[time-nuts] A silly question ...

Tom Van Baak tvb at LeapSecond.com
Thu Sep 27 14:18:16 EDT 2018


> "I think", that if for example, it takes 1 second to drift one cycle,
> that works out at 0.1 ppm. If it takes 2 seconds, it's 0.05 ppm, if it
> takes 5 seconds, it's 0.02 ppm etc. Is that correct?

Yes. At 10 MHz one full cycle is 100 ns. So if the cycles are drifting by 100 ns per second that's 100e-9 s / 1 s = 1e-7 = 0.1 ppm.

At these levels of frequency accuracy, using a 'scope is plenty good enough. In fact, it's more educational and somehow more enjoyable to watch analog sinewaves drift past each other than it is to see the digital display of boring frequency counter.

Where the 'scope method starts to break down is when the frequency error gets down to the ppb level. At 1e-9 it will take 100 s for the waveforms to drift by one cycle. And at 1e-12 you would have to wait an entire day (100 ns / 86400 s = 1.157e-12).

On the other hand, with frequency offsets this low you don't have to sit there the whole time. One trick would be to take a photo of the 'scope once an hour, or, say, once every 1000 s. If you played that back at 1 fps you'd have a 1000x "time magnifier".

/tvb



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