Hardware, Software, Testing Requirements, and Test Results
29
TIDUES1A – October 2019 – Revised February 2020
Copyright © 2019–2020, Texas Instruments Incorporated
EMC Compliant 10/100-Mbps Ethernet PHY Reference Design With IEEE
802.3at Type-1 (
≤
12.95 W) PoE-PD
3.2.2
Test Results
3.2.2.1
Eye Diagram
Because the serial communication links of today operate at very high transmission frequencies, a number
of variables that can affect the signal integrity are transmission-line effects, impedance mismatches, signal
routing, termination schemes, and grounding schemes. An eye diagram created using an oscilloscope
helps to quickly evaluate system performance and gain insight into the nature of channel imperfections
that can lead to errors when a receiver tries to interpret the value of a bit. As high-speed serial data
travels from a transmitter to a receiver, it can suffer impairments. The transmitter, PCB traces, connectors,
and cables will introduce interference that will degrade a signal both in its amplitude and timing.
An eye diagram is a common indicator of the quality of signals in high-speed digital transmissions and
might be helpful in locating the source of the problem. An oscilloscope generates an eye diagram by
superimposing positive and negative pulses. Overlaying many bit transitions over time result in an image
that looks like an eye-shaped pattern. An eye diagram can reveal important information. It can indicate the
best point for sampling, divulge the SNR (signal-to-noise ratio) at the sampling point, and indicate the
amount of jitter and distortion. Additionally, it can show the time variation at zero crossing, which is a
measure of jitter.
shows the eye diagram for the DP83825 Ethernet PHY.
shows
different eye diagram measurements.
NOTE:
A Tektronix FastAcq was used with positive edge trigger and infinite persistence that will
result in this stacked eye waveform.
Figure 28. Eye Diagram
