There used to be small IBM PC compatible "palmtop" computers capable of running for many days on a pair of AA batteries. HP 200LX was pretty cool. I think it had 80C186 running at 10 MHz or thereabouts, with 1MB of RAM, monochrome graphic LCD and PCMCIA cards for storage. Norton Commander and Turbo C run on it without any problems.
There was also the HP 95LX with a smaller screen (40x16, 240x128), a 5.37 MHz NEC V20 CPU, and 512K or 1MB of RAM.
Also, the 100LX with monochrome CGA (80x24, 640x200), a 7.91 MHz 80186-compatible CPU, and 1MB or 2MB of RAM but the extra was used for file storage as drive C:.
The 200LX was the same as the 100LX but had newer software in ROM.
The history of precision machinery is a steady ladder of using old machines to produce new more accurate components, and then new more accurate machines.
But simply making something smaller is not synonymous with it automatically becoming more accurate or capable of achieving higher resolution. If anything, very high precision machines are typically larger in size than the less accurate ones.
In a series of lectures in 1938, tv inventor Philo Farnsworth explained the need to develop "the tools to make the tools to make the tools" to make high tech stuff. He originated the phrase.
Sam Zeloof's new company is a serious business. He went from studying electrical engineering at CMU and doing semiconductor fabrication as a hobby, to running a well funded team to produce equipment for mini-fabs.
It is actually a great example which shows the difference between cobbling something together in a garage, to produce a one-off demonstration, and having a mini-fab which is capable of delivering the results reliably. The difference in the required effort is many orders of magnitude.
The "RAM in a shed" guy started with a Ph.D. in semiconductor technology, and was running a company before he made the improvised lab at home and started to make videos about it. He uses the lab to test some of the steps for the specialty devices which he invents.
Saw Jeri demo some of her early transistors, it was one of the most inspiring things seen in a hackerspace… 15 years ago now? Sheesh. Glad to see AtomicSemi/fab2 in existence, that is a relief. We simply have to get desk-side fabrication happening, faster, imho ..
Cold gas thrusters used in space applications typically produce 60-70 kgf of thrust for 1 kg/s of gas flow, which means the average axial velocity of the jet is just under 600-700 m/s. It is a little more when the gas expands into vacuum and a little less when it expands into ambient air at the sea level.
How fast the gas expands through an opening depends on the pressure ratio between entrance and exit, and for ordinary air the flow reaches the speed of sound at the narrowest part when the pressure ratio exceeds about a factor of two. The flow never becomes supersonic in the constriction itself, but after the constriction the flow can continue to accelerate to supersonic speeds, especially if the constriction is followed by an expanding nozzle (converging-diverging, or de Laval nozzle), although this is not strictly necessary. For example, one can observe shock diamonds in the air coming out of an ordinary plastic bottle filled with slightly compressed gas: https://www.youtube.com/watch?v=61tsXGUc6nU
Thanks for the clear explanation, and you deserve a separate upvote just for that video link. "slightly" -- compared to what Tesla is claiming, sure, but 5+ bar is no joke -- "you'll put an eye out, kid"
I wonder if the shock waves visible in the video are still due to the converging-diverging aspect, just with no bell around the diverging part.
If we look at the flow lines, they converge and diverge even when the "nozzle" is a section of a pipe.
In rockets, there is a so called "aerospike" engine [1], which does not have any bell-shaped nozzle at all, but instead uses the ambient air to always shape the flow for an ideal expansion ratio. In theory it is great, but in practice it remained mostly a pie in the sky.
Modern satellites, in particular Starlink, use optical transceivers that are identical to the ones used for the fiber optic links, with an addition of all the required parts to make them work across free space. So a link between a pair of satellites is very comparable to a single pair of optic fibers.
The cables can go the entire distance between the data centers. The satellites probably connect to their closest neighbors, and any long distance transmission would have to make multiple hops, and share the bandwidth with everything else in the system going through the same nodes.
Since there are hundreds of undersea cables, with a dozen fiber pairs or so each, the cable system has several orders of magnitude greater throughput compared to the current constellation of satellites.
The cable contains not only the fibers, but also optical amplifiers to compensate for the losses. Even for 24 fibers, the amplifier is a substantial and expensive piece of equipment. It also requires electric power, which is not trivial to provide along the entire span between continents.
And in short distance unrepeatered coastal festoon fiber systems where it's within normal DWDM system reach, node to node with no active underwater electronics, those are also often in areas with relatively low population/traffic that won't need more than 8 or 12 fibers at absolute max on any leg in a chain or ring. Such as the Connected Coast project being built in British Columbia right now.
