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

Magnus Danielson magnus at rubidium.dyndns.org
Mon Dec 1 16:48:59 EST 2014

```Back in the day, you built two devices measured the noise and then
assumed that they where so near each other that you just split the noise
in two and assigned the same noise to them both.

Doing the three-cornered hat allows you to make thee observations from
three sources, so breaking the sums up you can extract the noise of each
individual.

Another trick is to use cross-correlation of two independent channels.
The common source (DUT) is (idealy) the only signal correlating between
the channels, so when accumulating measures, the noise of the two
channels correlate out while common noise stays. Cross-correlation is a
very strong method, as you can measure noise at or below your references
noise, but you need to pay in measurement time.

Cheers,
Magnus

On 12/01/2014 01:41 PM, Bob Camp wrote:
> Hi
>
> The simple answer:
>
> 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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