[Discuss-gnuradio] Solved! Carrier Phasor Slides to Zero Phase

[WaveMaker]

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)
> 
-- 
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