Very true Sent from my iPhone
> On Apr 3, 2018, at 7:51 PM, Bob kb8tq <[email protected]> wrote: > > Hi > > 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 !! > > Bob > >> On Apr 3, 2018, at 5:29 AM, Dana Whitlow <[email protected]> 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, >> and >> 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. >> Junction >> 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 >> outside >> 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 >> wires >> inside the dewar were often found to have fairly sharp bends at improbable >> locations upon warmup for diagnostic purposes (or due to cooling system >> failure). >> >> 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. >> >> Dana >> >> >>> On Tue, Apr 3, 2018 at 12:26 AM, Mark Sims <[email protected]> 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. >>> _______________________________________________ >>> time-nuts mailing list -- [email protected] >>> To unsubscribe, go to https://www.febo.com/cgi-bin/ >>> mailman/listinfo/time-nuts >>> and follow the instructions there. >>> >> _______________________________________________ >> time-nuts mailing list -- [email protected] >> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >> and follow the instructions there. > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
