[evlatests] 3bit vs 8bit flux transfer

[Barry Clark]

The tcals for receiver X019 in ea06 were measured on 2013-nov-22.  The 
receiver was installed in ea06 on 2017-feb-16. Possible something 
happened to it when it was in the lab in 2017 that made it discrepant, 
and anyway, seven years stability is a lot to ask, though some things 
are still quite good at that age.

On 1/26/2021 5:15 PM, rperley via evlatests wrote:
> A short test run was take a few days ago, which compared flux transfer 
> accuracy between 3 and 8 bit quantizers, and which assessed the 
> accuracy of 'absolute' flux density calibration with the 8-bit samplers.
>
> Observations were made of 3C286 and J2007+4029 (the calibrator source 
> for Cygnus A) when the two sources were at the same elevation.  Bands 
> utilized were X, Ku, K, Ka, and Q.  For each band, observations were 
> made in both wideband (3-bit) and narrowband (8-bit) modes.  For the 
> latter, the two IFs were chosen to lie near the opposite ends of the 
> bands.
>
> Test1:  Compare flux transfer between the sources.
>
>    For the 8-bit mode the full switched power values were applied to 
> the data.
>    For the 3-bit mode, only the requantizer (RQ) gains were applied, 
> as it is known that the 3-bit digital switched power ('PDif') is not 
> linearly related to the analog power.
>
>   Result:  The determined flux density for J2007, based on 3C286, were 
> the same, within 1%, for both sampler modes.
>    This surprised me a bit -- I had expected a few percent 
> differences.  Evidently, the actual gain variations of the 
> amplifiers/IFs between the two sources (located far apart on the sky) 
> is very small.  So small that in fact, no gain correction at all is 
> required!  (At least, not for this day).
>
> Test 2:  How good is the gain calibration when based on knowledge of 
> the system gain constants (Tcal and antenna efficiency).
>
>    For this, the full switched power values were applied to the 8-bit 
> data, and 'CALIB' was run on the visibilities from 3C286. If 
> everything is perfect, the resulting gains will all be 1.0.
>
>    They weren't.
>
>    But for most antennas, they were quite close.
>
>    I plotted histograms of the voltage gains for the central SPW for 
> the two IFs in the 8-bit data, each polarization separately. Note that 
> the conversion from correlation to calibrated visibilitites uses the 
> Tcal and efficiency values which are externally determined (i.e., they 
> are tabled quantities).  The observed gain values will be greater than 
> 1.0 when the tabled Tcal is too low, or the tabled efficiency is too 
> high.  In other words, if the corrected visibility is too low, then 
> the actual Tcal is higher than that listed, and/or the actual 
> efficiency is lower than that listed.
>
>    a) X-band.  For both frequencies (8.3 and 11.3 GHz), the mean gain 
> values are between .99 and 1.01.  The spread is quite small (about 
> 0.12).  One antenna is seriously discrepant -- ea06, whose gain value 
> is about 1.3.
>    For the higher frequency, the gain voltage spread is much wider, 
> with the histogram very skewed to the high side.  Antennas 6, 21, 28, 
> and 1 all have abnormally high required gains.  Due to the skew, the 
> median gains are 0.96 and 0.98 for RCP and LCP, respectively.
>
>    b) Ku-band.  The means and medians are between .98 and 1.02. ea06 
> is again discrepant.  The spread in voltage gains is 0.1 to 0.2, with 
> the higher spread at the higher frequency.
>
>    c) K-band.  The means and medians are again .98 to 1.02, for both 
> frequencies.  The spread is very small at 19 GHz (0.1) slightly higher 
> at 25 GHz.  ea06 is better behaved here, but still too high.
>
>    d) Ka-band.  At 32 GHz, the medians are about 1.01, but the means 
> are higher, as there are some antennas with high corrections 
> required:  ea25, ea01, ea19, and ea21.  All are above 1.2.
>    At 37 GHz, the situation worsens.  The medians are about .98, but 
> the same antennas as listed at 32 GHz are quite discrepant.
>
>    e) Q-band.  The trends noted above continue.  Means and medians are 
> both above 1.0 -- and antennas 1, 6, 19, and 25 are truly bad.  (They 
> also have very bad vertical beam profiles).  Excluding the obvious 
> deviants, the mean corrections at 42 GHz will be very close to 1.0, 
> but at 48 GHz, it is about 1.1 -- a 20% error in flux.
>
>   By adjusting the Tcal or efficiency values, the spread in the gain 
> voltages can be greatly reduced, the medians (and probably the means) 
> brought to within a couple percent of 1.0.  The trick will be to 
> determine these adjusted values as a function of frequency.  There are 
> a lot of frequencies!
>
>
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