[time-nuts] quartz / liquid nitrogen
Dr. Ulrich L. Rohde
ka2weu at aol.com
Tue Apr 3 20:43:44 EDT 2018
Sent from my iPhone
> On Apr 3, 2018, at 7:51 PM, Bob kb8tq <kb8tq at n1k.org> wrote:
> If the objective is great phase noise far removed from carrier, there’s a gotcha.
> Let’s say you have a 10 dbm source at room and it’s broadband is at KTB of -174 + 1db.
> That gives you -183 dbc. You cool your oscillator to whatever and KTB goes down
> to -194. You do a bang up job at that temperature and get within a db there was well.
> You now have a 10 dbm source with -203 dbc.
> Run the super cooled signal through a coax out to the room environment. Pass it through
> a 50 ohm gizmo and …. KTB is back at -174. Your source is at -184 dbc. To *use* the
> signal, you likely need to cool whatever it’s driving as well.
> While one might say …. that pretty weird. Well, similar things do happen. Many an ultra
> low phase noise OCXO gets sold, only to find that the “next stage” isn’t as quiet as the
> system guys hand hoped. Hmmm …. errrr ….oops !!
>> On Apr 3, 2018, at 5:29 AM, Dana Whitlow <k8yumdoober at gmail.com> wrote:
>> Many years ago, circa 1977, I was moved to try some crude tests on a few
>> semiconductor devices at LN2 temperature (77K).
>> These tests were very crude, involving dunking the parts into the LN2 bath,
>> many failed outright. Most of the devices tested were in plastic packages.
>> Here are the results as I remember them, applicable only for the survivors:
>> Silicon bipolar transistors: The DC beta fell to very low values.
>> forward voltages rose considerably.
>> Silicon JFETs: Seemed to continue working reasonably well.
>> Silicon MOSFETs: Same as JFETs
>> Red LEDs: The junction forward voltages rose considerably, to about 5V as
>> I recall. The light output per unit current rose truly spectacularly.
>> My first experiences with seriously-cryogenic RF amplifiers were at the
>> Arecibo Observatory beginning about 11 years ago. These were all either
>> GaAs- or InP-based and we cooled them to ~15K, generally leading to
>> input-referred amplifier noise temperatures of ~3K. Many of the devices
>> needed continuous exposure to light to work properly when cold, and the
>> metal block amplifier packages had holes in the lid directly over the active
>> device chips. Small red LEDs in ordinary plastic packages were inserted
>> in the holes and were driven at a few mA, generally in a series string.
>> Since cool-down was fairly gradual over a span of at least a couple hours,
>> there was little problem with thermal shock and almost all LEDs survived
>> cooldown and warmup for the several cycles they experienced during
>> my 10 years at the observatory.
>> RF amplifier biasing was invariably done with opamp circuits to maintain
>> set drain currents and drain voltages, with said bias control circuits
>> the dewar at room ambient temperature. Failures were not too uncommon,
>> largely attributed to connector misbehavior at low temperature. Formation
>> of "ice" (really frozen air) inside the dewars was suspected because fine
>> inside the dewar were often found to have fairly sharp bends at improbable
>> locations upon warmup for diagnostic purposes (or due to cooling system
>> Cooling was done with a closed-cycle gaseous He system, using the
>> Gifford-McMahon cycle. Note that He does not liquefy (at reasonable
>> pressures) until around 4K. All dewars for this kind of work depend on
>> high vacuum inside for thermal insulation, with black body radiation
>> and direct conduction through wires and mounting structures being
>> the principal remaining heat leaks.
>> At these temperatures, maintenance of high vacuum inside the dewar was
>> essentially automatic because all components of the inward-leaking air
>> were known to freeze out. This could lead to a hazard because over time,
>> months or years, enough air could freeze out to result in dangerously high
>> internal pressures upon "thawing" when the dewar was warmed for any
>> reason. For this reason, all dewars were equipped with blowout plugs
>> to avoid high pressure's damaging the dewars themselves.
>>> On Tue, Apr 3, 2018 at 12:26 AM, Mark Sims <holrum at hotmail.com> wrote:
>>> And you want your semiconductors to be in ceramic/lided packages with the
>>> bond wires flapping in free air. Bond wires embedded in epoxy like to
>>> break... don't ask how I found this out ;-) ... it brings back bad
>>> memories... and makes bad memories... Quantum chips have very
>>> elaborate/specialized bonding to survive liquid helium... even with that,
>>> thermal cycling still breaks them.
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