A great deal of system performance information can be deduced from the eye pattern display. To interpret the eye pattern, consider Fig.7.1 and the simplified drawing shown in Fig. 7.2. The following information regarding the signal amplitude distortion, timing jitter, and system rise time can be derived:

• The width of the eye opening defines the time interval over which the received signal can be sampled without error due to interference from adjacent pulses (known as intersymbol interference).

• The best time to sample the received waveform is when the height of the eye opening is largest. This height is reduced as a result of amplitude distortion in the data signal. The vertical distance between the top of the eye opening and the maximum signal level gives the degree of distortion. The more the eye closes, the more difficult it is to distinguish between 1s and 0s in the signal.

The height of the eye opening at the specified sampling time shows the noise margin or immunity to noise. Noise margin is the percentage ratio of the peak signal voltage V1 for an alternating bit sequence (defined by the height of the eye opening) to the maximum signal voltage V2 as measured from the threshold level, as shown in Fig. 7.2 That is

Noise margin (percent)=$\frac{V_1}{V_2} ×100$ percent

Figure 7.1 General configuration of an eye diagram showing definitions of fundamental measurement parameters.

• The rate at which the eye closes as the sampling time is varied (i.e., the slope of the eye-pattern sides) determines the sensitivity of the system to timing errors. The possibility of timing errors increases as the slope becomes more horizontal.

  Timing jitter (percent)=$\frac{∆T}{T_b} × 100$ percent

  • Timing jitter (also referred to as eye jitter or phase distortion) in an optical fiber system arises from noise in the receiver and pulse distortion in the optical fiber. If the signal is sampled in the middle of the time interval (i.e., midway between the times when the signal crosses the threshold level), then the amount of distortion ΔT at the threshold level indicates the amount of jitter. Timing jitter is thus given by

  • Traditionally, the rise time is defined as the time interval between the point where the rising edge of the signal reaches 10 percent of its final amplitude to the time where it reaches 90 percent of its final amplitude. However, in measuring optical signals, these points are often obscured by noise and jitter effects. Thus, the more distinct values at the 20 percent and 80 percent threshold points normally are measured. To convert from the 20 to 80 percent rise time to a 10 to 90 percent rise time, one can use the approximate relationship.

$$T_{(10-90)}=1.25T_{(20-80)}$$

• Any nonlinearities of the channel transfer characteristics will create an asymmetry in the eye pattern. If a purely random data stream is passed through a purely linear system, all the eye openings will be identical and symmetric.