NI-Synch cRIO Holdover

[nkraemer]

I have a cRIO-9057 synchronized via NI-Synch and PTP to an external grandmaster. If the external grandmaster dies, what worst-case holdover performance can I expect from my cRIO assuming constant temperature? The datasheet claims the "Internal Real-Time Clock" clock has a 200ppm accuracy. How much (if at all) does using the NI-Synch functionality at a constant (but arbitrary) temperature improve upon this?

Any knowledge or experience in this area is appreciated!

[cweeks12]

Hi nkraemer,

I found this article that gets into the details of NI-Sync a little bit, https://www.ni.com/en/support/documentation/supplemental/09/timing-and-synchronization-systems.html and it looks like all NI-Sync will do is help multiple devices synchronize their clocks to the same time. Remove that grandmaster clock, and I don't think that NI-Sync will actually do anything to help the accuracy of the clock, because without the external clock, you don't have any reference and would have to rely on the RIO's hardware itself, probably pointing towards that 200ppm accuracy without the grandmaster.

[nkraemer]

Hi Connor,

Thanks for digging up that article for me. Do you (or anyone else) know if NI-Sync would be capable of detecting the difference between the oscillator's nominal frequency and its actual frequency and compensating for that in a Phased Locked Loop? I assume it must be doing something like that to synchronize to a PTP grandmaster within 10s of nanoseconds since a 200ppm or even 40ppm error in the oscillator would lead to de-synchronization on the order of microseconds over the course of a second. I was hoping NI-Sync would be capable of keeping the latest frequency 'calibration' even after losing the PTP grandmaster for improved holdover performance.

[cweeks12]

I unfortunately can't find any information on that around. Just as a side question, why is the grandmaster going to be taken away from the system? I would think it would be ideal to constantly be sync-ing to it.

[nkraemer]

I am looking at this from a reliability perspective. Ideally the grand master would always be present, but we would like to know how failure-tollerant we are.

Do you know where else I could look to find answers to this? Would you recommend opening up a Service Request with NI?