[evlatests] P-Band status

[Rick Perley]

    P-band was looked at yesterday, both in high-speed continuum, and in 
spectral line (but, due to my blunder, only if IFs A and C).    There 
are many problems, leading to the long and tedious listing below. 

    Major issues found:

    1) No fringes from antennas 4, 13, 16, and 25 -- all of which can be 
explained as due to lack of a receiver, or having the receiver turned off. 
    However -- I note that antenna 13 has perfectly normal values for 
Tsys!  Is there actually a receiver in this antenna, which is not 
connected? 
  
    2) Antenna 14 fringed nicely, but all the data were flagged. 

    3) VLA antenna 8 is very unstable, with large, instantaneous, and 
erratic changes in amplitude.  On the AC side, the amplitudes are 
generally too high by a factor of a few, with occasional drops to a 
'normal' amplitude.  The drops in channel A are not coincident with 
those in channel C.  On the BD side, the situation is reversed, with 
generally normal amplitudes, with occasional jumps to a level a few 
times larger.  Again, the jumps in 'B' are independent of those in 'D'. 
       The Tsys data clearly show the receiver is not working properly.  
On the AC side, Tsys is given as about 10,000K.  On the BD side, the 
values appear close to normal, with spikes of up to 100,000K (!) which 
are perfectly correlated with the big jumps seen in fringe power. 

    4) There are significant delay errors remaining  on many antennas -- 
typically 20 nsec (enough to give a radian of phase wrap within a 6 MHz 
bandwidth). 

    5) EVLA antenna 21A is 'pulsing', with generally good data 
insterspersed with large jumps up to a level about ten times higher, 
lasting a few seconds.  Meanwhile, 21B has such poor SNR that a gain 
solution was barely achievable.   21C looks fairly normal, but 21D is 
very noisy. 
       These effects are reflected in the Tsys data:  The 'pulses' in 
21A are matched by jumps in Tsys, to values up to 120,000 K.  (Indeed, 
these look just like what's happening in antenna 8).  The Tsys values on 
the BD side are very odd -- values steady about 670 K, -- so steady that 
the quantization of the measurement (~0.5K) are the only variations 
seen.  Something is broken here. 

    6)  Antenna 24A's bandpass is all wrong, and shows this IF is not 
working -- and the SNR of the fringes we see confirms this.  The other 
IFs from this antenna look relatively normal. 
    24A's Tsys values show the same effects seen in 21B and D -- a high 
value, essentially unchanging, clearly showing the measurement 
quantization as the only changes.  The other IFs look more normal. 

    7) Antenna 26, all IFs, give very weak fringes -- especially on the 
B,D side, where a gain solution was barely possible. 
    The Tsys values for this antenna -- on all four IFs -- show the same 
fixed, high values noted for antennas 21 and 24.  In addition, 26C has a 
quasi-sinusoidal variation (but very small -- 4K variation on a claimed 
840 K Tsys), with period about 20 seconds. 

    8) Antenna 17 gives fringes which are too high on all IFs, and are 
especially erratic on IF 'C'.   Tsys for this antenna is way too high -- 
a few thousand K -- but 'looks' normal otherwise.  The high Tsys is 
sufficient to explain the low statistical weights this antenna has. 

    In general, the derived statistical weights (a measure of estimated 
antenna G/T) are in keeping with the misbehavior noted above.  
Especially poor are antennas 11, 17, 21, and 26. 

    The phase behavior was in general quite good.  But four curious 
exceptions were seen (during the 3-minute observation):
       a) Antenna 3 showed a 50 degree jump, on IF 'D' only.
       b) Antenna 5 had a 30 degree jump up, then down a minute later, 
on IF 'C' only. 
       c) Antenna 10 had a show 40 degree phase winde, on IF 'D' only. 
       d) EVLA antenna 23 showed a 60 degree phase change, over 2 
minutes, on IFs C and D only.