both I and
Q components.  This has nothing to do with the RFX2400 and everything to
do with the fact that the USRP uses IQ signals.  This in no way a
limitation on the capabilities, you just need to understand analytic
signals.  There are many DSP books which cover the concepts, and if you
look back through the list archives you will find a couple of long
expositions by me on what IQ signals are.


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