[evlatests] `Wobbles' before/after T304?

[Rick Perley]

    In answer to Vivek's question, I offer one more plot.  Attached is a 
'waterfall' plot of the phases for a single baseline:  2 x 23 (N56 x 
N64).  This shows one of the slower (longer period), stronger 
'wobbles'.  Time increases vertically, and frequency horizontally.  The 
scale is -1 to -1 degree.  The source is 3C84 -- about 32 Jy.  (It's 
increasing in flux, by the way -- it was in the middle 20s a few months 
ago). 
   
    The smooth left half is IF 'C', right half is IF 'D'.  IF 'C' is 
centered near 14.5 GHz, IF 'C' near 17.5 GHz.  The blue boundaries 
(bottom and top) delineate the sixteen 'subbands'. 

    Note that the right side has a slow (period about one minute) 
sinusoidal variation of about +/- 1 degree, visible on most of the eight 
subbands, and more notably on the first two.  Note how much stronger the 
wobbles are in the first third of the first subband in IF 'D'.  This 
greatly increased amplitude is entirely due to the bandpass correction 
-- which is correcting for the attenuation due to the anti-aliasing 
filter in the 1 GHz path in the T304.   It argues strongly that the 
'wobble' is actually about equal amplitude across the subband. 

    This amplified variation is seen in *all* baselines which show the 
effect. 

    I also draw attention to the inter-subband boundaries on the left 
wide (IF 'C'), where a high frequency weak 'wobble' can barely be 
discerned!  This higher-frequency ripple is also visible in 'D' on (for 
example) the first subband. 

    The wobble is also visible in the calibrator (2.0 Jy) -- but is very 
much weaker.  Hence, it seems to be proportional to the correlated flux 
density. 


Vivek Dhawan wrote:
>
> Rick said he thinks it is downstream of the T304 but I don't remember
> why. And the fact that the wobbles are weak on the weak source makes it
> not an additive but a multiplicative mechanism.
>
> Sleeping on it now...
>
>
>   
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