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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 |
Bob-
> 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"
that
even had a formula to convert to jitter. I understand what you are saying now.
-Jeff
> 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
> >