AN 835: PAM4 Signaling Fundamentals

ID 683852
Date 3/12/2019
Public
Document Table of Contents

2.2. Eye Height (EH6) and Eye Width (EW6)

When using NRZ, the eye height and width are measured from the biggest opening of an eye. However, this is not the case for PAM4’s eye height and eye width.

For the PAM4 eye, one needs to locate the Tmid point first. See the following figure for more details. Tmid is the midpoint of the maximum horizontal eye opening for a 10-3 (this methodology is defined in section 16.3.10.2 of the OIF-CEI56G-VSR standard) inner eye contour (red) of the middle eye.

Figure 10. How EW6 and EH6 Are Measured

EH6 represents the eye height at a BER of 10-6 (green). After the Tmid point is found, draw a vertical line that intersects with the three eyes' 10-6 contour ring (green). EH6 is the vertical distance between two intersection points on the 10-6 contour ring in an eye. As shown in the previous figure, EH6 is not necessarily the maximum eye height.

EW6 represents the eye width at a BER of 10-6 (green). Taking the upper eye for example, you find the half point of the eye height, (EH6 upp)/2. Draw a horizontal line that intersects with the 10-6 contour ring (green). The EW6 of the upper eye is the horizontal distance between two intersection points on the 10-6 contour ring in the eye. The lower eye’s EW6 is measured in the same manner. From the figure, notice that the EW6 of each eye is not the widest opening. The asymmetry of the upper and lower eye causes the widest portion of the eye to be off-center. Compared with the widest portion, EW6 is considerably reduced.

This methodology can be used to determine the expected eye mask for a signal for a given BER as per the following figure.

Figure 11. PAM4 Horizontal Eye Mask Sample