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Re: [Discuss-gnuradio] OFDM results.

From: Jeff Brower
Subject: Re: [Discuss-gnuradio] OFDM results.
Date: Fri, 06 Jun 2008 13:55:47 -0500


> 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.

Ok thanks for your explanation.  Also I found a Wikipedia page on "phase noise" 
even had a formula to convert to jitter.  I understand what you are saying now.


> Jeff Brower wrote:
> > Bob-
> >
> >
> >> 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.
> >
> > -Jeff
> >

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