I'd be astonished if it runs into capacity problems with an ultimate upgrade capacity of being able to run at least 20 different 200 or 400 Gbps circuits per fiber pair if appropriately costly DWDM terminals are installed at each end of dark fiber. Not even if all the tiny towns and cities that it hits were to magically quadruple in population.
12 is ridiculously low without the repeater concern. It seriously should cost a rounding error more to put 400, maybe 4000 fibers in that cable because the cost of the fiber itself is literally nothing compared to the cable armour, boat hire and labour.
A while ago, NRO put on-line a bunch of declassified documents [1], including their internal books about the history of satellite reconnaissance. But it was all in one unsorted pile, often scans of barely legible pages. I think this was the original source of some of the documents shown in this article.
Although one could randomly stumble on some interesting anecdote here and there in the NRO archive, overall that collection was not made to be easy to navigate, and one would have to be very patient to sift through it thoroughly. Maybe the situation is better now.
There are different types of atomic clocks, but in most common types, the output comes from a crystal oscillator, or more generally frequency synthesizer, which is then slaved to some spectral feature in the "physics package". That is to say, the phase noise is as good as that of the crystal in the unit, but the longer term frequency stability is much improved by the slaving.
An exception is an active hydrogen maser, which directly outputs the frequency of atomic transition. It has very good phase noise, but is a rare beast, which is only used where it is absolutely necessary.
A very good 10 MHz ovenized crystal oscillator, Hewlett Packard 10811D, ages by up to a few Hertz per year, and has a mechanical capacitor for trimming the frequency by up to 10 Hz, and an electronic frequency adjustment by 1 Hz using a varactor. I am sure the varactor does not improve jitter, but considering that the full range of adjustment is 0.1 ppm, it also should not add that much jitter, assuming everything is well designed.
So we are talking about a reasonably high stability crystal to begin with, and a very narrow adjustment range. In an atomic clock, the feedback loop uses the electronic frequency adjustment to more or less completely remove the aging. This requires a very tiny and a rather slow acting feedback.
In the atomic clock, the output of the crystal is used as a reference for a microwave sweep generator, which then scans the spectrum of atomic transitions. The absorption peak in a cesium clock is something like a kiloherz wide, but with a good signal to noise ratio and with a lot of averaging, one can measure the position of the peak to a very tiny fraction of its width. Comparing the measured and the expected positions reveals the deviation of the reference frequency from its design value, and that is what generates the tuning feedback for the crystal. I do not know off the top of my head how slow the feedback is exactly -- it is something that one could look up in the Hewlett-Packard service manual, but I am sure it is slow enough to be irrelevant for the cycle-per-cycle jitter.
You can pull the frequency of a crystal resonator circuit by changing the loading capacitance, for example. You may use a varactor or any semiconductor junction. Doing so doesn't really affect phase noise.
I'm probably missing the right terminology. I would have thought the feedback loop creates jitter of its own. Phase locked VCOs are generally noisier than a crystal alone. But maybe I'm overthinking it and the loop bandwidth can be made very narrow with trimmed crystal and long integration times or whatever.
The contribution of the crystal to the overall tank circuit Q is vastly higher, by several orders of magnitude, than the contribution of the varactor diode.
So while making an oscillator steerable inevitably increases its phase noise slightly, the increase is either ignorable in practice or cancelled by the control loop itself.
The cone of Cherenkov radiation produces arcs of "bright pixels" in the detector.
There are very good illustrations of this if you search for "Super-Kamiokande Cherenkov ring." That's a Japanese neutrino telescope with a bunch of huge photomultipliers lining the walls of a giant underground cistern filled with water. Ice Cube has similar light detectors, probably from the same Japanese manufacturer, that were lowered on strings into wells melted into ice.
It was not just the VCs. There were also influential people in Pentagon who saw the inefficiency of the prime defense contractors, especially after having seen what SpaceX was able to do.
One of the consequences of this was the meteoric rise of the young Will Roper [1], who in a span of a few years went from a theoretical physicist to being in charge of the Air Force procurement program, and then tried to facilitate the access of startups to defense contracts, through AFWERX and "Defense Pitch Days".
Whether this worked or not is debatable, but the itch to change things was there. One example of a startup which was an early beneficiary of this top-down attention is Ursa Major [2], a rocket engine company. Unless one looks at them in the context of these military programs, their existence would seem quite puzzling.
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