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

Azelio Boriani azelio.boriani at gmail.com
Mon Dec 1 08:49:07 EST 2014

```and then, for the second part of the question, the 10^-12 is an Allan
deviation but usually the terms variance and deviation are
unfortunately used one in place of the other.

On Mon, Dec 1, 2014 at 1:41 PM, Bob Camp <kb8tq at n1k.org> wrote:
> 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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