Re: [Discuss-gnuradio] Carrier Phasor Slides to Zero Phase

[WaveMaker]

Where is this DC offset compensation implemented?  


Matt Ettus wrote:
> 
> 
> You are probably seeing the DC offset compensation removing your signal 
> when it looks like DC.
> 
> Matt
> 
> 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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