Figure 1: Focus on Electrical absolute calibration
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