[evlatests] New T-Data Procedure for EVLA Antennas

[Robert Hayward]

Hello All,

The purpose of this memo is to describe our proposed scheme for handling 
the receiver calibration data that will be required for the EVLA Antennas.

In the past when a new receiver is installed on an EVLA Antenna, someone 
from the FE Group (usually me) would send out a sanitized list of 
Receiver Temperature (Trx), Noise Calibration Temperature (Tcal) and 
Solar Calibration Temperature (Scal) values as determined by our SOIDA 
measurement system. I refer to this collective set of numbers as the 
receiver's "T-data".

Remember that the VLA only uses the Tcal & Scal values for the 2 default 
frequencies. The AC and BD pairs have frequency offsets of 50 to 100 
MHz, depending on the receiver band. For the old VLA antennas, we'd send 
Operations the 2 Tcal values (one for each polarization). If the 
receiver had a Solar noise diode, then the 2 Scal values would be added.

The EVLA & VLA receivers are continuously calibrated radiometers and 
these cal numbers are used to determine the System Temperature using the 
backend calibration formula:

    Tsys = 15 * TP/SP * Tcal

where TP is the gated Total Power and SP is the Switched Power as 
measured at the T5 module (since we no longer have F4 modules on the 
EVLA antennas, the T5 is the only synchronous detector available for the 
new system).

However, the EVLA will eventually be capable of handling Tcal tables. 
The VLBA already does this. For a new receiver on the VLBA, we send VLBA 
Operations the Tcal values at every 25 MHz across the band for the lower 
frequency receivers, and for every 100 MHz at the higher frequency 
bands. The VLBA software applies the appropriate Tcal number for what 
ever frequency is being observed. We intend to do something similar for 
the EVLA.

For the EVLA receivers installed to date, we have been sending tables of 
  Trx's, Tcal's and Scal's to VLA Ops. As the Modcomp isn't able to 
utilize all of this data, Ken Sowinski would then interpolate the cal 
numbers for the default frequencies and enter them into the Modcomp. So 
all the rest of the T-data was essentially ignored. However, at some 
point we will have to start using calibration tables so that the Tcals 
across the wide (up to 10 GHz) bandwidths can be applied appropriately.

Rather than send out an email that includes a table of T-data every time 
we install a new receiver, or replace a failed one, I propose to 
distribute a spreadsheet that contains the T-data for each of the 8 
cryogenic receivers on an EVLA antenna. As an added benefit, this Excel 
spreadsheet will contain plots of each receiver's Trx, Tcal, Scal and 
Gain curves. Additionally, it will contain a page that automatically 
calculates the Tcal & Scal for the standard default frequencies (which 
should make Ken happy). The worksheet will be provided in both the 
native Excel XLS format as well as a PDF file. I have attached the file 
for Antenna 14 as an example.

I'm hoping that this will allow Ops to easily find the T-data for 
updating the calibration parameters but also provides the in-house 
astronomers with a quick way to look at such things as:

- The Trx across the band. I often get queries from them asking how well 
a receiver works at the edge of some receiver band. They would merely 
need to look at the Trx and Gain plots to find out.

- Ditto for Tcal numbers. As we provide wider bandwidths on the EVLA 
receivers, the 2 default frequency values are somewhat meaningless if an 
astronomer is working at a band edge (like what Rick seems to be doing a 
lot these days). A look at the table of Tcals would allow a better value 
of Tcal to be quickly found and applied.

- Questions of receiver power levels could be resolved quickly by 
looking at the Gain data (i.e., how much less gain is there on a Q-Band 
receiver at the high end?). The Gain vs. frequency plot will allow the 
astronomer to get a handle on the RF band shape.

- Finally, the plots of Trx will allow one to make relative comparison 
of the receiver performance on various antennas (i.e., is Antenna 13's 
L-Band really worse than the one on Antenna 16?).

The spreadsheet is 33 pages long. For each of the 8 receiver bands, 
there are 2 pages for data tables, 2 pages for the Trx/Tcal and 
Gain/Scal plots, and 1 page for the Default cal values.

For the time being they will be located on filehost. PC users will find 
them at

\\Filehost\evla\techdocs\fe\T-Data

while Unix users can access them at

/home/evla/techdocs/fe/T-Data

This location may change when VLA Ops has time to digest the new scheme.

As noted earlier, there will be 2 versions. The first is the actual 
Excel spreadsheet which the FE Group will use to manage the SOIDA data 
and automatically generate the various graphs. The second is the PDF 
version of the Excel worksheets which just about everyone should be able 
to download and read. I think we should use a well defined file name 
convention like

    T-Data-EA14-2000-10-03-RHH.xls
       and
    T-Data-EA14-2000-10-03-RHH.pdf

which identifies the EVLA Antenna (EA14), the revision date (Oct 10, 
2006) and the initials of the person who last modified it (in this case, 
me).

Note that it will be a generic spreadsheet and will include sections for 
the receivers that are yet to be built (i.e., S, U & Ka-Band). Remember 
also that the wideband receivers are not yet in production, so the 
current L, C & X-Band systems are interim narrowband systems and their 
Trx curves look pretty pathetic when plotted against the full EVLA 
bandwidths.

Henceforth each time the FE Group installs a new receiver, or swaps one 
out, we'll send out a notice out of the particular change (e.g., a new 
Q-Band on Antenna 23). The notice will include the "Default" page (in 
pdf) as an attachment (i.e., "T-Date-EA14-Default-2000-10-03-RHH.pdf"). 
Ken (or whoever is the designated guardian of the Modcomp Tcals) can use 
it to find the new values to plug into the Modcomp. When WIDAR is up and 
running, someone in VLA Ops will have to extract the table values (in 
whatever delimited format is required) and enter them into the WIDAR 
TCal database.

Things will get way more complicated for Solar Cals once the Lilie 
Coupler-Fed scheme is implemented. This is because a programmable 
attenuator will be used used to dynamically control the noise diode 
power. Thus the WIDAR Scal database will require a noise level value 
versus attenuation setting (or, alternatively, a max noise level and a 
table of attenuation factors). Not much thought has gone into this yet.

I have placed copies of the new T-data files (i.e., Excel spreadsheet 
and a pdf versions) for Antennas 13, 14, 16, 18, 24 and 26 in the above 
noted directory. They reflect the present state of all of our installed 
L, C, X, K & Q-Band receivers.

As a final note, I believe that as some antennas come out of the Barn, 
the Modcomp may retain the Tcals from their earlier life as a VLA 
antenna so weird Tsys's may be reported when these erroneous Tcal values 
are applied to the newly installed receivers. It is probably worth 
having Ken (or his designate) check all the the Tcals shown on the 
"Default" sheet for each receiver on each antenna to make sure we have a 
complete set of proper Tcal values.

Please take an opportunity to look over the new T-data files. Comments 
and suggestions are welcome to we try improve the EVLA calibration 
experience.

Cheers,
Bob
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