Hello, everyone. This is Colin from Fiber Optics For Sale.
In this video, I will explain what is jitter in fiber optic telecom systems, why jitter is bad, what causes jitter, and three types of jitter testing. So let's get started.
What is jitter? Let's look at this illustration first.
The solid line signal is the ideal clock. It has a perfect period. The dashed line signal is jittered clock. We can see that the rising edges of the jittered clock have a displacement from the ideal clock. The timing difference is marked as J1, J2, J3, J4, and J5. This phenomenon of timing displacement from its ideal position is called jitter.
The amount of jitter varies periodically. The jitter period is the interval between two times of maximum effect or minimum effect. Jitter frequency is jitter period's inverse.
According to ITU-T G.810, jitter frequencies below 10 Hz is called wander, and at and above 10 Hz is called jitter.
The bottom figure shows the plot of the relative displacement versus time, this is called the jitter's time function.
So why is jitter bad for a fiber optic communication system?
Here is an eye diagram that shows the distortions caused by jitter.
In order to accurately determine whether a given bit is a one or a zero, the signal should be sampled at the point where the vertical eye opening is maximum. This decision point is set by the recovered clock signal from the data bits.
But a jitter causes this decision point to move away from the maximum eye opening point. So the decision of a one or zero can be wrong if the jitter is too big. This causes bit errors and degrades the performance of a transmission system.
Jitters can be roughly categorized into two types -- random jitter and systematic jitter.
Random jitter comes from the noise generated by the electronic components in telecom equipment. Random jitter is independent of the transmitted data pattern.
Systematic jitter is dependent on the transmitted data pattern. They are caused by the finite Q of the clock recovery circuit and its relationship to the transmitted data spectrum.
SONET and SDH standards specify the jitter requirements at the optical interface.
The transmission equipment jitter specifications are organized into three categories -- Maximum Tolerable Jitter, Jitter Transfer Function and Jitter Generation.
Jitter tolerance measures how well the receiver can tolerate a jittered incoming signal. For telecom equipment, jitter tolerance is specified using jitter tolerance template. Each template defines the region over which the equipment must operate while maintaining a better-than-specified Bit Error Ratio performance. This figure shows a OC-48 jitter tolerance template.
Jitter transfer function is the test of jitter transmitted from input to output of a regenerator. It is the ratio of the amplitude of the equipment's output jitter to the applied input jitter. This is to ensure that once installed in a system, the equipment won't cause an unacceptable increase in jitter. This bottom figure shows an OC-48 jitter transfer function test.
Jitter generation is also called intrinsic jitter. It measures the jitter at an equipment's output when there is no jitter applied at the input.
So there you have it. Please leave your comment below if you'd like to see other topics.
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