Figure 1: Focus on Electrical absolute calibration
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 = t A - t 1 = Δ tx - Δ PPS
When applying a loopback from Tx to Rx then:
- Δ RXcal = t A +t loop - t 4p = - Δ rx - Δ PPS
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.
By definition, the moment a Tx packet traverses the external time
reference plane is later than t 1 on the internal time scale,
hence Δ TXcal is positive proportional to Δ tx.
The opposite is true for the moment a Rx packet traverses the external
time reference plane since this is by definition before t 4p
on the internal timescale, hence Δ RXcal is negative
proportional to Δ rx. The internal timescale is ahead of the
external timescale by Δ PPS, which is taken into account in
both Δ TXcal and Δ RXcal. Applying these fixed
calibration parameters virtually shifts the internal time scale such
that it lines up with the external time scale as is shown by the green
arrow time scale in Figure 2.
The calibration parameters can be calculated since t A can be
measured, t loop is a known calibrated loopback delay while t
1 and t 4p are the timestamps that are recorded by
the WR
software.
Figure 2: Time scales and their relations
Documentation and Tools
-
White Rabbit Electrical Absolute Calibration
Procedure
- A step by step guide to perform electrical absolute calibration on your WR device.
- For absolute calibration a special SFP+ Loop Back Module (aka SFP+ timing calibration module) is needed.
-
SFP+ timing calibration
module
- Data_Sheet
- Manufacturing files (Note: PCB Design created with Mentor Graphics using a Xpedition License)
- White Rabbit SFP LoopBack Enclosure
- White Rabbit SFP Short Enclosure
- Procedure used for calibration of the SFP+ timing calibration modules
Project information
- Electrical absolute calibration inter-comparison (Nikhef, VTT, NPL) results are reported in:
- arXiv.org paper "First electrical White Rabbit absolute calibration inter-comparison" which is available under the Creative Commons Attribution-ShareAlike 4.0 International License.
- Frequently Asked Questions
Acknowledgement
Part of this work is funded by ASTERICS European Commission grant no 653477
Contacts
Peter Jansweijer, Henk Peek, Tjeerd Pinkert, Guido Visser
Commercial producers
SFP+ timing calibration module, INCAA Computers, Netherlands
Status
Date | Event |
---|---|
24-02-16 | Start of project |
22-07-16 | added description of White Rabbit Absolute Calibration Procedure |
17-07-17 | Made this sub sub-wiki page |
30-05-18 | Updated figure and formulas |
31-08-18 | Updated Calibration Procedure to V1.1 |
09-11-18 | SFP timing calibration Module |
05-03-20 | added module calibration procedure document |
17-09-20 | update SFP+ timing calibration module data sheet |
26-01-22 | add link to EE inter-comparison paper |
Last updated: 26 January 2022