[evlatests] No gain change?

[Barry Clark]

This is just the sort of complication we did this test for.  Reducing
the gain (and noise) by 20% would make the previous measurements of
correlator efficiency drop from the unbelievable 108% we saw with the
lobe rotators on to an interesting 88%.

But why 20%.  Can it be a factor of 0.75, the mean gain of the lobe
rotator?

Rick Perley wrote:
>     I reported earlier on the remarkable drop in rms noise when the 
> fringe rotators are turned off (in 3-bit mode).  I also noted that we 
> couldn't rule out the possibility that the gain changed, since turning 
> off the rotators destroys the coherence of the source flux. 
> 
>     This coherence loss won't affect the autocorrelations, so I looked 
> at these, from times when the rotators were on and when they were off. 
> 
>     Autocorrelation amplitudes remain the same for both.  Confidence 
> that these are actually meaningful numbers is provided by noting that 
> they change between the 'on-3C286' and 'off-3C286' observations by about 
> the right value -- 5 Jy (and remains this for fringe rotation on and 
> fringe rotation off). 
> 
>     I thus claim -- much more strongly now -- that the noise reduction 
> observed when the rotators are turned off is real, and is at about 20%. 
> 
>     Ken is now repeating this experiment using the 8-bit path. 
> 
>     Results should be ready shortly...
> 
> 
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