White Rabbit Calibration
Relative calibration
-
White Rabbit devices
calibration
- provides coarse calibration values for the official releases of WR Switch and WR PTP Core firmware. You can use them as defaults for matching hardware and gateware version. However, to achieve the best possible synchronization results you should perform the full calibration procedure for your WR hardware.
- WR Calibration Procedure
Absolute calibration
Overview
Absolute calibration is achieved by measuring the time relationship (Δ TXcal, Δ RXcal) between the external electrical phase planes of the PPS signal and the interface to the Electrical-Optical / Optical-Electrical converter (EO/OE, usually the electrical SFP connector) as shown in Figure 1. Note that EO/OE converters have their own calibration parameters that define the relationship between their electrical and optical phase planes. Calibration of EO/OE converters is not covered on this wiki page.
Figure 1: White Rabbit link delay model for absolute calibration*
Absolute calibration enables independent developers and/or vendors to
exchange their WR gear while achieving sub ns timing synchronization.
Absolute calibrated devices can be used as "golden standards" for the
relative calibration
procedure.
Principle
Precision Time Protocol (PTP) packets that are used for the link delay
calculation are timestamped inside WR devices and hence are defined on
the “internal time” scale (the red arrow in Figure 2). Delays exist
between the internal time scale and the electrical interfaces that are
defined on the “external time” scale (the blue arrow in Figure 2).
The external time scale uses the PPS signal as a time reference point
(t=0). The time it takes for a t 1 timestamped Tx packet to
traverse the external electrical reference plane is:
- Δ TXcal = A - t 1 = Δ PPS + Δ tx
When applying a loopback from Tx to Rx then:
- Δ RXcal = B - t 4p = Δ PPS + Δ rx
defines the time it takes for a t 4p timestamped Rx packet to
traverse the external electrical reference plane. Note that t
4p includes fine delay phase information.
A and B can be measured while t 1 and t 4p are the
timestamps that are recorded by the WR software (by
PPSi).
time-scales.png
Figure 2: Time scales and their relations*
Tools and Documentation
The "White Rabbit Absolute Calibration Procedure" is a step by step
guide to perform absolute calibration on your WR device. For absolute
calibration a special SFP+ Loop Back
Module
is needed.
Manufacturing files can be found here:
Acknowledgement
Part of this work is funded by ASTERICS European Commission grant no 653477
06 June 2016, Peter Jansweijer, Henk Peek, Tjeerd Pinkert, Guido Visser