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| syncronise multiple ADCs over an Ethernet network? |
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| nctnico:
--- Quote from: FenTiger on December 08, 2019, 06:39:44 pm ---10kHz is one sample every 100us. NTP might not reach that accuracy, but PTP (IEEE1588) will keep a pair of clocks synchronised much more closely than that over a single Ethernet hop. --- End quote --- For PtP for work well you need to use fiber ethernet connections and you'll need special add-on hardware which can synchronise to PtP to get a reasonable accuracy. Also PtP will lose accuracy if you pump a lot of data over the network. The most simple way is to distribute TOD (over RS485), 1PPS and 10MHz over seperate wiring (using a distribution amplifier) and timestamp the data using that. Once the data is timestamped it doesn't matter how long it takes to travel over ethernet. If more accuracy and/or longer distances are required then using White Rabbit switches is the only option to stay below 1us of jitter (typically White Rabbit can transfer time with <1ns jitter over hundreds of kilometers when calibrated). For example CERN uses White Rabbit to synchronise test equipment used for the LHC. |
| dmills:
Not been my experience with PTP at all, 1Gb/s with PTP aware switches and correctly configured traffic prioritisation and EEE turned off and I see a few hundred ns over multiple nodes even over copper and with several hundred Mb/s of other data running, go to 10 or 100Gb/s fabric and things get better again. Sure you need to configure the gear correctly and having switches with PTP clock capability helps a lot, but sub microsecond is EASY! Regards, Dan. |
| nctnico:
But did you do long term (days, weeks) testing with PtP using continuous monitoring? I have access to such data and have seen PtP misbehave. PtP can get upset quite easely so you need to have a good (expensive) local oscillator which is tracking the PtP slowly to average the excesses (and perhaps throw away large time jumps). Putting switches in between and using copper connections doesn't help to improve PtP synchronisation because in the end PtP still relies on packet delays. For example: copper PHYs also have internal packet buffering and re-synchronisation. This can easely add 100ns or more of time assymetry. All in all a system will be much more stable if you can distribute the time over wires directly. |
| Yansi:
--- Quote from: nctnico on December 11, 2019, 08:59:40 pm --- --- Quote from: FenTiger on December 08, 2019, 06:39:44 pm ---10kHz is one sample every 100us. NTP might not reach that accuracy, but PTP (IEEE1588) will keep a pair of clocks synchronised much more closely than that over a single Ethernet hop. --- End quote --- For PtP for work well you need to use fiber ethernet connections and you'll need special add-on hardware which can synchronise to PtP to get a reasonable accuracy. Also PtP will lose accuracy if you pump a lot of data over the network. --- End quote --- No, unless you try to sync for timenut nonsense precision. .. which more then likely is not required here, neither for generic audio applications. |
| OldEE:
Hi gang. Thank you for the follow up to my mention of Dante. I learned a lot from all you fine folks. Also I misspoke about routing Dante. What we actually did was to extend the Dante VLAN to another switch. That was a glitch in the brain to keyboard link. Larry |
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