Technical Bulletin N°11

Optical Complex Spectrum Analyzer AP2440A application note

The phase eye diagram

The eye pattern , commonly known as an eye diagram is widely used in the telecommunications industry to determine the quality of a transmission signal. It is a display in which a digital signal from a receiver is repetitively sampled and applied to the vertical input , while the data rate is used to trigger the horizontal sweep. As the name reveals, the digital pattern creates a look similar to a series of eyes between a railroad track. The shape of the eye provides useful information for system optimization.

The Apex Technologies Optical Complex Spectrum Analyzer has this feature build in as one of its basic functions and measures it in a completely different way from a Digital Sampling Oscilloscope. The APEX Technologies AP2440A series equipments are able to measure the intensity and the phase of a periodic signal as a function of time, then thanks to a bits superpositions software method we can display the eye diagram. The overall advantage of this method is that it enables the display of both the intensity and phase of the eye diagram, which is not possible by the conventional method of a digital oscilloscope.

A DPSK modulation example

With DPSK modulation, 2 different phase states can be transmitted. The '1s' and '0s' are represented by a 180° phase difference. With the AP2440A series OCSA's, one can display a graph of the phase as a function of the time, a constellation display and the phase eye diagram.

A QPSK modulation example

With QPSK, 4 different phase states are transmitted. Each phase state is separated by a 90° phase difference and corresponds to a symbol which carries 2 bits. With the AP2440A series OCSA's, one can display a graph of the phase as a function of the time, a constellation display and the phase eye diagram.

 
Every measurement displayed on this technical bulletin have been measured with an
AP2440A Optical Complex Spectrum Analyzer from APEX Technologies
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