This is assuming that your oscilloscope is set at 100 nS/DIV. My Tek 2465A
will sweep at 5 nS/DIV normally, and 500 pS/DIV when the sweep is set to X10.
If I'm figuring correctly, this will allow 1e-12 in 432 seconds. I use a stop
watch to time the zero crossings of the sine wave - something like:
Assuming the wave moved 5 divisions in 185 seconds: (500 pS * 5 DIV)/185
seconds = 13.5e-12 ppm. (I think this is correct. tvb?)
Since a 10 MHz sine wave tends to look like a flat line at 500 pS/DIV, I often
set the vertical V/DIV to 2 mV/DIV.
Tom
From: Tom Van Baak <[email protected]>
To: Discussion of precise time and frequency measurement
<[email protected]>
Sent: Thursday, September 27, 2018 11:19 AM
Subject: Re: [time-nuts] A silly question ...
> "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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