Editing Eye pattern (section)

=== Impedance Mismatches ===

Stubs, impedance mismatches, and other defects in a transmission line can cause [[Signal_reflection|reflections]] visible as defects in the edges of the signal. Reflections with a delay greater than one UI often render the eye completely unreadable due to [[Intersymbol_interference|inter-symbol interference (ISI)]], however those with a shorter delay can be easily seen in the shape of the eye.

In the image below, a roughly one inch (25.4 mm) open circuited stub is present in the line, causing an initial low-impedance effect (reduced amplitude) followed by a positive reflection from the end of the stub with a delay of about 320 ps or 0.4 UIs. This can be clearly seen as a "step" in the rising edge in which the signal rises to a fraction of the full value, levels off for the round trip delay of the stub, then rises to its full value when the reflection arrives.

[[File:Eye pattern mismatch.png|thumb|upright=1.5|none|Eye pattern of a 1.25&nbsp;Gbit/s NRZ signal with a one-inch stub]]

In the image below, an additional three inches of cable is added to the end of the same stub. The same "step" is present but is now four times as long, producing reflections at about 1280 ps or 1.6 UI. This produces extreme ISI (since the reflection of each UI arrives during the subsequent UI) which completely closes the eye.

[[File:Eye pattern long stub.png|thumb|upright=1.5|none|Eye pattern of a 1.25&nbsp;Gbit/s NRZ signal with a four-inch stub]]