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Re: [Discuss-gnuradio] OFDM results.

From: Bob McGwier
Subject: Re: [Discuss-gnuradio] OFDM results.
Date: Fri, 06 Jun 2008 14:43:05 -0400
User-agent: Thunderbird (Windows/20080421)


Good to hear from you again. I am distinguishing betwen clock recovery operations in the receiver and the oscillator feeding the down conversion mixers. Even though there is some commonality in the sources for these two DIFFERENT things on the USRP, implementation factors of the clock recovery, how well the DBS-RX takes the up conversion of the oscillator by MANY factors to make the downconversion mixer sources, etc. are almost enough different to be independent noise sources.

I believe the clock recovery is working just fine or these pictures would look like crap. The jitter of the constellation increasing with increasing distance from the origin is VERY indicative of angular phase noise in the downconversion oscillators in (say) the DBS-RX.


Jeff Brower wrote:

In your sixteen QAM and other figures I see two effects.

Notice just the slightest hint that arcs through the top four
constellation points in the 16 QAM is not straight.  This curvature is
caused by nonlinearity.

Your result almost surely can NOT be clock jitter.  If you had a lot of
clock jitter,  the pictures would look much worse.  Notice the
dispersion gets larger as your proceed away from the origin.  This is
almost surely phase noise in some oscillator.

"Phase noise" of an oscillator to me means jitter.  Can you clarify?  Do you 
mean a
sinusoid oscillator that has some issue with shape; i.e. non-linearity?  Thanks.


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