[time-nuts] an interesting problem

xaos at darksmile.net xaos at darksmile.net
Sat Feb 5 18:43:41 UTC 2011


I wonder if there is any value to performing a FFT on the data.

-GKH

Quoting jimlux <jimlux at earthlink.net>:

> Here's an interesting problem..
>
> I've got a system at work with an internal clock oscillator that I want
> to get some statistics on, but there's no direct visibility for the
> oscillator, nor do I have a convenient test point that I can probe.
>
> I can divide it down by an arbitrary number to generate pulses which I
> then send out via SpaceWire timecodes.  SpaceWire is a fast point to
> point digital data link and it has a special capability that
> essentially has a "tick in" signal at one end and a "tick out" signal
> at the other end.  The latency between tick in and tick out is random,
> but bounded and discrete.  the link runs at a clock rate derived from
> the same oscillator, and you have to wait until the current character
> being sent has been clocked out before you can send the special
> "timecode" token.
>
> THat is, I can detect the "tick out" pulse and it has a random N*[0-14]
> clock delay (distributed more towards 0 than 14) from when the "tick
> in" (which is synchronous with the clock I want to measure).  N is the
> ratio between my clock and the data rate on the wire ( 7, in this case,
> so the time step is about 100ns)
>
> So, by making measurements of the time when the "tick out" appears (or
> time between tick_out pulses) can I somehow "take out" the random
> variability of the link.
>
> It seems, since the clock isn't *terrible* that I could, for instance,
> accumulate statistics, and throw out the ones that have more than 0
> clock latency (which is probably a few 10s of percent of the ticks.. I
> haven't looked yet).  Or, given that the interval between ticks is one
> of 28 or 29 discrete values (plus the underlying clock variance), if
> the clock variance on a given pulse is <<clock rate, the
> histogram/probability distribution of times would look like a bunch of
> little humps, each with variance =clock variance.
>
> or, is a real oscillator going to have an instantaneous variance that
> is comparable to or greater than 1 clock pulse?
>
> tvb's site has some sample ADEV numbers, and I would imagine that his
> HTV-2 TCXO has comparable performance (at least over the short run at
> constant temperature).. He shows ADEV of 1E-9 from 1-1000 seconds...
>
>
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