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Re: [Discuss-gnuradio] help -- how to implement transmitting periodic on

From: madengr
Subject: Re: [Discuss-gnuradio] help -- how to implement transmitting periodic on/off tones
Date: Mon, 29 Nov 2010 12:40:45 -0800 (PST)

Yes, LO leakage, more specifically DC imbalance in the I and Q channels; I
assume you are using a daughter board with an IQ modulator?  This is
supposedly corrected for in the firmware, however, probably not done over
temperature.  You can probably tweak it by adding/subtracting a few LSB of
DC offset to your I and Q samples.  It's an iterative approach, tweaking
until you minimize it.  Won't be able to do better than what's spec'd in the
data sheet though.  I have never been able to get less than -60 dBc; not on
URSP hardware but my own stuff.  It's really only a problem in really
wide-band modulation (e.g. spread spectrum) where your leakage poke above
the modulation, or with tons of averaging and you are trying to hide your
signal.  Of course it's a problem for you, but best bet to blank off the
buffer (or switch the T/R switch) to improve isolation.
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