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 enforces standardization since it enables
independent developers and/or vendors to exchange their WR gear while
achieving absolute sub ns timing. Absolute calibrated devices can be
used as "golden standards" for the relative calibration
procedure.
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.
Figure 1: White Rabbit link delay model for absolute calibration*
EO/OE converters have their own calibration parameters that define the
relationship between their electrical and optical phase planes.
For some WR applications the electrical system can be broken into
different parts (for example a DIO
FMC to be plugged
onto a SPEC board). When the
electrical phase plane of the PPS signal is defined on such an
exchangeable module and future exchange is foreseen then separate
calibration of each of these parts is necessary.
Both, calibration procedures for EO/OE converters and exchangeable
modules, are outside the scope of this wiki page. However, it should be
noted that EO/OE converters and exchangeable modules should store their
calibration parameters in EPROM such that the system can automatically
reach absolute calibration during link initialization.
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).
/4679
Figure 2: Time scales and their relations*
Documentation and Tools
- White Rabbit Absolute Calibration Procedure, a step by step guide to perform absolute calibration on your WR device.
For absolute calibration a special SFP+ Loop Back
Module
(aka SFP+ timing calibration module) is needed.
Manufacturing files:
Acknowledgement
Part of this work is funded by ASTERICS European Commission grant no 653477
Contacts
Peter Jansweijer, Henk Peek, Tjeerd Pinkert, Guido Visser
Status
Date | Event |
24-02-16 | Start of project |
22-07-16 | added description of White Rabbit Absolute Calibration Procedure |
Last updated: 12 Nov 2016