Re: [time-nuts] Good references on holdover?
Hi Javier, As far as I understand in WR both references are synchronous. Why don't you try to track both references (or N references) simultanously? If you take care of the design, your performance should increase while locked and the transition from one reference to the other if you ever miss one should be smoother. Cheers, Pablo On Fri, Feb 6, 2015 at 10:21 AM, Javier Serrano javier.serrano.par...@gmail.com wrote: Dear all, We would like to start working on holdover performance for White Rabbit [1]. This is a new domain for us. Our main use case is a WR switch losing its reference because someone disconnects a fiber. We can have redundancy, but it will take some time for a switch to change over to another reference. During this time, the oscillator in that switch will be free-running. We want to minimize the phase drift during that interval, which we think should be a couple of seconds maximum. We have never worked on holdover, and I am wondering if we can do something smarter than the obvious feeding of some constant voltage to the VCXO, based on averaging during the locked state. Does anybody know of any good references on holdover? Thanks! Javier [1] http://www.ohwr.org/projects/white-rabbit/wiki ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Good references on holdover?
This is a great list. Thanks everyone! Much of the material relates to cases where good holdover needs to be maintained for several hours, but there's a lot of insight to be gained from the reading, and I am sure those techniques will come in handy for other projects. Thanks again! Javier On Fri, Feb 6, 2015 at 2:58 PM, Attila Kinali att...@kinali.ch wrote: On Fri, 6 Feb 2015 10:21:08 +0100 Javier Serrano javier.serrano.par...@gmail.com wrote: We would like to start working on holdover performance for White Rabbit [1]. This is a new domain for us. Our main use case is a WR switch losing its reference because someone disconnects a fiber. We can have redundancy, but it will take some time for a switch to change over to another reference. During this time, the oscillator in that switch will be free-running. We want to minimize the phase drift during that interval, which we think should be a couple of seconds maximum. We have never worked on holdover, and I am wondering if we can do something smarter than the obvious feeding of some constant voltage to the VCXO, based on averaging during the locked state. Does anybody know of any good references on holdover? I think you are looking for something like [1]. I think [2] could be also of help, although it's not as good as the Nicholls paper. Zhou's paper [3] seems to be very similar to what Nicholls did (i have not fully read it yet). HTH Attila Kinali [1] Adaptive OXCO Drift Correction Algorithm, by Nicholls and Carlton, 2004 http://dx.doi.org/10.1109/FREQ.2004.1418510 [2] A Frequency Model for OCXO for Holdover Mode of DP-PLL, by Hwang, Shin, Han, Kim, 2000 http://dx.doi.org/10.1109/SICE.2000.889649 [3] Adaptive Correction Method for an OCXO and Investagion of Analytical Cummulative Time Eror Upperbound, by Zhou, Kunz, Schwartz, 2011 http://www.sce.carleton.ca/faculty/schwartz/abstracts/HuiPaperschwartz.pdf -- It is upon moral qualities that a society is ultimately founded. All the prosperity and technological sophistication in the world is of no use without that foundation. -- Miss Matheson, The Diamond Age, Neil Stephenson ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Good references on holdover?
You may find the following Master's thesis useful: http://www.ko4bb.com/manuals/download.php?file=08_Stuff_Not_Sorted/8_Sept_28_2014_Uploads/Adaptive_OCXO_drift_correction_thesis_Zhou_2009.pdf Best regards, Charles ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Good references on holdover?
The state of the art 20 years ago is described here: http://www.hpl.hp.com/hpjournal/96dec/dec96a9.pdf They understood their OCXO very very well. And you can find EFC trends on the web for hundreds of different Z3801A's (and similar) if you want to see how the EFC trends (and occasionally jumps). It is very intuitive to watch the smoothed EFC graphs build over time. They were specifically meeting a telco spec for 24 hour holdover. Your 3 second requirement is, um, a bit different! I have no idea what your local clock is, or what you are locking to, if 3 seconds is a long time. Tim N3QE On Fri, Feb 6, 2015 at 4:21 AM, Javier Serrano javier.serrano.par...@gmail.com wrote: Dear all, We would like to start working on holdover performance for White Rabbit [1]. This is a new domain for us. Our main use case is a WR switch losing its reference because someone disconnects a fiber. We can have redundancy, but it will take some time for a switch to change over to another reference. During this time, the oscillator in that switch will be free-running. We want to minimize the phase drift during that interval, which we think should be a couple of seconds maximum. We have never worked on holdover, and I am wondering if we can do something smarter than the obvious feeding of some constant voltage to the VCXO, based on averaging during the locked state. Does anybody know of any good references on holdover? Thanks! Javier [1] http://www.ohwr.org/projects/white-rabbit/wiki ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Good references on holdover?
