[Gb-ccb] some lab test results

[Richard Lacasse]

Brian,

One suspicious thing that I noted in your plots is that your 60 Hz 
harmonics (60, 120 and 180) are all systematically low.  There must be a 
scaling factor off somewhere?

Rich

Brian Mason wrote:

> 
> hi all- I've been looking at the ccb lab data collected last week and 
> still have more to do, but here is a progress report.
> 
> 1) Tsys & Tcal measurements.  I collected data with hot and cold loads, 
> with no cals, one cal, and the other cal firing. Results are in the 
> attached ccbTsys pdf file (Tsys and Tcal values) as well as analys19oct, 
> which shows a more detailed consideration of non-linearity.  The png 
> file shows a plot of the non-saturated channels (10 and 11 saturate 
> against a hot load) Trx values vs frequency. At a level of 10 or 15% I 
> am not confident in the overall absolute calibration of Trx because our 
> cold load is a little sketchy, and my room temperature is just a nominal 
> 300 K. We'll do a better measurement on the telescope (at least record 
> the real ambient temperature).
> 
> More importantly there is dramatic gain compression in some of the 
> channels.  I measure this by comparing the cal diode on - cal diode off 
> signal vs a hot load and a cold load respectively-- the cal signal in 
> counts is in one case, against a hot load, only 50% of the cal signal 
> against the cold load (this is one of the channels with -20.7 dBm into 
> the detector). The raw total power counts against either hot or cold 
> load are stable over the duration of the measurements to ~1% so overall 
> gain drifts aren't a big factor here.
> 
> Without a model or measurements of how the linearity varies for the 
> system as a whole vs input power level it's hard to correct for 10+% 
> nonlinearities or to have confidence in the Trx or Tcal results.
> 
> In spite of the sometimes large gain compression I am reluctant to 
> recommend we consider hardware changes at present even were such changes 
> straightforward.  We have a good baseline of tests with the current 
> system; and we don't have a firm assessment of where we are with respect 
> to the post-detector noise floor yet.  Data with the full CCB on the 
> telescope (and possibly with the receiver off, much later) may be 
> helpful to form an overall picture of what's going on so we can come up 
> with a reasonable solution should that be needed. It is not very 
> surprising to me that over such a huge RF band it is a challenge to 
> balance things equally as well as meet our stringent noise requirements!
> 
> 2) power spectra: I collected 100 sec against a cold load with no cal 
> diode firing and derived tp and differenced power spectra. They are at:
> 
> http://wiki.gb.nrao.edu/bin/view/Projects/CcbNineteenOctSpectra
> 
> There is visible 60 Hz and other stuff, variable from channel to channel 
> and usually present in the differenced data.  Considered in quadrature 
> to the other noise, the line contamination typically introduces << 1% to 
> the total noise RMS in the differenced data although in a couple of 
> cases it's up to 2 or 3%. There are some low-frequency zooms on that 
> page as well to look for microphonics.
> 
> These tests were with the box not properly sealed up, and the old/bad 
> cables, so should be taken with a grain of salt. I will repeat the test 
> on the telescope with the final system.
> 
>  Brian
> --------------------------------------------------------------------
> Brian Mason                           | office: +1(304)456-2338
> Associate Scientist                   | fax:    +1(304)456-2229
> National Radio Astronomy Observatory  | mail:   PO Box 2 
> bmason at gb.nrao.edu                    |         Green Bank, WV 24944
> http://www.gb.nrao.edu/~bmason/       |
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