How to roughly observe the eye diagram on an oscilloscope without eye diagram analysis software

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

High-end oscilloscopes may come with eye diagram analysis software, but the price is often expensive. If the engineer has limited funds and does not have a high-end oscilloscope, but needs to observe the eye diagram on the oscilloscope to understand the general situation of the signal, then a general oscilloscope can also perform simple measurements as long as it is properly set up.

First, we use the edge trigger of the oscilloscope to stabilize the signal. Then we turn on the measurement item to measure the rise time of the oscilloscope.

Then we change the trigger mode of the oscilloscope to slope trigger. Slope trigger means that when the time for the waveform to reach another level from one level meets the set time condition, a trigger is generated. Therefore, we need to set the trigger level upper limit and trigger level lower limit.

We adjust the trigger threshold level, set the trigger level lower limit to 10%-30% of the signal amplitude, and the trigger level upper limit to around 70%-90%, to represent the threshold range of the signal rise and fall time trigger. As shown in the figure above, the trigger level upper limit is set to 20.8mV, and the trigger level lower limit is -12.2mV. At the same time, set the conditional maximum time, and select this time setting condition to be less than the previous signal rise time.

After setting the trigger, we also need to set the afterglow of the signal. The so-called afterglow is to let the signal accumulate on the screen according to certain setting conditions. It can be set to automatic, or set to normal to adjust the signal accumulation time by itself. As shown in the figure below, we have selected automatic.

For oscilloscopes with digital fluorescent display effects, as shown in the figure above, you can see that the signal has a sense of light and dark layers. The brighter the part, the higher the probability of the signal appearing. Some oscilloscopes also support color temperature mode, as shown in the figure below. In color temperature mode, the more orange the waveform color, the higher the probability of the signal appearing, and the bluer the waveform, the lower the probability of the signal appearing.

Finally, you can use the cursor and measurement functions of the oscilloscope to simply measure some eye diagram parameters. The figure above tests the eye diagram of the USB signal.

If we don't have a high-speed signal, we can simulate the eye diagram. We only need a signal generator that can generate two signals. One signal is used as a trigger source, which can be a 9MHz sine wave, and the other signal is used as an eye diagram display, which can be set to a 3MHz square wave. The signal frequency of the displayed eye diagram should be a multiple of the signal frequency of the trigger source, and then the slope trigger can be used.

In addition, considering the accuracy and stability of the eye diagram test, it is generally required to accumulate enough waveforms before analysis, which places relatively high requirements on the storage depth of the oscilloscope. The higher the storage depth of the oscilloscope, the more signal points the oscilloscope captures, and the more accurate the test results. Of course, the analysis speed of the oscilloscope will also be relatively slow. Therefore, when measuring the eye diagram and jitter of high-speed signals, try to increase the storage depth of the oscilloscope.