[USRP-users] LFTX output amplitude

Michael Rahaim mrahaim at bu.edu
Tue Dec 9 16:02:34 EST 2014

Hi all,

I'm working with GNURadio (Version 3.7.6git-142-g82779dce) and some USRP
N210's with LFTX / LFRX daughter cards. I've been running through some
basic tests and I came across something that seems odd to me:

I have a simple flowgraph that uses a signal generator and a UHD sink with
QT GUI sliders to adjust the carrier frequency and the signal frequency in
real time (imaginary value is set to 0).

If I set the frequency of either the carrier or the signal to 0 at the
start, I get the same voltage range from the LFTX every time I run the flow
graph; but if I'm (a) running the flow graph with no carrier, then add a
carrier and remove it or (b) running with a constant signal (signal
frequency=0) and increase the signal frequency then set it back to 0, the
resulting voltage range from the USRP changes every time.

The second scenario makes sense since turning the signal frequency on and
off sets the constant output of the signal block to the last value of the
signal before the change from frequency "f" to 0. What I don't understand
is why a signal input to the UHD sink with a range from -1 to 1 generates
various output ranges when adding / removing the carrier. My assumption was
that the LFTX acts as a pass through when carrier is set to 0, which
doesn't seem to be the case. Is this by design?

Thanks in advance for any information,

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