by Dr. Thomas A. Clark, W3IWI (clark @ tomcat.gsfc.nasa.gov)
In my professional life, I have been responsible for the global network of radio telescopes used in Very Long Baseline Interferometry (VLBI) for high accuracy (millimeters on a global scale) geodetic science (see our web site at http://lupus.gsfc.nasa.gov for some info). The geodetic VLBI network operates at S-band (2.2-2.4 GHz) and X-band (8.1-8.9 GHz) and all the stations use HEMTs (High Electron Mobility Transistors) operating at cryogenic temperatures (~20K). At S-band, the HEMT LNAs have amplifier noise temperatures greater than 5K, resulting in Tsys ~30-50K, and at X-band they contribute ~10K to the ~50K Tsys. Cooled HEMTs are used all the time in the radio astronomy world. As an example of the current state-of-the-art, take a look at the plot on cooled FET/HEMT performance on the NRAO web site at http://www.nrao.edu/engineering/ampplot_aug98.html.
To get the cryogenic temperatures, we use commercial 2- or 3-stage closed-cycle Joule-Thompson refrigerators. These refrigerators are rather similar to conventional air conditioners, except that Helium is used as the "working fluid" -- Freon would freeze hard as a rock! The 1st "warm" stage cools the ~300K ambient temperature down by a factor ~4-5 to ~60-70K. The 2nd "cold" stage cools the ~60-70K by another factor of 4-5 to ~15-20K. Some receivers use a 3rd stage to get down to the ~4K level.
Note that I used absolute temperatures in this description. A large portion of the cooling improvement comes from the reduction of the kTB noise contribution, where T is the absolute temperature. Thus cooling from ~300K ambient by ~30C (which is also 30K) results in only a 10% drop in the thermal noise contribution -- hardly worth the effort! Basically, the solid-state thermionic refrigerators just can't "pump" enough heat to make a significant improvement.
A much better approach has been used by optical astronomers for years to cool photomultiplier tubes (see, for example http://www.photocool.com/dricrf.htm ). Dry Ice can be obtained at your local Ice Cream store -- it's even advertised at local "Seven-Eleven" neighborhood stores (see http://www.dels24hours.com/ for an example in the Tampa, FL area!).
I'll tell an anecdote from ~15 years ago to illustrate how well Dry Ice works. It was at a Central States meeting with Barry Malawanchuk (VE4MA) and I competing to win the 1296 MHz Noise Figure contest. Barry brought his newest FET amplifier built with copper water pipe. I brought a 1420 MHz LNA we were using for radio astronomy. I put my 21cm LNA into a foam plastic box with only the coax & bias cables visible and filled the box with dry ice, and let it cool for a few minutes.
Barry was so proud of his LNA and was certain he would win. He was showing ~0.5 dB NF and ~20 dB of gain. My "black box" had more that a tenth dB better NF and about 40 dB of gain. It was also broad as a barn, with little difference anywhere in the 1200-1500 MHz range. My only "tweaker" was a gate-voltage bias pot.
Then Barry realized what I had done and decided to cool his LNA. Unfortunately, copper water pipe presents a huge thermal mass. And since his FET biases were optimized for ambient temperatures, his amplifier was a bitch to tune when it finally got cold. After a couple of hours of tweaking, Barry matched my LNA and we declared a tie.
So my advice, if you want to get a significant performance improvement, try putting your LNA into a foam plastic box; the kind that holds a 6-pack of beer is about the right size. To minimize moisture condensation problems, first dry the amplifier well first, then put it in your kitchen freezer. While still cold, seal the amplifier from moist air by putting it into a condom. Depending on how you bias the amplifier, you might want to bring the bias out separately so you can tweak it cold.
Editor's Note: See also Dr. SETI's column for another answer to the cooling question.
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this page last updated 28 December 2002
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