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Please explain scope delay lines?
Axtman:
So a Tektronix 365 scope has a delay line that is probably 20 feet (6 meters) long. Electricity travels at nearly the speed of light. I thought I read that light would circle the earth seven times at the Equator a second.
How can such a miniscule piece of wire have any sort of delay?
KungFuJosh:
"In its ideal state, electricity travels at the speed of light or one foot every nanosecond...Of course, this assumes that there is no resistance, no inductance, and no capacitance in the circuit."
johansen:
It takes time to trigger the scope and start the sweep, in that time, information is lost.
If the delay line is long enough you can put the information on the screen after the sweep has started, preserving linearity.
https://www.everythingrf.com/community/what-is-propagation-velocity-in-a-cable
bdunham7:
--- Quote from: Axtman on November 09, 2023, 02:41:33 am ---How can such a miniscule piece of wire have any sort of delay?
--- End quote ---
Well, certainly you agree that it has SOME delay, right? So if you signal goes 25,000 miles in one second, then it goes one foot in 1/(25,000 x 5280 x 7) seconds. That comes out to about 1 nanosecond per foot, thus 20 feet would result in a delay of about 20 nanoseconds. The signal moves through the typical wires a bit slower than light in a vacuum, about 1.5ns per foot. Delay lines are specially constructed and things move even slower. So a typical delay line provides 40ns or more of delay, enough for the trigger and sweep circuitry to start up.
Circlotron:
--- Quote from: Axtman on November 09, 2023, 02:41:33 am ---How can such a miniscule piece of wire have any sort of delay?
--- End quote ---
Even relatively short pcb tracks have a tiny but measurable delay. This often necessitates having equal length tracks so that one signal doesn't arrive before another related signal. One way of achieving this is to put wiggles in the tracks to extend their length to match their otherwise longer partners.
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