[time-nuts] GPSDO Sigma Tau at large observation times

Estanislao Aguayo eaguayo at thinksrs.com
Wed Oct 5 11:57:55 EDT 2016

Hello Time-Nuts,

My name is Tani Aguayo, I'm a newbie in this exciting precision timing 
world, and so I have lots of questions. So here I go, first, let me 
describe my experimental set-up for you.  I've been measuring the Adev 
of several GPSDOs against each other, and once they are locked with a 
suitable TC, the Adev plots show two regimes of operation (See 
attached), the first one, at short time scales, has a lot to do with the 
oscillator being disciplined characteristics, temperature, pressure.. 
and there is a second one, where the Adev just drops linearly (in a 
log-log scale). Each oscillator gets to the second regime earlier or 
later, depending on the GPS PLL TC. In  the attached plot, the TCs of 
the units compared, have been carefully selected depending on the 
individual oscillator behavior. My thinking was that once in this second 
regime, every unit is following the same clock, with the jitter of the 
worst clock being compared, and since the sigma-tau makes no sense to me 
for a single clock, I need some help with this concept. My question is, 
when you are in the second regime, you are basically measuring the 
ability of each clock to follow each other (is like there is just one 
clock!, with the worst one setting the Adev value), does then the 
sigma-tau becomes meaningless for GPSDOs at time scales larger than the 
TC of the oscillator being locked to GPS?

I just wanted to thank the active members of this list, I feel like I'm 
learning a lot with the different discussions and I wanted to say thank you.

- Tani Aguayo

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