[evlatests] Results from 20-db LSC converter attenuator

[Rick Perley]

    We ran the 'solar attenuator' test script on April 4.  Stephen White 
extracted the essential results, which I am only now able to review.  
There are some 'interesting curiosities'...

    To review:  The test is quite simple:

    Observe 3C84 and the sun at standard L, S, and C band frequencies.  
For each, spend a couple minutes observing in normal ('cold sky') mode, 
then turn on the 20-db solar attenuators, and watch the system react.  
In principle (and in an ideal world), there should be a dramatic 
factor-of-100 drop in power delivered to the samplers, which is restored 
within a few seconds as the T304's attenuators react.  The fringe 
visibilities should be about the same in either state ( a couple of dB 
different will be fine, as the switched power monitor should correct the 
difference), as should be the calculated system temperature.  (In fact, 
because we have not any solar cals yet, the calculated Tsys, while on 
the sun, will be only noise, as the regular cal will be swamped by the 
sun's emission.)

    What we saw:

    A)  L-Band

    The total power (PSum) change while on 3C84 matches the 
expectations:  A dramatic drop, lasting three seconds, followed by a 
rapid (1 or 2 seconds) rise to about the same level as before. 
    The behavior upon transition while on the sun is entirely 
different!  There is the drop as before, but instead of a rapid rise, 
ther is (usually) a slow rise, lasting 20 seconds or more, culminating 
in a power which is as much as six times the original value!!  After 
this, a dramatic drop to (about) the correct level is seen.   For some 
antennas, an entirely different behavior is seen, with a large up *up* 
(rather than down), followed by a slow stepwise decline to the correct 
level. 
    Why is the behavior while on the sun different?  The actual power 
level is the same (the solar power having been attenuated by the T304's 
own attenuators).  So how does the T304 'know' that the power it is 
seeing is from the sun, rather than cold sky, and why is it reacting 
differently? 
    The other curiosity is in the before/after values of Tsys, when on 
3C84.  Since the T304 and T302 should not add any appreciable noise, the 
Tsys values should be mostly independent of the actual attenuator 
setting.  This is generally true -- but there are notable exceptions:  
(All are in LCP, as these are the plots Stephen left for me):
    ea15, Tsys doubled after transition (Tsys is higher with the 20 dB 
T302 attenuator in). 
    For ea08 and ea24, there was also a rise, but by about 10%. 
    For all other antennas, the change was not perceptible. 

    B) S-Band

    We would hope the behavior would be similar to that for L-band, but 
we would be wrong in thinking it so ...
    On 3C84, the power levels drop as expected, but rather than stay 
there for 3 seconds (as at L-band), they typically  stay there for 5 to 
7 seconds, then jump back up to the more-or-less correct level. 
    The behavior on the transition while on the sun is generally similar 
to L-band, except antennas which do not have the drop plus slow 
rise/overshoot at L-band, do while at S-band, and vice versa. 
    The change in Tsys while on 3C84 is similar to L-band -- most 
antennas return, some (like ea08) do not. 

    C)  C-Band

    The power transition while on 3C84 is like L-band:  3 or 4 seconds 
down, then rapid return to expected levels. 
    The power transition while on the sun is generally like L and S 
bands, but appears to be faster -- the 'slow rise' is terminated 
earlier, with less overshoot. 
    The rise in system temperature upon transition on 3C84 is more 
notable at C-band:  Antennas with large (25% or more) changes include:  
ea08, ea14, ea17, ea18, ea22, ea26.  (Notable is that for ea15, which 
doubled its Tsys at L-band, there is no perceptible change upon 
transition at C-band). 

    Many thanks to Stephen, for his diligence in getting these plots out.