Re: low-noise amplifiers for radio astronomy (was: Re: Ranging and Pioneer)

John (Liberty) Bell wrote:
Jonathan Thornburg -- remove -animal to reply wrote:

Here are two descriptions (again courtesy of google) of reasonably modern
low-noise amplfiiers for radio astronomy uses:
http://gard04.mc2.chalmers.se/papers/GHz2001.pdf
http://www.skatelescope.org/documents/WBLNA98MTTSPaper.pdf

Very interesting (and very familiar). Yes, the noise reducer will work
in this application too

.....Provided the input impedance of the device can be adequately
matched to the source impedance, at the desired frequencies.

I could be a bit out of my depth here, but if we take
http://www.skatelescope.org/documents/WBLNA98MTTSPaper.pdf as an
example, it appears that this may already be a problem, for published
designs, at such frequencies.
If we assume a modest(?) capacitance of 1pF for the gate of the first
transistor (and connecting wire). this gives an input impedance of 17
Ohms at 10 GHz. You thus need to throw away 2/3 of the available
antenna signal voltage in a choke, prior to amplifier input, just to
obtain impedance matching with the transmission line.

Clearly a lower noise amplifier won't help much, if you can't get most
of the signal to its input.


John Bell
http://accelerators.co.uk
(Change John to Liberty to respond by email)
.

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