Solved! The answer is incredibly simple: Oscope.py has a triggered mode, and when you go to XY display mode it STILL uses the trigger... So what I saw was an artifact of a slowly rotating colony of derotated samples, but the view was partially obscured by the triggering effect. So, after clicking on "Auto", I saw what one would expect.
In summary, I think "we" need a nice VectorScope.py :-) Happy gnu-radio-ing! WaveMaker wrote: > > I took a 256,000 samples per second stream from the USRP and directly > displayed the I/Q stream to the X/Y display of a (slightly modified) > oscope.py. > > With a -60dBm unmodulated carrier as RF input, and with the receiver tuned > within 0.1 Hertz of the the carrier, I expected to see a tight > constellation of samples slowly rotating around the origin at the rate of > (Fcarrier - Flocal_oscillator). > > To my surprise however, when the carrier & receive frequencies are within > 0.1 hz of each other, the displayed constellation of I/Q only lives around > [x=0, y=Amplitude]. > > I've been scratching my head on exactly why this is and have concluded it > has something to do with the finite length of the Hilbert transformer - we > don't really have true input phase because the Hilbert transformer is of > finite length. > > Can a Wise One put into concise words what's causing this phenomena? > > > Then, a second, even more baffling observation: Every few seconds, the > phase of the constellation seems to slip by 180 degrees and the > constellation jumps to [x=0, y=-1] for a while. It seems to me that this > phase jump is at the rate at which the constellation SHOULD have slowly > rotated, i.e. the constellation rotation angle is somehow constrained to > being only around those two points. In other words, instead of the phasor > slowly rotating at 0.1Hz, it jumps between [0,1] and [0,-1]. Thinking > about this duzi, I conclude that this is somehow related the the above > insight, but as for the phase JUMP, maybe someone with good fundamental > insight can share it in simple words for a simple man? > > > (PS: my modifications to oscope.py was limited to allowing manual scaling > of both axis in XY mode, so i'm pretty sure i haven't messed up something > significant in the xy display that could cause the above phenomena) > -- View this message in context: http://www.nabble.com/Carrier-Phasor-Slides-to-Zero-Phase-tf1951297.html#a5383094 Sent from the GnuRadio forum at Nabble.com. _______________________________________________ Discuss-gnuradio mailing list [email protected] http://lists.gnu.org/mailman/listinfo/discuss-gnuradio
