[time-nuts] Tbolt issues

Charles Steinmetz csteinmetz at yandex.com
Thu Sep 1 18:26:21 EDT 2016

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,


More information about the time-nuts mailing list