What is the use of eye diagrams in PCB design?

The eye diagram is due to the afterglow effect of the oscilloscope, which overlaps the waveforms of each symbol obtained by scanning to form an eye diagram.

This article will lead you to understand what the eye diagram on the PCB is, how the eye diagram is formed, what information is contained in the eye diagram, and how to distinguish the signal quality according to the eye diagram.

To understand the eye diagram of an oscilloscope, you need to master the following 4 points:

1. What is an eye diagram?

The eye diagram is a series of digital signals accumulated and displayed on the oscilloscope. It contains rich information. From the eye diagram, the influence of inter-symbol crosstalk and noise can be observed, reflecting the overall characteristics of the digital signal, so as to estimate the optimal system performance. Therefore, eye diagram analysis is the core of signal integrity analysis of high-speed interconnection systems.

In addition, this graph can also be used to adjust the characteristics of the receiving filter to reduce the inter-symbol crosstalk and improve the transmission performance of the system.

Connect an oscilloscope across the output of the receiving filter, and then adjust the oscilloscope scanning period to synchronize the horizontal scanning period of the oscilloscope with the period of the received symbols. At this time, the graph seen on the oscilloscope screen is called the eye diagram.

The signal measured by the oscilloscope is generally the waveform of some bits or a certain period of time, which reflects more detailed information, while the eye diagram reflects the overall characteristics of all digital signals transmitted on the link.

The way to observe the eye diagram is: connect an oscilloscope across the output of the receiving filter, and then adjust the oscilloscope scanning period to synchronize the horizontal scanning period of the oscilloscope with the period of the received symbols. eyes, so it is called "eye diagram".

From the "eye diagram", the influence of inter-symbol crosstalk and noise can be observed, so as to estimate the pros and cons of the system. In addition, this graph can also be used to adjust the characteristics of the receiving filter to reduce inter-symbol crosstalk and improve the transmission performance of the system.

Second, how is the eye diagram formed?

For digital signals, the changes of high and low levels can be combined in various sequences. Taking 3 bits as an example, there can be 8 combinations of 000-111. In the time domain, align enough of the above sequences to a certain reference point, and then superimpose their waveforms to form an eye diagram.

As shown below. For the test instrument, the clock signal of the signal is first recovered from the signal to be tested, and then the eye diagram is superimposed according to the clock reference, and finally displayed.

3. What information is contained in the eye diagram?

For a real eye diagram, as shown in the figure below, first we can see the average rise time (RiseTime), fall time (FallTime), overshoot (Overshoot), undershoot (Undershoot), and threshold level (Threshold) of the digital waveform. /CrossingPercent) and other basic level conversion parameters.

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

Fall Time (FallTime): The fall time of the pulse signal refers to the time interval from 90% to 10% of the peak amplitude of the pulse.

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

Undershoot: Refers to the next valley or peak. Excessive overshoot can cause the protection diode to operate, resulting in premature failure. Excessive undershoot can cause false clocks or data errors.

Threshold level (Threshold/CrossingPercent): It refers to the minimum receiving level that the receiver can achieve when the system transmission characteristics are inferior to a certain bit error rate.

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

It is impossible for the signal to maintain the exact same voltage value of each high and low level, and it cannot guarantee 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 thicker, and the phenomenon of blur appears.

Therefore, the eye diagram also reflects the noise and jitter of the signal: on the vertical voltage axis, it is reflected as voltage noise (VoltageNoise); on the horizontal time axis, it is reflected as time domain jitter (Jitter). As shown below.

When noise is present, the noise will be superimposed on the signal and the traces of the observed eye pattern will become blurred. If there is intersymbol interference at the same time, the "eyes" will be opened smaller. Generally, the wider the eyes of the eye diagram are, the higher the eye height of the eye diagram, the better the signal quality.

The eye diagram can be obtained by signal simulation, and then the quality of the signal can be judged according to the eye diagram. If the eye diagram is not good, the hardware design or PCB design can be adjusted to make the eye height of the eye diagram higher to ensure the signal quality of the produced products.