[time-nuts] GPSDO Sigma Tau at large observation times
eaguayo at thinksrs.com
Wed Oct 5 11:57:55 EDT 2016
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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