[evlatests] Aliasing in narrow-BW observations

[Barry Clark]

> 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.)