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The Thing

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Re: The Thing

#101

Post by acqq »

minipci1321 wrote: post #97
acqq wrote: post #11

The creator is interesting: https://en.wikipedia.org/wiki/Léon_Theremin And his most famous creation: https://en.wikipedia.org/wiki/Theremin

Interesting? Wikipedia article doesn't even get close. There are various accounts of his life and achievements, memoirs of witnesses, people who knew him in various contexts, the descendants/family, records of his own recollections -- all available on Internet. Unfortunately, most of it is in Russian. Reading it gives you that messy, undefinable feeling of experiencing a genius.

> Unfortunately, most of it is in Russian.

You can still post the links that are most interesting to you, please, the google translate service is quite usable.

Re: The Thing

#102

Post by Nition »

alexhornbake wrote: post #28
gibspaulding wrote: post #23

Earlier quoted context omitted.

There are a surprising number of popular songs which have actually used the theremin. Perhaps the most recognizable is "Good Vibrations" by the Beach Boys. You can hear the theremin especially clearly in the background of the first chorus. https://www.youtube.com/watch?v=Eab_beh07HU

Common mistake. Good vibrations did not actually use a theremin. It was a ribbon controller type instrument that was designed to mimic the sound. Theremins are hard. https://en.m.wikipedia.org/wiki/Electro-Theremin

In terms of actual Theremin music, here's Clara Rockmore's Nocturne In C# Minor [1975]: https://www.youtube.com/watch?v=ghWdgcYIcSk

Re: The Thing

#103

Post by gnopgnip »

maxlybbert wrote: post #41
wyc wrote: post #33

Earlier quoted context omitted.

Once on an international flight, I overheard some construction workers describe how the US government must fly in US citizenship-holding personnel and US sourced materials in construction of certain foreign projects for security reasons. Does anyone know more about this?

I worked on a software project that was targeted at the federal government and required all programmers to be US citizens. It didn’t require us to have special security training or anything, though; and there was a different project that required programmers to leave their cell phones in lockers during the day. I’m convinced that the government doesn’t believe citizens are any more loyal or hard to bribe than other p…

>I’m convinced that the government doesn’t believe citizens are any more loyal or hard to bribe than other people, but insisting on citizenship would make it easier to charge us with particular crimes if the need arose.

It is not always about the security side. There are certain requirements to employ US citizens based on the funding source. The charitable reasoning is ensuring the program can continue to run even if are at war.

Re: The Thing

#104

Post by upofadown »

The description of the operation of this thing always seems incomplete. From the Wikipedia article:

>The length of the antenna and the dimensions of the cavity were engineered in order to make the re-broadcast signal a higher harmonic of the illuminating frequency.

That wouldn't actually work by itself. There would have to be some sort of non-linear element to cause the harmonics. Chances are there was something like a varactor in there. The original technical description probably omitted critical details because spooks are like that...

Re: The Thing

#105

Post by madengr »

unnouinceput wrote: post #20
madengr wrote: post #5

Maybe someday the Soviet side of things will be declassified, and we can read the technical account from that end. Would be interesting. There seem to be some technical unknowns in the article. I don’t think you can get FM back from the passive cavity, just AM, unless you can pump the cavity with feedback. Same goes for re-radiating at a harmonic. Maybe if the Q of the cavity were super high. Again, would be interest…

There is nothing unknown in the article. You can do this at home as well. Is very well explained how it works. For start I would suggest you get technical of how Yagi antennas work, it's the same principle coupled with cavity resonator + membrane movement used in a microphone. In the end you get your high frequency beamed back modulated by the sound. And is not AM modulation, is FM modulation. But what article indeed…

Ha ha, I know how a Yagi works; 25 years of RF/Microwave circuit and antenna design. I don't see how you are going to get FM modulation out of this thing. This is akin to passing a CW tone through a filter at it's cutoff, then modulating the filter response. You get AM sidebands as the passband varies with modulation. The frequency does not change.

In order to get FM modulation from a passive resonator, the decay time of that resonator has to be much longer than the modulation; i.e. the resonator Q has to be very high, much narrower bandwidth than the modulation.

