Re: [Discuss-gnuradio] PNG's of a neutral hydrogen observation at PARI with a USRP.

[Lamar Owen]

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On Monday 06 June 2005 14:58, Marcus Leech wrote:
> >As to the DBSRX, I mentioned to Charles Matt's statement of the 'less than
> > 8dB noise figure' and Charles wasn't real impressed;  need pretty good
> > amplification ahead of it to get a useful noise temp.  We have a few
> > front ends with less than 3 dB NF here, and we will soon have better.  No
> > cryo here, yet.

> That's true of a lot of receivers, particularly wideband ones.  What's
> the noise figure of
>   the R8500? Probably in the same ballpark.

Hmmm, about 6-8 dB I believe.  But that's for 70 MHz IF reception; we have a 
good downconverter and LNA at the dish.

> Add roughly 25dB of gain, with a N.F. better than about 0.6dB in front
> of the
>   receive chain, and along with good spectral integration, you'll have a
> pretty
>   good system.

Yes, this is true.

> I'm surprised that PARI doesn't have any of the better-quality LNAs for
> 21cm--
>   they're readily available and cheap.  Both D.E.M. and R.A.S. make some
>   excellent LNAs at this frequency.

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.
-- 
Lamar Owen
Director of Information Technology
Pisgah Astronomical Research Institute
1 PARI Drive
Rosman, NC  28772
(828)862-5554
www.pari.edu