[evlatests] No Wobbles at S-Band!!!

[Rob Long]

We can spend money to eliminate the known problem at the risk of needing 
to "re-spend" money to fix the as of yet undiscovered problems. We were 
waiting on the RFI sweep to ensure we get the correct filters to resolve 
all the issues.  We know what we think we need to solve the 128 birdies, 
but what if there are other issues that the filter we buy doesn't 
correct. Not my decision of course, but that was our rationale.

Rob

Rick Perley wrote:
>     The origin of the 'wobbles' seen yesterday is now understood. 
> 
>     It is due to an RFI spike (actually, an internal 'birdie') at 2944 
> MHz, strong enough to beat with the continuum emission of 3C147, and 
> visible only on the north-arm baselines because of the (relatively) slow 
> fringe rate at the time. 
> 
>     A bit of numerology:  2944 = 23 * 128.  This is the 23rd harmonic of 
> a 128 MHz fundamental tone.  The problem is well known -- indeed, 
> perusal of the spectrum shows a harmonic in *every* S-band subband 
> (conveniently, each is 128 MHz wide, so it's easy to find).  The odd 
> multiples are stronger, as expected. 
> 
>     Not only is the origin of this problem known, but so is its 
> solution.  We have been waiting for a long time to survey the spectrum 
> for other such internal birdies, but it seems that this 'RFI Sweep' 
> survey is still some time away.   Perhaps it's time to get on with 
> eliminating this known problem? 
> 
>    
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