[evlatests] Linearity at P-band

[Rick Perley]

     The P-band test done last week permits a close look at our system 
linearity over power ranges of ~ 5.
     Short observations of Cygnus A, 3C48, 3C380 and 3C123 were taken.  
The power settings (both for the T304 attenuators, and the requantizers) 
were set on Cygnus A.  This source was chosen for the setup since it 
increases system power by a factor of about four, and it seems prudent 
to do the system gain setups on the strongest source one observes.  (See 
below for justification).
     Two antennas were out of the array at the time:  ea08 and ea11.

     Two tests were then done utilizing the data from subband #6 -- 312 
MHz (BW = 16 MHz).

     1) PDif compression.
     2) Flux density transfer from 3C48/3C380 to Cygnus A.

Results:

1)  PDif ratios were computed in the usual way:  PDif(Cygnus 
A)/PDif(3C380).  The PDif values from two antennas were not useable:  
ea13 has zero mean PDif (diode not switching?), and ea18 has erratic 
values on the 'L' side, and very odd values on the 'R'.

           For the remaining 24 antennas, more than 80% of the PDif 
ratios were between 0.97 and 1.03.  This range seems to encompass the 
typical variations in these values at this frequency band.  The values 
outside this range were all on the 'low' (compressed on Cyg A) side, but 
none was less than 0.93.  Results from L-band indicate that there is no 
discernible effect on the visibilities when the compression is above 
0.90, so I felt safe in applying the switched power to the data.

     This distribution of PDif compression is, I believe, better than 
any other band, when the power jump is at similar levels (a factor of 4 
to 5).


2)  I then calibrated the data, utilizing well-tested methods: delay 
calibration, bandpass calibration, gain calibration.  For the last step, 
the flux densities from Scaife and Heald were utilized for 3C48 and 
3C380.  Note that these data were taken in D configuration, and the 
available models do not include the effects of background sources.   The 
effects of confusing sources are quite evident in the visibilities.  I 
calibrated Cygnus A utilizing a simple average over the antenna gains 
solutions from 3C48 and 3C380.  Minimal (= almost no) editing was 
attempted.

     The results of this are shown in the attached figure, showing the 
visibility amplitudes in the 'RR' (horizontal) correlation, over a u,v 
range of zero through 1000 wavelengths.  The partial resolution of 
Cygnus A is evident.

     There are two major points made by this plot:

     A) Virtually all visibility points on the short spacings are 
tightly clustered and heading for a zero intercept (total flux) of 6500 
Jy.  The Baars et al. value for Cyg A at 312 MHz is 6305 Jy -- about 3% 
less than ours.  This is of no concern since:
                (i) The Baars scale for Cygnus A is not this accurate,
                (ii) No accounting for background sources was made in 
the calibration
                (iii) The Scaife and Heald scale is also uncertain to 
this level, at least.

     B) There are some visibility values significantly below the 
others.  All of these come from ea28.  What is special about this 
antenna?  It did not get a requantizer power adjustment, since it was 
out of the array when this setting was made.  In consequence, its power 
level was set for the 'cold' sky sources -- when on Cygnus A the power 
was four times higher than the standard.  This result clearly indicates 
that visibilities are strongly affected when the (digital) power levels 
are higher than about 30 counts.  In this case, an overpower of a factor 
of four reduces visibilities by about 25%.  (If two antennas had the 
same overpower, the visibilities from that baseline would have been cut 
in half).

     I then ran a self-calibration, using a simple trial image.  The 
resulting visibility function is shown in the other attached figure.  
The gain corrections are nearly all in the range of 0.96 to 1.04 -- 
these are typical variations at this band for any source -- whether it's 
Cygnus A or a weak point source.  This is a very nice result indeed.

*****************************************************

     Finally -- a major appeal to get the requantizer settings made part 
of the general system setup for this band:

     I reviewed the PSum powers as a function of subband and antenna, 
prior to the requantizer settings.  The range is astonishing:  A high of 
over 100, and a low of less than one.  (!!!!!).  These are extrema.  A 
fairer test is the typical:  A high of 40 to 50, and a low of 5.  
Invariably the low is in the 16th subband (highest frequency).  The high 
is always in subbands 4 and 5.  (Subbands 2 and 3 are even higher, but 
RFI is part of the reason).  What we are seeing here is the spectral 
range of input power.  The high end is weak both because of the receiver 
bandpass/gain, and from the steep spectral index of synchrotron 
emission.  By overpowering at the low frequency end, and underpowering 
at the high end, we are seriously affecting the science quality of the 
data taken.

     I claim it's time we implement an effective regimen for setting the 
requantizer gains at P-band.




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