Jeff:
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.
Bob
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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