On Fri, 6 Feb 2015 10:21:08 +0100 Javier Serrano javier.serrano.par...@gmail.com wrote: We would like to start working on holdover performance for White Rabbit [1]. This is a new domain for us. Our main use case is a WR switch losing its reference because someone disconnects a fiber. We can have redundancy, but it will take some time for a switch to change over to another reference. During this time, the oscillator in that switch will be free-running. We want to minimize the phase drift during that interval, which we think should be a couple of seconds maximum. We have never worked on holdover, and I am wondering if we can do something smarter than the obvious feeding of some constant voltage to the VCXO, based on averaging during the locked state. Does anybody know of any good references on holdover? I think you are looking for something like [1]. I think [2] could be also of help, although it's not as good as the Nicholls paper. Zhou's paper [3] seems to be very similar to what Nicholls did (i have not fully read it yet). HTH Attila Kinali [1] Adaptive OXCO Drift Correction Algorithm, by Nicholls and Carlton, 2004 http://dx.doi.org/10.1109/FREQ.2004.1418510 [2] A Frequency Model for OCXO for Holdover Mode of DP-PLL, by Hwang, Shin, Han, Kim, 2000 http://dx.doi.org/10.1109/SICE.2000.889649 [3] Adaptive Correction Method for an OCXO and Investagion of Analytical Cummulative Time Eror Upperbound, by Zhou, Kunz, Schwartz, 2011 http://www.sce.carleton.ca/faculty/schwartz/abstracts/HuiPaperschwartz.pdf -- It is upon moral qualities that a society is ultimately founded. All the prosperity and technological sophistication in the world is of no use without that foundation. -- Miss Matheson, The Diamond Age, Neil Stephenson ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Good references on holdover?
Javier, If you are aim to do hold-over as you switch between two sources, you are looking at reasonably short times, then just keep a fixed voltage to the oscillator suffice. Even if you need a little longer times, say 10-20 s, it suffice. Temperature changes and oscillator drift may be the main problems for longer times. Cheers, Magnus On 02/06/2015 10:21 AM, Javier Serrano wrote: Dear all, We would like to start working on holdover performance for White Rabbit [1]. This is a new domain for us. Our main use case is a WR switch losing its reference because someone disconnects a fiber. We can have redundancy, but it will take some time for a switch to change over to another reference. During this time, the oscillator in that switch will be free-running. We want to minimize the phase drift during that interval, which we think should be a couple of seconds maximum. We have never worked on holdover, and I am wondering if we can do something smarter than the obvious feeding of some constant voltage to the VCXO, based on averaging during the locked state. Does anybody know of any good references on holdover? Thanks! Javier [1] http://www.ohwr.org/projects/white-rabbit/wiki ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Good references on holdover?
On 2/6/15 1:21 AM, Javier Serrano wrote: Dear all, We would like to start working on holdover performance for White Rabbit [1]. This is a new domain for us. Our main use case is a WR switch losing its reference because someone disconnects a fiber. We can have redundancy, but it will take some time for a switch to change over to another reference. During this time, the oscillator in that switch will be free-running. We want to minimize the phase drift during that interval, which we think should be a couple of seconds maximum. We have never worked on holdover, and I am wondering if we can do something smarter than the obvious feeding of some constant voltage to the VCXO, based on averaging during the locked state. Does anybody know of any good references on holdover? The only thing that might be better is if you have a control loop that forms a model and generates corrections based on that, so that while you holdover, it's not just a constant tuning voltage, but perhaps a slow ramp. For instance, if your control loop has estimated the aging rate (and you have an oscillator that ages really, really fast), you could do that. or if your control loop ingests the temperature and has calculated the temperature/frequency transfer function. But it seems that you have a fairly short holdover time requirement (seconds, maybe a few minutes?) I'm not sure the environment changes all that much in that time span. A simple hold the control voltage might be as good as it gets. I've looked into more complex schemes for situations where you are tracking out something that changes fairly rapidly (Doppler shift for an orbiter) and in a way that isn't a nice linear slope (which could be done nicely by a PID kind of loop). ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Good references on holdover?
Thanks for your ideas. On Fri, Feb 6, 2015 at 11:09 AM, Hal Murray hmur...@megapathdsl.net wrote: Is your PLL analog or digital? I'll assume digital since it's hard to hold analog voltages stable for several seconds. Yes, it is digital. It's even software. It runs on an LM32 [1] soft core inside an FPGA. Then there's a DAC and a VCXO. Our goal is to achieve sub-ns synchronization with respect to a master reference even during switch-over between redundant paths. Cheers, Javier [1] http://www.ohwr.org/projects/lm32 ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Good references on holdover?
javier.serrano.par...@gmail.com said: We have never worked on holdover, and I am wondering if we can do something smarter than the obvious feeding of some constant voltage to the VCXO, based on averaging during the locked state. Does anybody know of any good references on holdover? I doubt if you will come up with anything better. Maybe if you have big temperature changes, it might help to track the recent slope. Is your PLL analog or digital? I'll assume digital since it's hard to hold analog voltages stable for several seconds. I suggest capturing some data from a live system. Say 1 minute or 1/2 hour, whatever you can get with minimal work. Then scan it by eye to get a feel for what it does in normal operations. Then maybe patch your collector to only collect data for big values of the types of stuff that look interesting. Do you see any jumps? What's the biggest change over your worst case holdover? (or a bit longer for caution) I'm assuming you can compute or measure the frequency change per count. Do you see anything that will go over your limits? I assume you have some target limits rather than are doing this just because it feels good. If not, compute the predicted limits and see if your users are happy with that. One thing to watch out for is recovery. Do you have something like a PPS? If so, you have a decision to make. Do you want the PPS back to the correct timing as fast as possible or can it get there slowly? The PPS fixup time will be the integral under the frequency offset curve. You can have a tall narrow spike, or a short wide bump. For the spike, the frequency will be way off, but not for long. For the bump, the frequency will always be close, but it will be off for a long time. -- These are my opinions. I hate spam. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
