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Comment by bcaa7f3a8bbc | Hacker News Reader

Fun fact: for the most demanding RF applications, namely, radio astronomy, the front-end low-noise amplifiers are indeed cooled to cryogenic temperature by liquid nitrogen. Here's how it's done at NASA for the Deep Space Network [0]. It's a long paper, see Chapter 4 Cryogenic Refrigeration Systems, PDF page 179 (text page 159). Also, nice photos in page 183 and 188.

[0] https://descanso.jpl.nasa.gov/monograph/series10/Reid_DESCAN...

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The reverse problem is interesting. Consider Voyager 1- its high gain antenna is pointed essentially directly at the sun, a powerful wide-band noise source. How does it detect anything from earth? The DSN has to outshine the sun within the tiny S-band window that Voyager listens to: 20 kW and 62 dB antenna gain.

https://descanso.jpl.nasa.gov/DPSummary/Descanso4--Voyager_e...

https://ipnpr.jpl.nasa.gov/progress_report/42-175/175E.pdf

I haven't crunched the numbers, but I seem to remember that LIGO has the RF guys beat in terms of requirements.

Fortunately for the rest of us, that meant they had to make some measurements, which they were kind enough to share: https://dcc.ligo.org/LIGO-T0900200/public

Hew boy, don't click through to that site unless you have a few hours to check out all the custom LIGO part drawings …

Thanks for the link.

Since the amplifier has both voltage and current input noise sources, there is an optimum source impedance that provides a minimum noise figure, and it’s not an impedance match. This is called a minimum noise match, along with associated noise contours where noise is traded off with impedance match.

Also, the noise from an antenna is dependent on it’s efficiency and what it’s pointing at. Even if it’s input impedance is 50 Ohms, it can generate far less noise than the equivalent resistor.

>This is called a minimum noise match, along with associated noise contours where noise is traded off with impedance match.

You also get to trade off your impedances with gain and stability. RF design is fun!

It’s not which is why I now write software :)

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