[time-nuts] Tbolt issues

Bob kb8tq kb8tq at n1k.org
Thu Sep 1 19:21:28 EDT 2016


Hi

Frequency is a "change over time". If delta time is zero it is undefined. As you observe it in shorter time periods, the accuracy / stability gets worse. Since the error bars expand there isn't much of a limit as you go shorter. They are not quite the same thing, but they are related.

Bob

> On Sep 1, 2016, at 6:35 PM, Nick Sayer via time-nuts <time-nuts at febo.com> wrote:
> 
> Just a stupid question...
> 
> On a theoretical basis, can one speak of the limit of the frequency observed as tau approaches zero?
> 
> Might that in some way be the "instantaneous frequency" which people often think of?
> 
> I rather suspect the answer is "no," but I'll ask anyway. 
> 
> Sent from my iPhone
> 
>> On Sep 1, 2016, at 3:26 PM, Charles Steinmetz <csteinmetz at yandex.com> wrote:
>> 
>> Bert wrote:
>> 
>>> maybe some one smarter than us can working with the parameters that Tbolt
>>> makes available better performance can be achieved
>> 
>> I am quite sure of that
>> 
>>> the frequency is being changed to compensate for time
>> 
>> Yes, the PPS is steered by making slight adjustments to the OCXO frequency.  But you can make these adjustments as arbitrarily small as you want with the setup parameters.  I run my Tbolts with pretty tight limits on the frequency adjustments.
>> 
>>> and we do not care about ADEV, we care about the actual
>>> frequency at that moment it goes in to the measuring device
>> 
>> There is no "there" there.  One never makes a frequency measurement at just one instant -- the measurement will ALWAYS be done over a macro time interval (very often, one second, sometimes 0.1, 10, 100, or 1000 seconds).  We never observe, and have no way to know, the instantaneous frequency (as you put it, "the actual frequency at that moment it goes into the measuring device") -- so how can we care about it?  The only thing relevant (or even meaningful) is the average frequency during our measurement interval.
>> 
>> xDEV tells us half of what we want to know -- how stable our oscillator is from one measurement interval to another.  We would also like to know what frequency it is wobbling around -- the "centroid" frequency, if you will (to borrow a geometric term).  (Mathematicians can argue for days about which type of "average" is appropriate here -- the rest of us just pick one and carry on.)  ADEV does not tell us this "centroid" frequency directly, but it can be extracted from the same measurements we took to calculate ADEV.
>> 
>> I think you are being misled by a belief that the linguistic construct, "instantaneous frequency," has real meaning in the world.  It doesn't.
>> 
>> Best regards,
>> 
>> Charles
>> 
>> 
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