Lux, Jim via time-nuts writes:

Use a window and optical transmission ?

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On 12/13/22 1:24 PM, Poul-Henning Kamp wrote:

It's hard to get power in with optical transmission.

Say you have your favorite 10811 based GPSDO and you want to see how
it's going to work in your diving bell loaded with heliox (with a coax
cable to a floating buoy with the GNSS choke ring antenna) - but not
really...

This is sort of different from the whole He, H2 question - but it raised
the interesting question of operating devices at pressures other than
room temp.  We've talked a lot on the list about schemes of varying
complexity to thermally isolate an oscillator - well, putting it in even
a partial vacuum might  be effective, one really wants to get the MFP to
"bigger" than the gap between device and wall, so convection stops being
an issue: then it's just radiation, but even if you pump down to, say,
1/100th bar, the bulk thermal conductivity will be (I think) 1/100th of
what it is in "room air".

On 2022-12-13 04:10 PM, Lux, Jim via time-nuts wrote:

I recall from a number of decades ago when I worked on aerospace systems
that used large amounts of H2, that the lower explosive limit for
hydrogen in air was 4%, so we had all our sensors set to alarm at 2%.
Keep in mind, though, that if hydrogen gets loose it can burn, but if
pure oxygen gets loose, EVERYTHING will burn. And as others said, a tiny
quantity only has so much energy available.

Steve Hendrix

Hi

Given that you can keep feeding gas to the pot ( = minor leaks in the back yard
aren’t that big a deal …), grab a tube of epoxy and attack the pressurized side of
whatever connector happens to be handy. Probably do several layers. Run
carefully spaced bare wires through the epoxy.

My choice for a connector would be something circular. I’d go that way  just to
make the mounting hole easy to drill and later maybe plug with a bolt. A 4 pin “mic
connector” comes to mind ( and there’s one sitting loose right over there ….).
Power, ground, signal out, signal in maybe.

Epoxy may be overkill in this case. RTV might do the trick. It certainly would be
easier to strip off afterwords. A lot depends on just how “valuable” the pressure
pot happens to be …...

Bob

Lux, Jim writes:

The thing about vacuum is that you have to make sure you still get
rid of enough heat to not cook your devices.

I know of at least one early cube-sat which did not deploy it's
antenna, because somebody didnt heat-sink the FET which should have
delivered power to melt the launch-restraints.

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Poul-Henning Kamp      | UNIX since Zilog Zeus 3.20
phk@FreeBSD.ORG        | TCP/IP since RFC 956
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Never attribute to malice what can adequately be explained by incompetence.

Hi

If we are off to a stabilized / vacuum environment, it’s a pretty good bet that there
will be multiple sources of leaks / outgassing / whatever to mess things up. You
will have a pump of some sort to deal with what’s going on. The better you can make
things, the smaller a pump you would need. It’s still a pump and under some sort
of servo control ….. loop back to “pressure sensor is out of whack” a week or two
ago :)

Bob

Steve Hendrix, P.E. via time-nuts writes:

There is a very informative monograph from NASA (sorry, dont have a link
at hand) about their early experiences with cryogenic rocket fuel.

A particular interesting episode is where they have a leak in a LOX tank,
and then the firetruck of the emergency response catches fire because
they are downwind.

--
Poul-Henning Kamp      | UNIX since Zilog Zeus 3.20
phk@FreeBSD.ORG        | TCP/IP since RFC 956
FreeBSD committer      | BSD since 4.3-tahoe
Never attribute to malice what can adequately be explained by incompetence.

Hi

I am embarking on building up a block - a common collector Butler
oscillator with an overtone  crystal at 98.304 MHz

on on FR4 , I've used the Philips NPN type  BFS17.

The results <=10 Hz phase noise were quite disappointing. (-69 @ 98 MHz)

Does anyone have any good suggestions. Maybe something slower (larger)
like a 2N5179 (or its SMT counterpart MMBT5179) will be better.

regards

glen

On 13/12/2022 11:14 am, Lux, Jim via time-nuts wrote:

Hi

If you are inside 10 ppm, it’s a good bet that you are in a region that is still influenced by the
resonator / oscillator Q. At 98 MHz this is an offset of about 1 KHz. First thing to worry about
is the Q of your resonator.

Past that, transistors vary a lot. This brand vs that brand matters. VHF transistors are rarely
well characterized for audio noise. Their performance vs spec is not a great thing to use as a
selection parameter.

Simple answer is always going to be: try it and see.

Probably better to look at 100Hz noise. At least there, you have pretty well defined limits as
to what’s ok, good, great, and impossible. As you go from -120 to -140 dbc/Hz, you pretty much
cover that entire range.

Bob

Generally speaking, the 1/f noise at, say 10 Hz, for a crystal
oscillator is determined

entirely by the intrinsic noise of the crystal.  It's really hard to
screw up an oscillator

circuit so badly that it actually contributes anything to the oscillator
1/f noise.

For instance, I'm sure we will all agree that the HP 10811 is a pretty
decent oscillator.

FYI, essentially the same crystal is used in the E1938A that I designed.
Those two

oscillators have ENTIRELY different circuits.  Yet both achieve the same
close in

phase noise as what I measured for the intrinsic noise of the crystal.
By the way,

the HP 10811 uses a selected 2N5179 transistor.  But is NOT SELECTED FOR

1/f NOISE.  Instead it is selected for good gain at high DC collector
current, because

the HP engineer who designed it had trouble with the oscillator starting
up correctly,

in conjunction with the ALC circuit.

If you use the same output circuit for your oscillator as the 10811, you
will also

have no oscillator transistor contribution to far out phase noise.
This is all

very well documented if you want to read the details.

In a previous life, I designed many many 5th overtone oscillators.  Your
crystal

like the Croven ones I used in those days is not of the same caliber as
the

10811 crystal, so therefore it is even more unlikely that the transistor

you choose is going to make any difference.  I always locked my 5th OT

crystals to something like a 10544 oscillator to clean them up close in

using PLL synthesizers.  The 10811 hadn't been invented yet.

I hope this gets you back on the right track.

Rick Karlquist
N6RK

On 2023-01-16 14:26, glen english LIST via time-nuts wrote: