[time-nuts] Measurements - Phase vs TDEV vs ADEV/MDEV
jimlux at earthlink.net
Sun Feb 21 11:04:57 EST 2016
On 2/20/16 10:22 AM, Attila Kinali wrote:
> 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).
I don't know that the crystal and oscillator on the Teensy is all that
wonderful. Your Abracon is an integrated unit, so I would expect it to
be substantially better.
Don't forget that the teensy measurements were made by latching the 16
MHz clock in the ARM core when the 1pps from the Rb arrived. So there's
all the issues with threshold noise/risetime wrapped in there. There was
no attempt to have terminations or anything (although the coax from the
Rb to the teensys was about a meter long). The teensys are powered off
I was more interested in "teensy:teensy" matching (and of course, it's
fun to just measure stuff).
> 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.
I wouldn't be surprised by this at all. The teensys were run in a very
non-ideal environment, so their performance is probably substantially
worse than the crystal that's on the board.
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