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 > > _______________________________________________ Discuss-gnuradio mailing list Discuss-gnuradio@gnu.org http://lists.gnu.org/mailman/listinfo/discuss-gnuradio
