[evlatests] Results from T304 Attenuator and Requantizer Tests

[Keith Morris]

> 
>                 Expt. 1:  Large Stokes V visibilities, with 6 x 17 and 
> 14 x 17 having values nearly equal to the total flux.  (This is caused 
> by the LCP apparently overflowing, giving visibilities near zero). 

Antennas 6, 14, and 17, IFC only, had sampler RMS's that were high by
60% or so.  T304 input attenuators are set in the 0 - 4dB positions.
Output attenuators are pinned at 31.  The T304s are unable to provide
the correct power to the samplers.  Reducing the input power target will
re-center the adjustment range of the attenuators.

>                 Expt. 2:  All Stokes V visibilities are less than 10 Jy 
> (which is less than 1% of the Stokes I). 


>                 Expt. 3:  Large Stokes V found on 17 x 23 and 6 x 17.  
> Smaller, but still significant values are found on many other correlators. 
Antennas 6, 17, and 23 all have reasonable attenuator values and
reasonable output power on both calibrator and Cygnus.

>                 Expt 4:   A single antenna -- ea27 -- had modest (10 to 
> 30 Jy) values of 'V' on its baselines to 2, 3, 4, 12, 18, 25, and 26 -- 
> a behavior quite different than any of the other experiments.  All other 
> baselines showed no 'V' at all.

Nothing unusual about attenuator values, total power, or digitizer rms
on any of these antennas. The same antenna/IFs (6, 14, 17)that had high
sampler rms in Test 1 continue to show high sampler rms in test 4.  Of
these, only ea06 shows up in the table below.
> 
 >The following table lists the antennas and
> baselines, and approximate residual:
>        Antennas       Baseline       Residual
> ---------------------------------------------------
>       1 x 5               w7 x w8            110 Jy
>       1 x 15             w7 x w9            110 Jy
>       5 x 15             w8 x w9              50 Jy
>       3 x 12              e8 x e9               50 Jy
> 
>     The following baselines have residuals between 30 and 50 Jy:
>       15 x 20           w9 x n9
>       1 x 26              w6 x w7
>       3 x 26              w9 x w6
>       1 x 4                w5 x w7
>       3 x 4                e9 x w5
>       3 x 25              w4 x e9
>       20 x 28            n8 x n9
>       3 x 6                e9 x n3
> --------------------------------------------------------------------------------------


If anyone would like more detail, please see or email me.  Michael and I
have discussed a test in which we combine tests 2, 3, and 4 into a
single test, i.e.

a) compensate the T304 input power for the reduced bandwidth of L-band,
b) use the more powerful source to find attenuator settings, and
c) use the more powerful source to find requantizer settings.








>     The distribution of the afflicted antennas is very odd, with a great 
> preponderance of antennas on the west arm. 
>     This is not RFI -- the calibrator scans -- for all setups -- gave 
> lovely data. 
>     This is not due to delay errors -- the analyzed data are from a 
> single 2 Mhz channel at the middle of the band.  (SNR is not an issue 
> for this source!)
>     This is not due to an error in the 'golden' image -- a 2nd 
> self-calibration, using the image made from the data itself, produced no 
> changes to the antenna gains. 
> 
>     *** Summary ***
> 
>     While we were overdriving the requantizers when observing Cygnus A 
> -- this is not the cause of the two central problems: 
>        a)  The compression in the switched power when observing a strong 
> source,
>        b)  The apparent introduction of non-closing errors when 
> observing a strong source. 
> 
>   
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-- 
Keith Morris
National Radio Astronomy Observatory
1003 Lopezville Rd.
Socorro, NM 87801
575-835-7060 (phone)
575-835-7027 (fax)