[evlatests] 3 and 8 bit comparisons

[Rick Perley]

    Vivek gave a brief report on yesterday's short tests. 
    I've also reviewed the data, and have a few additional observations. 

    The observations were in an OSRO-type mode, a single subband for the 
3-bit path, and a pair of subbands for the 8-bit path.  CBand, near 5 
GHz.  BW = 128 MHz, Channelwidth = 2 MHz.  Alternating between 3C286 and 
a nearby blank field, one minute per field.  About 30 minutes duration 
for each. 

    A)  Amplitude Stability. 

    Excellent for 8-bit -- no gain changes of any kind. 
    For 3-bit, we have the usual change of gain at the beginning, due to 
warming up.  Because of antenna slew, we did not see the first 2 minutes 
of the run -- when the antennas got on source, all samplers *except* 22R 
and 27R were fully warmed up, as no coherent gain changes were seen in 
the data. 
    Gain stability from scan to scan at 3-bit varies a lot.  15R, 15L, 
and 22L are as good as the 8-bit path.  27R is almost as good.  22R, 
27L, 28R, and 28L show notable changes from scan to scan, typically of 
order 5% in power, sometimes a few times higher. 

    B) Gain Amplitudes. 

    For 8-bit, all gains were nearly the same.  But for 3-bit, they were 
notably different.  A table will be useful ...

             8-bit             3-bit
-----------------------------------------
    15R   6.25             6.95
    15L   6.15             7.2
    22R   5.80             12.0   < -- NB!
    22L   6.65               5.4
    27R   6.30             11.0   < -- NB!
    27L   7.00             6.3
    28R   6.10             7.7
    28L   6.20             6.60
--------------------------------------
    The sense of these gains is that a high gain means a low power --- 
22R and 27R are weak, by a factor of about 5 dB in power.    Remember 
this when we get to the sensitivity plots ...

    C)  Phase stability.

    Good, and the same, for both paths.    (Except for antenna 27 on the 
BD IFs -- I'll send a special note about this). 

    D) Bandpass stability

    Very good for both paths.  No changes over the 30 minutes above 1%, 
and probably much better than this, as these short scans don't give very 
high SNR.  However, the individual solutions for the 3-bit path are 
clearly noisier than those for 8-bit.  For an explanation, read on ...

    E)  Sensitivity. 

    3C286's spectrum is known, so we can determine the absolute 
sensitivity by using the blank field noise statistics.  The 
histogram-plotting program UVHGM is excellent for this purpose.  I 
selected 15 minutes of the test when all gains are stable, and 
determined the baseline noise statistics, after applying both the gain 
and bandpass corrections.  I utilized the central 45 channels to improve 
the statistics.  The results are below:

    rms noise, in mJy, per baseline/polarization for the 3-bit path.  
RCP in the upper right, LCP in the lower left. 

        15         22          27             28
------------------------------------------------
15 |   xx         310       240        210
22 |  190          xx       370        310
27 |   210       170         xx        250
28 |  230        190       210          xx
----------------------------------------------

    For the 8-bit path,  no matrix is required -- all combinations gave 
160 mJy. 

    There is an excellent correlation between the high noise entries in 
the above table and the high gain corrections listed in Section B.  
Indeed, the only entry in the noise matrix which is at the correct level 
(22 x 27 in LCP) is from the two antennas-IFs with the lowest gain 
corrections.  And the inverse is most certainly true -- the two 
antenna-IFs with the highest required gain corrections (22 and 27 in 
RCP) have by far the highest noise in the cross-correlations.  From this 
I deduce that the powers into the samplers are -- in effect -- too low.  
What is puzzling to me is Ken's statement that the powers were carefully 
adjusted before this experiment to give the same width in the state 
counts.  Do I misunderstand something?