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How Do You Measure A Billionth Of A Second?


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#1 Vapor Trails

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Posted 19 November 2011 - 05:04 PM

Well? And what purpose does it serve?

:unsure:
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#2 Hambil

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Posted 19 November 2011 - 06:12 PM

In five hundred twenty-five thousand
Six hundred minutes
How do you measure
A year in the life?

Are we talking about the light experiment or something else? Orpheus will have to take a crack at that for you, because the equipment is so complex they are arguing whether the results could have been effected by gravity.

#3 Christopher

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Posted 19 November 2011 - 07:14 PM

View PostVapor Trails, on 19 November 2011 - 05:04 PM, said:

Well? And what purpose does it serve?

:unsure:

An atomic clock is based on the oscillation frequency of an excited cesium atom, which vibrates over 9 billion times a second.  So they can presumably be used to measure intervals shorter than a nanosecond.

Also, a nanosecond would be the time it takes light to travel 29.9792458 centimeters (or just under 1 foot).  You might be able to measure that by something involving a laser, a beam splitter, and interference fringes, or something.

As for the purpose, naturally, precision timekeeping allows for precision measurement of all sorts of things.  A lot of stuff in physics happens on a very short timescale, like nuclear reactions, high-energy particle collisions, explosions, things like that.

There's also GPS.  The way the Global Positioning System calculates position is by comparing clock readings from several different satellites, which all relativistically time-dilated by different -- and very small -- amounts due to their different velocities relative to the measuring device.  The GPS uses relativistic equations to figure out its position relative to the satellites based on their different clock readings.  But it can only gauge that position accurately if the clocks are accurate to within 50 nanoseconds.
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#4 Orpheus

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Posted 19 November 2011 - 09:55 PM

On a more pragmatic note, I recently got a Tektronix TEK2440. This is a digital sampling oscilloscope that cost more than most cars did when it came out, but I got it for about $100, from an auction clearing out an estate sale. These units (and other fancy science gear) are not uncommonly seen on sale for ridiculously small amounts because no one seems to know what to do with them when they break.

I haven't had time to go past diagnosis, but I plan to write it up in Tinker's Corner. My estimate is that the repair should cost no more than $35 and take maybe an hour or two, including full recalibration. About $5 of that will be a circuit to assure a 1 ns (one-billionth of a second) risetime on a square wave for the recalibration.

One easy way to measure *in* nanoseconds is to use a 1GHz clock: just feed it into a 74143 chip (costs maybe a quarter on-line) and a 7-segment LED display (a common "calculator digit" display). For more digits, just cascade an additional 74143 and 7-segment LED for each added digit to the overflow pin of the preceding one. How do you generate a 1Ghz wave for a clock? Well, that's a whole subject, but ask any radio ham: it isn't hard.

One-billionth of a second is pretty routine. The computer you are using probably runs faster than one GHz, meaning it acts on a sub-nanosecond scale. Most microwave ovens run at 2.45 GHz, using an electromechanical circuit of maybe half a dozen simple parts (I'm dissecting a couple for projects this winter. I'll post pics)

I probably didn't answer the question you were asking, but if you clarify, I can be more specific. If you want to measure in binary (for use in a computer) instead of 7-segment decimal LEDs, you need only use a different cheap chip. If you want to measure intervals that are around 1 ns (say 0.1-10 ns in 0.1 ns increments) you can easily build a 10GHz oscillator with point to point wiring (above that, it gets tricky).

Edited by Orpheus, 19 November 2011 - 10:00 PM.


#5 Christopher

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Posted 19 November 2011 - 10:39 PM

Just to clarify, a GHz = gigahertz = a billion cycles per second = one cycle per billionth of a second = one cycle per nanosecond.

And yeah, that is a pretty low frequency for the EM spectrum.  It's in the UHF radio band.  Infrared is in the terahertz range (trillion cycles per second) and above, and visible light in the hundreds of THz.
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#6 Orpheus

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Posted 20 November 2011 - 12:36 AM

Yeah -- and don't let "UHF" (= "Ultra High Frequency") scare you. It was named "ultra" when your grandpa was a pup. It's no biggie today. I mean, okay, you have to use special chips -- i.e. I'd use an Advanced or Fast Schottky diode 74143 vs a plain-jane 74143, but that's what I priced when I said "about a quarter" ($0.25). In other words, yes, you really can build a gigahertz counter in a matter of minutes, for maybe a buck or two, without special skill. The LED display would cost more than the chips

Okay, it might cost a non-electronics-geek up to $10 and take a week or more, because you'd have to pay (and wait for) shipping, instead of reaching into your junkbox or tacking it onto an existing order. But you get the idea.

#7 Orpheus

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Posted 24 January 2019 - 09:12 PM

View PostVapor Trails, on 19 November 2011 - 05:04 PM, said:

Well? And what purpose does it serve?

Posted Image

I haven't re-read the entire thread (I'm in a rush), but the response to "what purpose does it serve?" wasn't useful, and it's been bugging me for years -- maybe once every week or two, because that's roughly how often I use nanosecond timing for Perfectly Normal Things.

Earlier this week, we had a serious cold snap, and one of the zones on a friend's home heating dropped out. The furnace guy diagnosed it as a broken wire, and wouldn't do anything about it. He referred my friend to an electrician who said he'd have to "fish" a new run of thermostat wire (running a new wire inside intact walls is called "fishing", because the rigid and semi rigid tools you use to run wires inside walls and through existing holes in studs, etc. are call "fish" -- or is it the other way around. Look, I don't have time to fix the entire messed-up English language for you!) and quoted him a healthy price -- but still wouldn't be able to do it for several days, which is way too long when your heat goes out at -4F, putting your pipes at risk of freezing and subsequent flooding (which means even more heathy bills for plumbing pipe replacement, replastering, carpet treatment, etc.)

Somehow I got roped in to deal with this, so I took an oscilloscope (the same one I recalibrated in my earlier response to this post, in fact) and some other gear to their house. Without going into to much detail: I disconnected the broken wire from the furnace controller and connected it to my oscilloscope, then sent a sharp pulse of low voltage electricity down it. When the pulse hit the broken part, it couldn't go any further, and "reflected" back. On my oscilloscope, I could see the original pulse and the reflection. From the nanoseconds of lag, I could tell roughly how many feet down the wire the break was. In a vacuum, "the speed of light" (or electic fields) is close to 1 foot/nanosecond. In a copper wire or PC board, it's close to 6 inches/nanosecond. [This is called "Time Domain Reflectometry", if you want to look it up.]

rummaging through is basement, I was able to measure the length of the wire from the furnace to the point where it went up through the main floor wall, and could therefore make a pretty good guess where in the wall the break was. I cut a fairly modest hole, found the spot where a badly-stapled wire had gotten pinched during construction (decades ago), and fixed just the broken bit. The repair still left my friend with a 3ft x 6inch (100 x 15 cm) hole in his plaster, but that's something a handy homeowner can fix in an hour or so -- compared to the damage/repair/tradesman expense of running a new wire through existing walls and/or fixing entire walls afterward.

I think that's a pretty down-to-earth incident that could happen to anyone -- and normally costs big bucks when it does.

So maybe, instead of thinking of "billionths of a second", you can think of a nanosecond as "a foot of light" or "six inches of electricity in copper".


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