[evlatests] New T-Data Procedure for EVLA Antennas

[Pat Van Buskirk]

Do these data go into Modcomp files or into the EVLA parameters 
database? I don't believe we currently have a procedure for the VLA 
operators to update these values. Who should we work with to develop these?

Thanks for the info and data,
Pat


Robert Hayward wrote:
> 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