# ? re state of the art time resolution

• posted

I hope this is an easy question. Assume if you will there's a spark somewhere in a fairly small volume -- say in a given cubic foot. If I'm allowed to put some form of omni sensor (crude phrasing to mean it can detect amplitude but not direction with any precision --- PMT, photodiode, whatever) just by observing the time differences among them I should be able to determine of the spark. The question is, with what precision? Are 10s of picoseconds resolution reasonable? I started my engineering career when transistors came in bottles called 12AU7s that glowed in the dark, and admit to being dated.

Thanks

AJW

• posted

The broadband radio waves generated by a spark would travel at c (about 1 ft/nsec). So if your timing can be accurate down to 10 psec (10-2 nsec), you *should* be able to find the difference in distance of the spark from the receivers down to 10-2 foot (about 0.10 inch).

On the other hand, if the atmosphere is stable and you measure a sound 'event', you could get by with looser timing requirements. Because sound only travels at about 1 ft/ms (roughly 1/10e6 of light), you could get 0.10 inch resolution with just tens of microseconds. But sound speed varies with a lot of things, so calibration could be more of a problem.

Of course, with two receivers, you can only determine a plane of points that the 'event' may have occured along. To pin point the location in 2D space, you need a third 'receiver'. And if you want to determine within 3D space, you need a fourth. Some analytical geometry will show that if the 'event' is between the receivers, your results will be more accurate than if the event is a long way from both receivers.

At any rate, it would be important that all receivers have the same 'trigger' threshold and that threshold of each receiver doesn't 'drift'.

good luck

daestrom

• posted

I understand the geometric issues of this problem, the question I still have is related to laboratory precisions in measuring the time differences between the sensor's observation of a single 'sharp' pulse. Does one get 'off the shelf'

10pS resolution, or 100? The measurement of small time differences (without heroics) is obviously key to solving an instrumentation problem. It's one possible implimentation, there are others.

Thanks,

ajw

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