From: "Pablo Alvarez Sanchez" <[EMAIL PROTECTED]> Subject: Re: [time-nuts] Timing on Ethernet (Magnus Danielson) Date: Fri, 3 Aug 2007 18:25:58 +0200 Message-ID: <[EMAIL PROTECTED]>
Hi Pablo, > First of all thanks a lot for your answers. Thanks. > > For a facility like CERN, I think that normal cabel > > assymetries will be sufficiently low such that they would not > > require explicit handling, unless you have higher > > synchronisation demands, but in that case I would strongly > > suggest a different solution. > > You are right. Most of our aplications just demand ~1ms accuracy. Right > now our requirement is only 1us accuracy, which is actually not so bad > if you consider we have a 27Km accelaretor to monitor. But this is a > long term project, so we have to try to do it as good as possible. I > think ~1ns is nice goal. ~1 ns should be acheivable without too much hazzle if one pays some attention to details. > > Looking at the IEEE 1588 while implementing in your own FPGA > > seems like an odd choice. It is an option, but you could > > fairly easy cook up something which fits your needs. It is > > not too hard actually. > > We may also connect to this network some "foreing" equipment like PLCs. > Then IEEE 1588 would be quite helpful. Certainly. But you can also see that as long as those see a IEEE 1588 interface you don't have to use that internally to the system. Double systems add cost, but the benefit may be there anyways. > > The most important issue is what kind of performance do you need? > > Maximum time-errors? > Not defined yet > > > Stability requirements? > What does this really mean? Stability of a node during continous tracking. Maximum drift during holdover is another aspect. For your kind of operation I would consider redundancy in links and nodes. Not necessarilly within the nodes. > Free running drift if we unplug the cable. 1ms over 30 minutes is ok. > But I would go for something like ~30us. Should be manageable. +/- 1E-6 in frequency error. > Jitter between home made modules ~100ps rms That is large. But if that is sufficient, I won't argue with it. > > Full UTC time or only UTC coordinated PPS? > Full UTC. We use UTC to tag external events and for Post Mortem > analysis. If there is anything wrong around the machine, a pulse is sent > to our receivers and time tagged. With this we can reconstruct the > situation that originated the problem. OK. > > 10 MHz clock? > Right now we use a 40MHz clock in our receivers. We can delays using > this clock, or start counters with external clocks. Sounds reasnoble. Gives 25 ns resolution without any interpolators. Just a simple interpolator can give you significant improvement, but so will averaging. Cheers, Magnus _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
