[evlatests] Aliasing in narrow-BW observations

[Walter Brisken]

Maybe Ken or someone else knows more than I do about this, but I remember 
seeing that the VLA supports some narrow band modes with the band of 
interest offset from the DC edge of 50 MHz sampled band by the width of 
the filter.  I think these are BW codes 7 and 9 if I'm not mistaken.  If 
such a mode exists, would it be a possible work around for this problem?

-Walter

On Wed, 10 Oct 2007, Mike Revnell wrote:

> The problem is the finite width of the transition band of the second
> stage of the transition filter. (at least as regards the transition
> filter, the global delay error at 6.25MHz is a different problem.)
>
> The polyphase filter architecture used in the transition filter has some
> nice features that make it possible to build the filter at all. You must
> pay for what you get though. The transition band width is determined by
> the number of taps in the filter which translates directly into how big
> the FPGA must be.
>
> A fundamental characteristic of these filters is aliasing of the half of
> the transition band in the stop band side of the response into the pass
> band side. This is a consequence of the fact that down sampling is
> performed essentially at the input of the filter.
>
> Mike
>
> Jim Jackson wrote:
>> Rick,
>>
>> I'll put on both systems engineering and project management hats to
>> answer this.  I spoke with Mike Revnell about this as soon as we saw
>> your message.  We are both convinced the problem is almost certainly
>> with the FIR filters implemented in the D351 deformatters. The
>> solution would likely be a more complex filter in a larger FPGA. A
>> larger FPGA means redesigning the deformatter which would obsolete
>> the roughly half project worth of them we've already built as well as
>> the expensive FPGA's we already bought for the rest of them.  It
>> would likely also involve about a year of Mike's time to build it and
>> possibly mean descoping a receiver from the project to pay for it.  I
>> think we will need to find a way to live with this until the WIDAR
>> comes along or find a way to correct for it in post processing (if
>> that's possible).
>>
>> Jim
>>
>>
>> At 04:43 PM 10/9/2007, Rick Perley wrote:
>>
>>>     Miller Goss ran a spectral line test last Thursday which has
>>> revealed an heretofore unsuspected problem which has significant
>>> consequences for 'transition' observations.
>>>
>>>     In his test, Miller observed in a very narrow bandwidth -- 780 kHz,
>>> in mode '2AC', producing 256 spectral channels for each of the two
>>> correlations.  After generating the bandpasses (which looked entirely
>>> normal),  he soon found that all, and only, the EVLA-EVLA baselines had
>>> large offsets in their bandpass-corrected visibility amplitudes.
>>> 'Large' here means of order 20% or more.  VLA to VLA baselines were
>>> fine, as were EVLA to VLA baselines.
>>>
>>>     There are only two ways that I could imagine this happening --
>>> either there is RFI in EVLA antennas, which only correlates between
>>> them, or there is some lower level broadband signal which is present in
>>> EVLA antennas only.
>>>
>>>     The first hypothesis was easily dismissed, as all the spectra are
>>> clean.  There is no RFI.
>>>
>>>     For the 2nd hypothesis to be viable, it has to only affect narrow
>>> bandwidth observations, as numerous (and nearly innumerable) tests and
>>> observations have been made at wider bandwidths, without any hint of an
>>> EVLA-EVLA extraneous correlation.  From this consideration, it was
>>> suggested that perhaps we are seeing some aliasing around the baseband
>>> end, which is present only in EVLA antennas, and which is significant
>>> only in the bottom ~500 kHz of so of the passband.
>>>
>>>     A test for this was executed this afternoon, and has clearly shown
>>> that this is the origin of the large offsets seen in Miller's data.  In
>>> this test, I observed a strong calibrator in mode '1A', at BW = 3 MHz.
>>> (Other BW were observed also -- and some other problems were found -- to
>>> be reported on later).  There are then 256 channels across the spectrum,
>>> for  12 kHz resolution.  Good-looking bandpass solutions were found.
>>> Bandpass-calibrated correlator spectra were then generated, which
>>> clearly shows the extent of the problem:
>>>        Extending up from baseband, for about 700 kHz, there is a large
>>> (up to a factor of two!) additional correlation in the signal.  Both
>>> amplitude and phase are affected, but the effect is much stronger in
>>> amplitude.  Both 'positive' and 'negative' additions are seen.  Beyond
>>> about 1 MHz from the baseband end, the effect is no longer detectable in
>>> any of the individual spectra.
>>>
>>>     This is a serious 'transition' issue.  The effect does not originate
>>> in the EVLA's antenna or DTS electronics, nor in the VLA's baseband
>>> filters.  It must be due to aliasing in the digital-to-analog conversion
>>> system we use to enable EVLA signals to be used for science observing
>>> (and tests!).   Unlike most 'transition' issues, this one will not
>>> lessen in time -- it grows with the number of EVLA to EVLA pairs we have
>>> -- and will not be eliminated until the new correlator arrives.
>>>     I believe the effect is time-variable (based on a small amount of
>>> data), so a simple 'closure' correction will not be easy, and perhaps
>>> not viable at all.  It is perhaps important to measure this variability,
>>> to see if it exists, and what its characteristics are.
>>>     This aliasing effect is small for wide-band continuum observations,
>>> and for relatively wideband spectral line observations, one can always
>>> choose to remove the affected channels on EVLA-EVLA baselines.
>>>     The effect is quite serious for narrow BW spectral observations --
>>> anything taken at BW less than ~3 MHz will be notably degraded.
>>>
>>>
>>>
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>>>
>>
>>
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