... | ... | @@ -55,66 +55,10 @@ dispersion. This can be measured and monitored in-situ. |
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Calibration of WR devices is spans three main calibration topics, each
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described on the following wiki pages:
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- [Absolute
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calibration](https://www.ohwr.org/project/wr-calibration/wiki/absolute-calibration)
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- [Absolute calibration](/absolute-calibration)
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- Electrical-Optical / Optical Electrical
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- In-situ determination of alpha
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### **Principle**
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Precision Time Protocol (PTP) packets that are used for the link delay
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calculation are timestamped inside WR devices and hence are defined on
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the “internal time” scale (the red arrow in Figure 2). Delays exist
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between the internal time scale and the electrical interfaces that are
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defined on the “external time” scale (the blue arrow in Figure 2).
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The external time scale uses the PPS signal as a time reference point
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(t=0). The time it takes for a t <sub>1</sub> timestamped Tx packet to
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traverse the external electrical reference plane is:
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- Δ <sub>TXcal</sub> = A - t <sub>1</sub> = Δ <sub>PPS</sub> + Δ
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<sub>tx</sub>
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When applying a loopback from Tx to Rx then:
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- Δ <sub>RXcal</sub> = B - t <sub>4p</sub> = Δ <sub>PPS</sub> + Δ
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<sub>rx</sub>
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defines the time it takes for a t <sub>4p</sub> timestamped Rx packet to
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traverse the external electrical reference plane. Note that t
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<sub>4p</sub> includes fine delay phase information.
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A and B can be measured while t <sub>1</sub> and t <sub>4p</sub> are the
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timestamps that are recorded by the WR software (by
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[PPSi](https://www.ohwr.org/project/ppsi/wiki)).
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/4679
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*Figure 2: Time scales and their relations**
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### **Documentation and Tools**
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- [White Rabbit Absolute Calibration
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Procedure](https://www.ohwr.org/project/wr-calibration/uploads/4b3ace7abe41d2c64aa8fffbf3194f7e/WhiteRabbitAbsoluteCalibrationProcedure.pdf),
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a step by step guide to perform absolute calibration on your WR
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device.
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For absolute calibration a special [SFP+ Loop Back
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Module](https://www.ohwr.org/project/white-rabbit/uploads/f6d07a9f48cfbaf3846f0b9b985571e4/SFP__timing_calibration_module.pdf)
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(aka SFP+ timing calibration module) is needed.
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[Manufacturing files](https://www.ohwr.org/project/wr-calibration/wikis/Documents/Manufacturing-Data):
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- White Rabbit SFP LoopBack
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[PCB](https://www.ohwr.org/4699)
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and
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[Enclosure](https://www.ohwr.org/4697)
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- White Rabbit SFP Short
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[PCB](https://www.ohwr.org/4700)
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and
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[Enclosure](https://www.ohwr.org/4698)
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### **Acknowledgement**
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Part of this work is funded by [ASTERICS](https://www.asterics2020.eu/)
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European Commission grant no 653477
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-----
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## Contacts
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