# [time-nuts] How can one measure ADEV of a good oscillator?

Bob Camp kb8tq at n1k.org
Mon Dec 1 07:41:12 EST 2014

```Hi

1) There are setups that increase the resolution of a counter

2) There are devices that are far more accurate at measuring frequency than a SR620

3) If you have three reasonably identical samples of a device you can indeed inter compare them once the resolution is there.

Bob

> On Dec 1, 2014, at 2:09 AM, Dr. David Kirkby (Kirkby Microwave Ltd) <drkirkby at kirkbymicrowave.co.uk> wrote:
>
> I think I have a flaw in my understanding of this.
>
> How can something like an SR620 measure the ADEV of an oscillator,  if the
> oscillator is of a similar or better than the reference fed into the SR620?
>
> I see plots of ADEV  for hydrogen masers, but I can't understand how this
> can be measured from the phase data unless the reference is better than the
> DUT, which is not going to be possible with a good hydrogen maser.
>
> I was thinking it might be possible if one has 3 oscillators and 3 time
> interval counters to perhaps solve 3 simultaneous equations. I can't prove
> that, but it seems intuitively correct.
>
> I must be missing something!
>
> Also I have seen graphs of both Allan variance and Allen deviation.  Both
> are typically 10^-12 for a decent oscillator, but given the variance and
> standard deviation are related by a square root, they can't both be around
> 10^-12.  I would expect to see values of 10^-6 or 10^-24, but I don't see
> such dramatic differences from 10^-12.
>
> If I see numbers around 10^-12 on an OCXO,  is that the Allen variance or
> Allen Deviation?
>
> Dave
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```