Noise figure measurements--tire kicking requested

It made it hot. Since it wasn't vegetable oil I could not cook fries in it ...

Oh, oh, when the ATF guys come out then trouble can loom ... :-)

I should have kept mine. IIRC it had over 100 resistors, soldered to a fighter wing shaped copper structure that made sure the SWR was ok up to

Since my budget was tight as a student I used an old honey bucket which had rather thin steel. Eventually it rusted out from the outside in. But it has lasted 30 years.

Dunno about your neighborhood, but LN2 was readily available to the public in mine, and surprisingly cheap. (~$1/l, IIRC).

-- Cheers, James Arthur

I've been told that LN2 tends to convert itself into liquid oxygen, and the temperature creeps up in the process.

John

The examples I've seen (being waved around in a sales guy's hand, nothing more) cooled from 300K down to 110K, and looked like a Chinese pagota they narrowed so much. I think it was nine stages. I have no idea what they did for heat spreading -- but given that they were competing with split Stirling coolers that cost between $5K and $20K, they could bring some pretty exotic materials to bear.

You used to be able to get blocks of the stuff from Baskin & Robins stores, to pack around big buys of ice cream in a styrofoam cooler. I don't know if you still can -- but you won't get arrested for asking.

Hopefully. They need to not lose so much power that the phone becomes uneconomical. You need to not let things bleed in and out enough that your measurement is good enough.

It occurs to me that with flexible cable your loss is going to vary as you move things around. I have no idea how much -- but one certain advantage of your TE cooler idea is that you can let everything sit still.

Having thought of that, I think that if I did it the high-school-science way I'd clamp everything up so the cables didn't move, and just slide my beakers of hot or cold liquids into place without disturbing the cable.

I'll be damned -- you didn't. I'm just so used to R being resistance that my brain latched onto it.

ND/1248299

Looks good, if it's not inductive.

Please post your results, by the way -- this looks like a cool project, even if I have absolutely zero need for something like this at the moment.

Wow. Now that's impressive. Do Pons and Fleischmann know about this? I would expect that since the stable isotopes of nitrogen would convert to quite unstable (and thus more energetic) isotopes of oxygen that the energy loss to extra mass would actually cool things down considerably -- and things working against the laws of thermodynamics would heat up some physicists.

Or did you mean that the LN2 condenses O2 out of the air, thereby raising the boiling temperature of the fluid? If that's the case then as long as your flask is designed to always blow out enough N2 vapor to prevent ambient air from entering, it should take a good long time for the temperature to climb.

And monitoring the temperature is probably a good thing if it matters, although 77K is probably cold enough that you don't need to know the temperature precisely.

Air liquefies a bit above LN2 temperature, so yeah, eventually you get some LOX mixed in if you don't keep the cork in the hole.

You see it when you pour LN2 into the metal funnel for the cold trap of an evaporator--liquid air condenses on the underside and on the tubing.

My shop is up about a dozen steps from the street, so getting a 60-litre dewar up here would be no joke. Besides, if it were to drop, it would probably wipe out the office downstairs, which has windows set into wells slightly below ground level. That would make me a little bit unpopular. The nice 20 litre ones you can carry are the ticket, but it's still a bit on the pricey side.

Part of the point of all this is to provide calibrations for some of my other test gear. HP stuff holds its calibration extremely well until the relays in the attenuators croak, but I'd be a lot happier with an independent check of some of this stuff.

Cheers

Phil Hobbs

So let us know whether you see any simulated gravity waves. ;)

Cheers

Phil Hobbs

How does it do that? Liquid oxygen... "with a freezing point of 50.5 K (-368.77 °F; -222.65 °C) and a boiling point of 90.19 K (-297.33 °F, -182.96 °C) at 101.325 kPa (760 mmHg)"

Perhaps the LN2 picks up moisture, or more likely it's another of Larkin's bits of misinformation... aka wet dreams?

...Jim Thompson

I've done something like that for this client--I have a Skyworks SKY65050 pHEMT cascoded by a BF862, with a 500-ohm drain resistor, driving a fast op amp.

The noise is about 0.4 nV/sqrt(Hz), based on measuring the noise with the input open-circuit and then with one, two, three, and four 50-ohm feedthrough attenuators in cascade. The noise is noticeably less at open circuit than with 12.5 ohms on the input.

