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

[Manikantan Ramadas]

Hi Adam,

Thanks a lot for the clarifications!

- Manikantan

On Tue, 20 Feb 2018 at 9:34 AM, Adam Deller <adeller at astro.swin.edu.au>
wrote:

> Hi Manikantan,
>
> On 20 February 2018 at 14:11, Manikantan Ramadas <mramadas at gmail.com>
> wrote:
>
>> 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.
>>
>
> There are two corrections being made: delay and phase.  Delay is made up
> of integer sample delay and fractional sample delay, to get to the ideal
> delay correction.  Phase can be corrected exactly in one step with simply
> the ideal delay times the LO.
>
> So fractional sample correction has nothing to do with fringe rotation
> (you would need to do it even if you had no frequency conversion, in which
> case no fringe rotation would be needed, but delay still would be).
>
> If you were doing fringe rotation with the integer sample delay, you'd
> only ever be shifting the phase by multiples of 250ns * Vlo, which given
> your LO of 2300 MHz would always be an integer number of turns of phase,
> which means your fringe rotation is basically doing nothing and hence the
> fringe is entirely washed out.
>
>
>>
>> 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.
>>
>
> Any implicit frequency conversion which is done by channelisation at the
> intermediate frequency counts towards to the LO sum.  Imagine you're
> observing at 1500-1516 MHz, and you set the LO to be 1400 MHz.  So your
> band of interest is now mixed down to 100-116 MHz.  Now you sample it at
> 256 Msamples/s with a real sampler, giving you the range 0-128 MHz, and
> then you use a digital downconverter to bandpass filter and shift down the
> frequency range 100-116 MHz by 100 MHz, so your subband is now at 0-16
> MHz.  The signed sum of the LOs in this case would be 1400+100=1500 MHz.
>
> If your digital downconversion from the IF really does center your band of
> interest on DC (so it has both positive and negative frequencies), then
> indeed the LO would be as you describe.
>
> Cheers,
> Adam
>
>
>>
>>
>>
>>> 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
>> ------------------------------------------------------------------
>>
>
> --
> !=============================================================!
> Dr. Adam Deller
> ARC Future Fellow, Senior Lecturer
> Centre for Astrophysics & Supercomputing
> Swinburne University of Technology
> John St, Hawthorn VIC 3122 Australia
> phone: +61 3 9214 5307
> fax: +61 3 9214 8797
>
> office days (usually): Mon-Thu
> !=============================================================!
>
-- 
"By being pleasant always and smiling, it takes you nearer to God, nearer
than any prayer." - Sri Ramakrishna Paramahamsa
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