[evlatests] Aliasing in narrow-BW observations

[Mike Revnell]

With DSP there always seems to be some kind of trick and you never know 
until you try.

In this case even to explore the possibilities will take a non-trivial 
(and completely unknown) amount of work. It needs to be determined how 
important this capability is required during the remaining transition 
period and a determination on the part of the referees (aka management) 
whether the distraction from other projects is justified.

Some background.

The culprit is undoubtedly the second stage of the transition filter. 
This filter is implemented with lookup table multipliers in one of the 
deformatter FPGAs. It would require reconfiguring the FPGA for a change 
of tap weights and recovering any state information lost during the 
reconfiguration. The mechanisms are in place to do this and would take 
less than a second to accomplish but may take some changes to software.

The phase correction FIR is only 64 taps so probably isn't too useful in 
this case.

mike.

Barry Clark wrote:
>> From evlatests-bounces at donar.cv.nrao.edu  Wed Oct 10 07:49:10 2007
>> Date: Wed, 10 Oct 2007 07:46:45 -0600
>> To: Rick Perley <rperley at nrao.edu>, evlatests at aoc.nrao.edu
>> From: Jim Jackson <jjackson at nrao.edu>
>>
>> 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
>>     
>
> Can this edge be sharpened at the expense of letting the rest of the 
> band go to pot?  We might have separate sets of filter coefficients 
> for narrow band.  (The phase correction FIR could be used, also.)
> _______________________________________________
> evlatests mailing list
> evlatests at listmgr.cv.nrao.edu
> http://listmgr.cv.nrao.edu/mailman/listinfo/evlatests
>
>