I'm going to try cascoding it with another pHEMT and with a SiGe bipolar instead, and see if I can get it to be quieter. The 1/f noise corner is about 10 MHz, but the input current is very close to zero--just a couple of nanoamps. Whether the current noise is correspondingly low is part of the question at hand.

Cheers

Phil Hobbs

LN2 has a boiling point of 77K, which is below the boiling point of LOX. So it'll condense LOX out of the air.

In fact you can do this with equipment available in a high-school chemistry lab. I know -- my high school chemistry teacher did it! I think he just used a big test tube with LN2, maybe jammed in some styrofoam (or maybe a styrofoam cup of LN2), with a little test tube held in the LN2. LOX condenses inside. You get a pale blue liquid (just the shade of the sky, for some reason) that's really cold, and that really promotes combustion.

Maybe that's why Dewar flasks have such a narrow neck?

Test tube, yes. Dewar, won't the LOX sink? Or are they perfectly miscible in the liquid state? I don't know. ...Jim Thompson

We had a guy come do a demo like that in my Grade 11 chemistry class. He did the usual LN2 demos--freeze a banana and use it to hammer in a nail, shatter a piece of rubber tubing, craze a handful of marbles....

But then he did something really fun, that I'm perfectly sure he never did before or since: he made some LOX with a little lecture bottle and a test tube, then took some steel wool, heated it with a torch, and

Joerg, you should have been there--I can't do justice to the sound effects. Nobody was hurt, amazingly, but afterwards we were digging little steel balls out of the back wall of the classroom for souvenirs.

Cheers

Phil Hobbs

Ooh. I feel cheated, now.

Interesting idea. I still have some MIM diodes in stock, and their zero-bias resistance is down in the 100-ohm range.

I have some wide bandwidth photodetectors, so I could just plumb them together with a calibrated attenuator. Problem is, they'd have to be fed with a laser, and laser intensity noise is always well above the shot noise. A flashlight and a photodiode is the ticket at lower frequency, for sure.

Thanks. I still have some YSI interchangeable calibrated thermistors I can use.

That's good wisdom, thanks.

Cheers

Phil Hobbs

On a sunny day (Wed, 14 Dec 2011 14:13:46 -0500) it happened Phil Hobbs wrote in :

May not be 4 you, but I bought the Stirling cryocooler on ebay, it was part of a super conductor filter unit, about 275 $, plus 175 $ shipping IIRC. So I can just cool any small object to any temp minimum around 65 K, make my own LOX or LN2, vapour chamber.. lots of nice experiments... Well worth the money, they are new around 10,000$ They will run on 110V AC 60Hz with a variac too, about 120W. I tried an audio amp and Linux siggen at 60 Hz, works perfectly. and of course it has all the thermocouples and drive electronics in the unit as well. It even has a RS232 interface... You have to test ebay every now and then for something like this, these are superconducting filters used in cellphone towers, so if they tare down a tower it may appear on ebay. I think every lab should have one, especially if you do things where low temps help a lot.

Oh, and you can make icecream too with it:-)

I had a similar experience in high school. I was assisting a visiting demonstrator of liquid air. We had a table on the auditorium stage on which he had placed a ceramic crucible. He filled the crucible with liquid oxygen made from the LN2, heated a ball of steel wool to red heat and dropped it in the crucible. It gave a remarkable display of molten steel and then burned itself through the crucible bottom and then the wood table top. Fortunately, it burned itself out before hitting the stage floor.

Anyone remember the GM science thing that toured the US in the mid '50's doing all the demonstrations previously mentioned, plus "cars of the future", "kitchens of the future", etc. ?? ...Jim Thompson

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Are diode lasers noisy? I looked at the noise around threshold once,

Excess noise near the threshold and then it looks like it comes back to the shot noise limit... of course I didn't get much above threshold... 10uA is pretty wimpy.

(The line was drawn by eye, and no real analysis done. Oh, Zero light amplifier noise was subtracted.)

George H.

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Yes, they're generally between 15 and 40 dB above shot noise, depending on the type. And that assumes there's no mode hopping, of course.

Cheers

Phil