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Re: [Discuss-gnuradio] Basic RX/TX

From: Matt Ettus
Subject: Re: [Discuss-gnuradio] Basic RX/TX
Date: Mon, 29 May 2006 23:40:25 -0700

On Sat, 2006-05-27 at 12:16 -0700, Thomas Schmid wrote:
> Hi,
> I have a question about the basic RX/TX boards. What should I see if I
> hook the basic TX board to an oscilloscope if I send out a QPSK
> signal? Here is what I do:
> - I generate a QPSK signal in GnuRadio
> - I send the signal to a complex file source and the signal looks right
> - I send the signal to the complex USRP source and have a BasicTX on
> it. The outputs TXA and TXB are hooked up to a oscilloscope on channel
> 1 and 2 respectively.
> Now, what should I see on the oscilloscope? Is it the baseband signal?
> How does the I and Q phases get mixed together? Does that depend on
> the mux values? Unfortunately I couldn't find a good block diagram of
> the TX path on the wiki, i.e., the link to [A system block diagram of
> the USRP receiver and transmitter path] on the wiki page
> "http://www.comsec.com/wiki?UniversalSoftwareRadioPeripheral"; is dead.

If you set the carrier frequency to 0 you will see the I signal on the I
channel out and the Q signal on the Q channel out.  This will only be
useful with the LFTX board since the BasicTX will block DC.

If you set the carrier frequency to something other than 0, then the I
and Q signals will be upconverted and sent to the outputs.  As long as
the carrier frequency is greater than your signal bandwidth, then you
only need either the I output or the Q output since they are now
double-sided real signals.  You can still use the I and Q connectors
with an image-reject mixer if you need.


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