Editing Eye pattern (section)

== Channel effects ==

Many properties of a [[Communication_channel|channel]] can be seen in the eye pattern.

=== Emphasis ===

[[Emphasis (telecommunications)|Emphasis]] applied to a signal produces an additional level for each value of the signal which is higher (for pre-emphasis) or lower (for de-emphasis) than the nominal value.

The eye pattern for a signal with emphasis may be mistaken for that of a PAM signal at first glance, however closer inspection reveals some key differences. Most notably, an emphasized signal has a limited set of legal transitions:

* Strong state to corresponding weak state (1-1 or 0-0 bit pattern)
* Strong state to opposite strong state (second transition of a 1-0-1 or 0-1-0 bit pattern)
* Weak state to opposite strong state (second transition of a 1-1-0 or 0-0-1 bit pattern)

An emphasized signal will never transition from a weak state to the corresponding strong state, a weak state to another weak state, or remain in the same strong state for more than one UI. A PAM signal also normally has equally spaced levels while emphasized levels are normally closer to the nominal signal level.

[[File:Eye pattern emphasis.png|thumb|upright=1.5|none|Eye pattern of a 1.25&nbsp;Gbps NRZ signal with 6 dB of pre-emphasis]]

=== 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.

=== 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]]