[Gb-ccb] some lab test results

[Brian Mason]

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
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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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