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## Absolute calibration
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### **Overview**
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Absolute calibration is achieved by measuring the time relationship (Δ
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<sub>TXcal</sub>, Δ <sub>RXcal</sub>) between the external electrical
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phase planes of the PPS signal and the interface to the
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*Figure1: White Rabbit link delay model for absolute calibration**
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Absolute calibration enables independent developers and/or vendors to
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exchange their WR gear while achieving sub ns timing synchronisation.
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exchange their WR gear while achieving sub ns timing synchronization.
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Absolute calibrated devices can be used as "golden standards" for the
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[relative calibration
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procedure](https://www.ohwr.org/project/white-rabbit/wikis/Calibration).
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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 t1 timestamped Tx packet to traverse the
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external electrical reference plane is:
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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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- 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 t4p timestamped Rx packet to traverse
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the external electrical reference plane. Note that t4p includes fine
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delay phase information.
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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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time-scales.png
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*Figure2: Time scales and their relations**
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- [WR calibration - ASTERICS -
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Nikhef](https://redmine.nikhef.nl/et/project/wr_cal/wiki)
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- Aims to provide a method of absolute calibration of all elements
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of a WR device (preliminary).
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### **Tools and Documentation**
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For absolute calibration special hardware is needed:
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The "White Rabbit Absolute Calibration Procedure" is a step by step
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guide to perform absolute calibration on your WR device. For absolute
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calibration a special [SFP+ Loop Back
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Module](https://www.ohwr.org/4466)
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is needed.
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Manufacturing files can be found here:
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- [White Rabbit SFP
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LoopBack](https://www.ohwr.org/4464)
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-----
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19 May 2016
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06 June 2016, Peter Jansweijer, Guido Visser
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