What is the use of eye diagrams in PCB design?
The eye diagram is due to the afterglow effect of the oscilloscope, which overlaps each symbol waveform obtained by scanning to form an eye diagram.
Now I 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 situation of the eye diagram.
To understand the oscilloscope eye diagram, 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 a wealth of information. From the eye diagram, the influence of crosstalk and noise between codes can be observed. The degree of inferiority, so 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 crosstalk between codes and improve the transmission performance of the system.
Connect an oscilloscope across the output of the receive filter, and then adjust the oscilloscope scan period to synchronize the oscilloscope horizontal scan period with the period of the received symbol. At this time, the graph seen on the oscilloscope screen is called an eye diagram.
The signal that the oscilloscope generally measures is a waveform of some bits or a certain period of time, which reflects more detailed information, and the eye diagram reflects the overall characteristics of all digital signals transmitted on the link.
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 to synchronize the oscilloscope horizontal scanning period with the receiving symbol period. Eyes, so called "eye diagram".
From the "eye diagram", the influence of crosstalk and noise between codes can be observed, so as to estimate the system's advantages and disadvantages. In addition, this graph can also be used to adjust the characteristics of the receiving filter to reduce crosstalk between codes and improve the transmission performance of the system.
2. How is the eye pattern formed?
For digital signals, there can be multiple sequence combinations of high-level and low-level changes. 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 at a certain reference point, and then the waveforms are superimposed to form an eye diagram.
As shown below. For the test instrument, first recover the clock signal of the signal from the signal to be measured, then superimpose the eye diagram according to the clock reference, and finally display it.
3. What information is contained in the eye diagram?
For a real eye diagram, as shown below, first we can see the average rise time (RiseTime), fall time (FallTime), overshoot (Undershoot), threshold level (Threshold) of the digital waveform / CrossingPercent) and other basic level conversion parameters.
Rise Time: The rise time of the 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 overcharging, the first peak or valley value exceeds the set voltage, which is mainly manifested 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. Overshooting can cause false clock or data errors.
Threshold level (Threshold / CrossingPercent): refers to the lowest receiving level that the receiver can reach when the system transmission characteristics are worse than a certain bit error rate.
Fourth, how to distinguish the signal quality according to the eye diagram
It is impossible for the signal to maintain the same voltage value at every high and low level, nor can it be 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 phenomenon of blur 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 (VoltageNoise); on the horizontal axis time axis, it is reflected in the time domain jitter. As shown below.
When there is noise, the noise will be superimposed on the signal, and the traces of the observed eye pattern will become blurred. If there is crosstalk between codes at the same time, the "eyes" will be opened smaller. Generally, the larger the eye opening of the eye diagram, the higher the eye height of the eye diagram, which means the better the signal quality.
You can get the eye diagram situation by doing signal simulation, and then judge the signal quality according to the eye diagram situation. If the eye diagram situation 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 quality of the product signal produced.
