[Fpga-synth] Interesting

[Eric Brombaugh]

Heed the words of the master!

Thanks for the advice - I don't usually spend much time worrying about  
jitter, but this is helpful. I'll take the suggestions of a guy with  
atomic clocks as gospel.

I've used DCMs several times and one thing I might suggest: If the  
reference clock is ever stopped they tend to go out into the weeds.  
It's helpful to have the reset input brought out to a controllable  
point to correct this.

Eric


On Nov 18, 2007, at 6:21 PM, Magnus Danielson wrote:
>
> A few words of advice:
>
> 1) Do not stare yourself totally to death on Peak-to-Peak jitter  
> values. Those
>   numbers have a high degree of bogusity about them. The reason is  
> that the
>   noise part of jitter is following Gaussian law, so if you wait  
> long enought
>   the peak-to-peak values will increase. Gaussian jitter can not be  
> measured
>   by peak-to-peak, but you want to measure it in terms of RMS. The
>   peak-to-peak is a probability function and the rule-of-thumb value  
> of 14
>   times the RMS value gives you 1.0E-12 in BER.
>
> 2) Don't fool yourself to beleive all jitter is thermal, i.e. having  
> gaussian
>   distribution. The total time-error (or peak-to-peak) jitter sum  
> (you can
>   calculate it this way for BER/window-size values) will be the  
> random jitter
>   plus the deterministic jitter. Deterministic jitter includes stuff  
> like
>   Inter-Symbol-Interference (i.e. amplitude skew due to memory  
> function of the
>   channel, which adds level to the signal such that propper timing  
> can not be
>   acheived) and fixed frequency modulations. The later kind will eat  
> you for
>   breakfast if you don't pay attention. Not all crystal oscillators  
> have low
>   jitter. You will find that plenty of them for higher frequencies  
> actually
>   uses a lower frequency crystal and then uses a DCM or PLL to get  
> the higher
>   frequency. Not very supprising will there be a modulation at the  
> comparator
>   frequency, and yes, I have tossed such oscillators out of our  
> designs many
>   times. There are those that will properly filter in the step-up.  
> Infact,
>   some of these cheap oscillators perform better for some  
> frequencies. You can
>   however never know until you have measured it at the intended  
> frequency.
>
> 3) Messen is wissen.
>
>   Higher speeds do require fancier tools. I have pretty fancy tools.  
> Infact,
>   for clock measurement I have fancier tools at home than at work.  
> Maybe that
>   is why they sometimes have me measuring things at home from work.
>
> 4) Don't forget the step-up relation. The time-jitter you have at  
> the reference
>   may remain the same for some setups, so the reference jitter needs  
> to meet
>   the end jitter requirement in time, even if the cycle period is  
> much bigger.
>   The relative jitter thus changes as the carrier frequency is  
> stepped up.
>
> 5) Read up on jitter. The Fibre Channel Jitter Mesurement Methology  
> document is
>   a good read. Tektronix has a nice document too. While these are more
>   transmission oriented, they are more advanced in analysis than  
> many of the
>   digital and FPGA papers are.
>
> 6) Don't forget normal signal integrity. The Handbook of black  
> magic, and the
>   followup on Advanced black magic should be a good read too. The  
> world is a
>   harsh analog world for the poor digital engineers to figure out  
> why their
>   0s and 1s isn't commming through properly.
>
> Cheers,
> Magnus
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