[Difx-users] Reg. Fringe Rotation in general..

[Manikantan Ramadas]

Hi Adam,

Thanks for the response! Please find my replies inline.


> 1.The delay in the fringeRotationVal should be T, not T'.
>
>
 So, I should use the accurate delay value estimate T instead of the
rounded-off value T'? OK can do. But my logic was that this error is what I
am correcting in Fractional sample correction, and I should be doing
fringe-rotation for the same delay d that I am using in delay-tracking of
the other station's data stream.

Also, fractional sample correction is normally done differently in a
> baseline based correlator, but the scheme you describe of going into the
> frequency domain, correcting, and then back should work (albeit somewhat
> computationally expensively).  However, you have either a typo or an error
> in your equation: the phasor exponent should be -i*2*PI*channelfreq*Tfc,
> not -i*2*PI*Vlo*Tf.  Channelfreq is the frequency of the channel, which
> would range from 0 to 4 MHz (for upper sideband data), -4 to 0 MHz (for
> lower sideband data), or -2 to 2 MHz (if this complex subband really has
> been mixed down to be centred on 0 Hz - see next point).  So the frequency
> you are multiplying by is not fixed (otherwise what would be the point of
> going to the frequency domain?), it is a ramp across the band.  If this is
> really an error and not a typo, then this is certainly destroying your
> fringes!
>

This is a typo! I am using channelfreq as you have mentioned. This is a
complex sub-band varying from -2 MHz to 2 MHz and centered at 0. In sky
freq-range, our central channel is centred on 2.3 GHz varying from 2.298
GHz (-2MHz) to 2.302 (+2MHz). The channelfreq that I am using in fractional
bit-shift correction is 0 for the central 2.3 GHz sub-channel, +2MHz at the
upper-end of the band, -2 MHz at the lower-end of the band and channel freq
steps of 4 MHz/64 in between range (FFT size is 64).


> Finally, Vlo (the local oscillator frequency) should be the signed sum of
> the effective LO, which is likely at the edge of the 4 MHz band, not in the
> middle.  If you have this wrong, you'll then have a residual fringe rate of
> half your bandwidth, or 2 MHz in this case.
>
> By "signed sum of the effective LO", you mean the LO values set at each
stage if I am doing it at multiple stages, right? In this case, it is a
single step conversion from RF to IF (70MHz).
In my complex sub-band (mixed to be centred at 0 Hz), I am using the
central local oscillator frequency - 2.3 GHz (for my central channel). This
is OK right? I am doing same way for other channels as well.



> Don't worry about the interpolators in DiFX: they are just down-sampling
> the 5th order polynomial which is valid for 120 seconds to a 1st or 2nd
> order polynomial which is valid for of order a few microseconds, for
> computational efficiency.
>
> Thanks for clarifying that!


> Note that autocorrelations are never really a great test, since they are
> insensitive to the phase, which is where almost every mistake is made :)
>

Yeah, right! :-)


>
> 2.  The fractional bandwidth is irrelevant for fringe rotation: you can
> think of the chunk of spectrum as having been shifted in frequency by Vlo
> in the downconversion process.  That holds true for every frequency within
> the downconverted subband: assuming upper sideband data, then the lowest
> band edge has been shifted down from Vlo Hz to 0 Hz, and the upper band
> edge has been shifted down from Vlo + B Hz to B Hz.  Hence they've all been
> shifted by Vlo, and that single correction is good for all of the
> frequencies within the subband.
>
> OK!

- Manikantan.

-- 
"By being pleasant always and smiling, it takes you nearer to God, nearer
than any prayer." - Sri Ramakrishna Paramahamsa
------------------------------------------------------------------
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://listmgr.nrao.edu/pipermail/difx-users/attachments/20180220/4c15af67/attachment-0001.html>