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Re: [Discuss-gnuradio] PNG's of a neutral hydrogen observation at PARI w
From: |
Marcus Leech |
Subject: |
Re: [Discuss-gnuradio] PNG's of a neutral hydrogen observation at PARI with a USRP. |
Date: |
Mon, 06 Jun 2005 16:23:23 -0400 |
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Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.4) Gecko/20030624 Netscape/7.1 |
Lamar Owen wrote:
The one on 26 West is a RAS LNA and feed; NF has been measured at 0.38dB. Our
standard of comparison for 70MHz spectroscopy is a RAS SpectraCyber
custom-built for us by RAS. We also have a rack mounted 1420-70MHz
downconverter in the control room, but find it better to downconvert in the
feedbox to 70MHz; there's 2,000 feet of 7/8's heliax in the line to contend
with, and even 7/8's loss at 1420 isn't great. At 70 it's negligible.
In all of those situations, the noise-figure of the back-end receiving
system is largely
irrelevant, since whatever noise-figure you have in the back end is
overwhelmed by
the gain of the front end. Front-end noise figure is the absolute
critical parameter
in determining system noise temperature.
But other factors in the back-end, like short and long-term stability
are important.
Noise figure isn't really one of them. For example, a gain change in
the back end,
due to temperature drift, etc, of only 0.05dB can completely swamp
your radiometer-mode
observations, particularly if such changes are occuring at similar
periods to the signals
you're looking for. Techniques like beamswitching, etc, are
traditional in dealing with
these issues. But having a very stable, temperature-controlled,
back-end helps
tremendously :-)
--
Marcus Leech Mail: Dept 1A12, M/S: 04352P16
Security Standards Advisor Phone: (ESN) 393-9145 +1 613 763 9145
Advanced Technology Research
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