Editing Eye pattern (section)

=== High-Frequency Loss ===

Loss of printed circuit board traces and cables increases with frequency due to [[dielectric loss]], which causes the channel to behave as a [[low-pass filter]]. The effect of this is an increase in signal rise/fall time. If the data rate is high enough or the channel is lossy enough, the signal may not even reach its full value during a fast 0-1-0 or 1-0-1 transition, and only stabilize after a run of several identical bits. This results in vertical closure of the eye.

The image below shows a 1.25&nbsp;Gbit/s NRZ signal after passing through a lossy channel - an RG-188 coaxial cable approximately 12 feet (3.65 meters) in length. This channel has loss increasing in a fairly linear fashion from 0.1 dB at DC to 9 dB at 6&nbsp;GHz.

The top and bottom "rails" of the eye show the final voltage the signal reaches after several consecutive bits with the same value. Since the channel has minimal loss at DC, the maximum signal amplitude is largely unaffected. Looking at the rising edge of the signal (a 0-1 pattern) we can see that the signal starts to level off around -300 [[Picosecond|ps]], but continues to rise slowly over the duration of the UI. At around +300 ps, the signal either begins falling again (a 0-1-0 pattern) or continues rising slowly (an 0-1-1 pattern).

[[File:Eye pattern LPF.png|thumb|upright=1.5|none|Eye pattern of a 1.25&nbsp;Gbit/s NRZ signal through a lossy channel]]

As high frequency losses increase the overall shape of the eye gradually degrades into a sinusoid (once higher frequency harmonics of the data has been eliminated, all that remains is the fundamental) and decreases in amplitude.