[time-nuts] A silly question ...

bill.riches at verizon.net bill.riches at verizon.net
Fri Sep 28 07:33:29 EDT 2018


This works for me:

1. 10 Mhz to be measured input to vertical channel
2. Standard input to ext trig.
3. Set scope for 10 ns per div. (1 cycle of 10 Mhz will fill up whole
screen)
4 Time the time it takes for trace to move 1 div.  (not the whole cycle -
just one of its sides.)
5. Divide these seconds into 1x10-8 using your calculator.  ( I set 1x10-8
into one of the memories)
6. The answer will give you your  offset.
7. Example:  If it takes 20 seconds for one of the sides of the cycle to
move, your offset is 5x10-10.   
It does not take all day to get a reading,

73

Bill, WA2DVU
Cape May

-----Original Message-----
From: time-nuts <time-nuts-bounces at lists.febo.com> On Behalf Of Tom Van Baak
Sent: Thursday, September 27, 2018 2:18 PM
To: Discussion of precise time and frequency measurement
<time-nuts at lists.febo.com>
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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