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What is the use of eye diagrams in PCB design?

The eye diagram is due to the afterglow effect of the oscilloscope. Each symbol waveform obtained by scanning is superimposed together to form an eye diagram.

1.Definition of eye diagram

Eye diagram refers to a pattern observed on an oscilloscope when using experimental methods to estimate and improve (by adjusting) the performance of a transmission system.The method of observing the eye diagram is: use an oscilloscope to connect to the output end of the receiving filter, and then adjust the oscilloscope scanning period so that the horizontal scanning period of the oscilloscope is synchronized with the period of the receiving symbols. At this time, the graphic on the oscilloscope screen looks like a human Eye, so called "eye diagram".From the "eye diagram", you can observe the influence of inter-symbol crosstalk and noise, so as to estimate the quality of the system. In addition, you can use this graph to adjust the characteristics of the receiving filter to reduce inter-symbol interference and improve the transmission performance of the system.

2.Formation of eye diagram

For digital signals, the sequence of high-level and low-level changes can be various. Taking 3 bits as an example, there can be a total of 8 combinations of 000-111. In the time domain, enough of the above sequences are aligned according to a certain reference point, and then the waveforms are superimposed to form an eye diagram.

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3.Information contained in eye diagrams

A real eye diagram, we can see the average rise time (Rise Time), fall time (Fall Time), overshoot, undershoot, threshold level (Threshold / Crossing Percent) of the digital waveform ) And other basic level conversion parameters.

Rise Time: The rise time of the pulse signal refers to the interval between the instants when the instantaneous value of the pulse first reaches the specified lower limit and the specified upper limit. Unless otherwise specified, the lower and upper limits are set to 10% and 90% of the peak amplitude of the pulse, respectively.

Fall Time: The fall time of the pulse signal refers to the time interval from 90% to 10% of the peak value of the pulse.

Overshoot: Also called overshoot is the first peak or valley value exceeds the set voltage, which mainly manifests as a sharp pulse, and can cause the failure of circuit components.

Undershoot: refers to the next valley or peak. Excessive overshoot can cause the protection diode to work, leading to premature failure. Excessive undershoot can cause false clock or data errors.

Threshold level (Threshold / Crossing Percent): It refers to the lowest receiving level that the receiver can achieve when the transmission characteristics of the system are worse than a certain bit error rate.

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4.How to distinguish the signal quality according to the eye diagram

It is impossible for the signal to keep the voltage values ​​of the high and low levels exactly the same each time, and it is not guaranteed that the rising and falling edges of each high and low level are at the same time.Due to the superposition of multiple signals, the signal line of the eye diagram becomes thick, and a blur phenomenon occurs. Therefore, the eye diagram also reflects the noise and jitter of the signal: on the vertical axis voltage axis, it is reflected as voltage noise (Voltage Noise); on the horizontal axis time axis, it is reflected as time domain jitter (Jitter).

When noise is present, the noise will be superimposed on the signal, and the stitches of the observed eye diagram will become blurred. If there is inter-symbol crosstalk at the same time, the "eyes" will be opened wider. Generally, the bigger the eye of the eye diagram is, the higher the eye height of the eye diagram is, the better the signal quality is.

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Do signal simulation to obtain the eye diagram, and then judge the signal quality according to the eye diagram. If the eye diagram is not good, you can adjust the hardware design or PCB design to make the eye height of the eye diagram higher, and ensure the signal quality of the product.