Re: XAUI frequency tolerance time window
Hi
It seems to me it would be unnecessary to go to Hertz range for jitter
measurement and tolerance. SONET does test in the Hertz range, but in
XAUI we have the elastic FIFO which will handle the clock deviation.
Theoretically it is possible to loose sync even at 1 Hertz, but I would
argue you have sever case of power supply fluctuation and probably
bigger
problem than jitter.
Another way to under run or over-run your FIFO would be if you operate
your link
longer, especially link distances possible varying from 85m to 40 Km..
Most FIFO
should handle 25-100 KHz and depending on the design.
Thanks,
Ali
Tom Lindsay wrote:
>
> This email is in response to:
> 1.2 Very low frequency jitter definition frequency and amplitude
>
> A- Comment to be entered
>
> Tom to send email concerning explanation of difference between low
>
> frequency jitter and clock tolerances of 100ppm clock tolerance.
> Agreed.
> ___________
>
> A clock will have a frequency, averaged over a period of time. It may
> fluctuate within that average. A receiver should track that average,
> such that the CDR (VCO) is at the same average frequency as the
> incoming data rate. Generally, this average frequency drives
> requirements for elasiticity buffering, and it must be controlled
> through a clock tolerance specification (+/-100ppm).
>
> The fluctuations within the average can be thought of as jitter -
> jitter also must be controlled.
>
> The time period (or some number of bits, etc.) for measuring clock
> tolerance should be specified. Generally, if the time period is
> increased, more lower frequencies of jitter will be averaged (filtered
> from the measurement); if the time period is decreased, more lower
> frequencies of jitter will be unfiltered and seen as frequency errors
> which could be controlled by the 100ppm spec. Note that using 100ppm
> frequency as jitter control in this manner is more stringent than
> present DJ specifications.
>
> Complete control of the frequency spectrum is recommended. This is to
> prevent elasticity buffering overruns and excessive jitter beyond what
> can be tracked. To provide complete control, frequencies above the
> frequency that corresponds to this time duration should be controlled
> with jitter specifications; lower frequencies would be controlled by
> the 100ppm spec.
>
> Define T = the measurement time duration for clock tolerance
> measurement. Then, require that a clock frequency must be within
> 100ppm of ideal when averaged over any interval >=T. Next (this is
> without rigor, a gut feel, someone should confirm), I would suggest
> that jitter be controlled down to freq <1/(2*T). This implies testing
> jitter output for a period >2*T , and testing tolerance with sine
> frequencies down < 1/(2*T).
>
> What should the value be for T? For practical lab measurements, the
> time may want to be on the order of at least 1 second. This implies
> measuring jitter output for >2 seconds, which is still reasonably
> practical. However, for tolerance, this implies sweeping down to <0.5
> Hz, which is not practical. Fibre channel defined T as 200,000 bits,
> which equates to approx 200 usec at 1Gbit/sec. This implies sweeping
> sine jitter down to approx 2.5 kHz for that data rate.
>
> I don't have a good feel on what to do from here.
>
> * Is this worth pursuing, or this unncessary specification/control
> for a non-problem?
> * If we set T, should its value be short so that sine sweeps can
> stop at higher frequencies? If so, what frequency? If T is too
> short, it becomes impossible to verify frequency in the lab; if T
> is too long, then we run into practical limitations of sine
> jitter sweeps.
>
> Comments, reactions, suggestions?
>
> Thanks, Tom Lindsay
> Vixel
> 425/806-4074
>
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