Measurements: the eye pattern

 

The eye pattern allows you to assess in a very simple way the quality of the received digital signals before they reach the demodulation component. It is about one of basic measurements in digital transmission.

Achievement :

 

The received signal, after its conditioning by the receive Nyquist filter, is connected on the Y input of an oscilloscope.

The oscilloscope is placed in the external synchronization mode. The the oscilloscope time base is synchronized by the clock signal.

One set up the vertical gain and the sweep rate to see a signal as below.

 

 

Interpretation

" The opening of the eye " characterizes the noise and the distortions brought by the transmission chain. The sampling operation must occur at the precise moment where the eye is the most open. A quite open eye will allow the demodulation device to easily distinguish the transmitted symbols. On the other hand, a closed eye will inevitably result in many transmission errors.

The closing of the eye is the result:

- of noise: it is then necessary to improve the quality of the radio signal

- of intersymbol interference: the amplitude-frequency characteristics and/or phase-frequency of the transmission chain are not correct (NB: this type of defect also affects the transmission quality in television). It is necessary to characterize them, find and correct the defects. One will refer usefully to the page relating to the Nyquist filter.

An example of distorted signal:

 

 

 

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