[time-nuts] Measurements - Phase vs TDEV vs ADEV/MDEV

[Attila Kinali]

Moin,

We are building here some clock synchronization system and have done
some measurements. I am trying to do some sanity checks, but I am
not sure whether what I am interpreting into the data is actualy true.
Hence I would like to ask for your help.

Our system consists of nodes that send out a pulse at a rate of 20kHz
to every other node. This pulse is used to synchronize the phase of
the pulses using a TDC implemented into a FPGA (based on the OHWR TDC core
from CERN[1]). From the self-calibration and the timing estimates we got
from the synthesizer, we guestimate that the largest bin is approximately
140ps in size.

Our measurements have been done using an SR620 with an trigger signal
generated by one of the nodes with a rate of 10Hz, about 5us before
the pulse. The output pulse of one node was connected to the start
input and another to the stop input. (see picture)

Looking at the phase data, you can see that jitter of the pulses
of two nodes is <+/-170ps, with the average being pretty much in a window
that is samler than +/-70ps.

For the other *DEV plots, I also added measruements of the Teensy 3.1
oscillator stabilty that I got from Jim Lux a while ago. There are
two traces, one is the raw data and another one with the temperature
dependence removed using a linear fit. As the oscillator we used is
an Abracon ASVTX-09, I expect our stability to be better, but not
a too big difference (at most an order of magnitude at 1s).


The TDEV we get for 0.1s is 4.6e-11 and for 1s it's 1.3e-11, going nicely
down as an (almost) straight line until about 100s (we have a longer 
measurement that shows us that long term effects do not kick in until >100s).

The ADEV starts at 8.0e-10 @0.1s passes by 7.8e-11 @ 1s and goes
down as a straight line. The straight line is to be expected, as
the two nodes are phase "locked", hence their frequency error 
averages out over time.


My interpretation of the TDEV is, that it's the 1sigma error of the interval
in seconds, when measuring a period of tau. And for the ADEV it's the 1sigma
error of the fractional frequency deviation when measuring the frequency.
So, my assumption was that ADEV*tau is approximately the 1sigma error in
seconds I would get when measuring a time interval of tau. But looking at this
data, this doesn't hold true for my clock sync system, but matches the
Teensy oscillator data quite well. Yes, TDEV is derived from MDEV and
not from ADEV. But what creates this big difference?

Also, extrapolating the Teensy oscillator ADEV back to 0.1, it seems
like the clock sync generates a synchronity that is two orders of
magnitude better than what the cheap crystals of the Teensy do.
I would expect the ASVTX-09 to be better (it's a TCVXO after with
a phase noise spec of <-135dBc at 1kHz and a temp dependence <2.5ppm over
operating range) and also the clock sync to have an effect. But getting
two orders of magnitude improvement seems to be too much. Hence I am
not sure whether the measurement setup we had is flawed and gives
us better numbers than we actually achieve.

Any help in clarifying these points and correcting my interpretations
are very much appreciated.

				Attila Kinali


[1] http://www.ohwr.org/projects/tdc-core/wiki
-- 
Reading can seriously damage your ignorance.
		-- unknown
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