A back of the envelope calculation, for 3 kHz audio bandwidth, and a 300 MHz resonator, that sets the unloaded Q=100E3, which you can't get even close with a passive resonator, short of a superconductor. Not to mention the loaded Q from the monopole radiation is going to lower it even more.

You can get FM modulation out of an oscillator because the feedback is generating a negative resistance to compensate for the resonator resistance; the active circuit is a Q multiplier, getting you that massive Q.

The interrogator would have to set up not only a standing wave, but provide coherent feedback. Sort of a stand-off oscillator. Even if you had low-level FM modulation, it's still going to have the spectrum of DSB AM as the Bessel components are pretty far down.

Maybe it is doing that stand-off oscillator, but my hunch is the receiver is a coherent AM demodulator. You have the carrier available , so might as well use it.

Re: The Thing

#106

Post by madengr »

VLM wrote: post #99
madengr wrote: post #5

Maybe someday the Soviet side of things will be declassified, and we can read the technical account from that end. Would be interesting. There seem to be some technical unknowns in the article. I don’t think you can get FM back from the passive cavity, just AM, unless you can pump the cavity with feedback. Same goes for re-radiating at a harmonic. Maybe if the Q of the cavity were super high. Again, would be interest…

My guess is the excitation power is broadband, which explains the unusually high power required. Smooth white noise with a spectrum 50 KHz wide centered on the theoretical center of the resonator, then the 3rd harmonic can rebroacast a resonant tiny high Q slice of up to 150 KHz deviation FM as the microphone diaphragm wiggles with sound. Imagine a transmitter design where you make a very powerful white noise signal,…

See my response above about the FM modulation; I don't think it will work like that; it would be AM. Assuming the resonator diaphragm is linear and small signal (WRT to the sound pressure), you won't get a radiated carrier 2nd harmonic. A cavity resonator (2nd order) would produce a 2nd harmonic in AM audio as the carrier rides that resonator response.

If the diaphragm goes non-linear (say from high sound pressure), then perhaps you'd get harmonics and maybe an f/2 response. At this point it's acting like a parametric amplifier, with your carrier frequency and pump frequency. Parametric amps need a strongly non-linear reactance, hence pumping a varactor diode with a strong signal.

Re: The Thing

#107

Post by piyush_soni »

piyush_soni wrote: post #55
deanclatworthy wrote: post #6

Earlier quoted context omitted.

Three of the four links here, have no discussion. One has hardly any. Posting interesting topics which have not received much prior discussion is encouraged here.

I didn't know it too. How do we define interesting though, given three of the four times it garnered no interest (do we keep on posting until it does)? I'm not sure if GP deserves downvotes for pointing them out at the least.

So a genuine question downvoted too? People are so reasonable here :).

Re: The Thing

#108

Post by minipci1321 »

acqq wrote: post #101
minipci1321 wrote: post #97

Earlier quoted context omitted.

Interesting? Wikipedia article doesn't even get close. There are various accounts of his life and achievements, memoirs of witnesses, people who knew him in various contexts, the descendants/family, records of his own recollections -- all available on Internet. Unfortunately, most of it is in Russian. Reading it gives you that messy, undefinable feeling of experiencing a genius.

> Unfortunately, most of it is in Russian. You can still post the links that are most interesting to you, please, the google translate service is quite usable.

See, for example, https://statehistory.ru/1038/Lev-Termen/

More links are at the bottom.

Re: The Thing

#110

Post by unnouinceput »

TheOtherHobbes wrote: post #54
unnouinceput wrote: post #20

Earlier quoted context omitted.

There is nothing unknown in the article. You can do this at home as well. Is very well explained how it works. For start I would suggest you get technical of how Yagi antennas work, it's the same principle coupled with cavity resonator + membrane movement used in a microphone. In the end you get your high frequency beamed back modulated by the sound. And is not AM modulation, is FM modulation. But what article indeed…

I'm finding it hard to imagine no one noticed an intermittent 1kW at 330MHz from somewhere close.

Well, the article does say a radio-amateur stumbled upon it and alerted the embassy. So in the end someone did. Since Soviets did the surveillance on key moments, and not continuous, that's what enabled for device to remain undetected for so long
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