Eye diagram, which is formed by overlapping the waveforms of each code element obtained from the scan due to the afterglow of the oscilloscope.
It contains a wealth of information, from the eye diagram can be observed on the impact of inter-code crosstalk and noise, reflecting the overall characteristics of the digital signal, so as to estimate the degree of system advantages and disadvantages, and thus the eye diagram analysis is the core of high-speed interconnect system signal integrity analysis.
In addition, this graph can also be used to adjust the characteristics of the receiver filter to reduce inter-code crosstalk and improve the transmission performance of the system.
Oscilloscopes generally measure the signal is a number of bits or a certain period of time waveform, more reflective of the details of the information, while the eye diagram reflects the overall characteristics of all digital signals transmitted on the link.
Observe the method of eye diagram is: an oscilloscope across the output of the receiver filter, and then adjust the oscilloscope scanning period, so that the oscilloscope horizontal scanning period and receive the cycle of the code element synchronization, then the oscilloscope screen to see the graphics like the human eye, it is known as the “eye diagram”.
From the “eye diagram” on the crosstalk between the code can be observed and the impact of noise, thereby estimating the degree of system advantages and disadvantages.
In addition, this graph can also be used to adjust the characteristics of the receiver filter to reduce inter-code crosstalk and improve the transmission performance of the system.
How Are Eye Diagram Formed?
For digital signals, there can be multiple combinations of sequences of high 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 to a certain reference point, and then their waveforms are superimposed to form an eye diagram.
The figure below. For test instruments, the clock 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.
What Information Is Contained In The Eye Diagram?
For a real eye diagram, as shown below, we can first see the parameters of the basic level shift such as the average rise time (RiseTime), fall time (FallTime), upswing (Overshoot), downswing (Undershoot), and threshold level (Threshold/CrossingPercent) of a digital waveform.
Rise time: The rise time of the pulse signal is the interval between the two moments 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 peak amplitude of the pulse, respectively.
Fall time: The fall time of the pulse signal is defined as the time interval from 90% of the peak pulse amplitude down to 10%.
Overshoot: Also known as overshoot is the first peak or valley value over the set voltage, mainly manifested as a tip 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, leading to premature failure. Excessive undershoot can cause false clock or data errors.
Threshold level (Threshold/CrossingPercent): Refers to the system transmission characteristics inferior to a particular BER case, the minimum level of reception can be achieved by the receiver.
How To Distinguish Signal Quality Based On Eye Diagram Conditions
The signal cannot keep the voltage value of high and low levels exactly the same every time, nor can it be guaranteed that the rising and falling edges of high and low levels are at the same moment every time.
Due to the superposition of multiple signals, the signal line of the eye diagram becomes thicker and appears blurred (Blur).
So the eye diagram also reflects the noise and jitter of the signal: in the vertical axis voltage axis, embodied in the voltage noise (VoltageNoise); in the horizontal axis time axis, embodied in the time domain of the jitter (Jitter). This is illustrated in the figure below.
When noise is present, the noise will be superimposed on the signal and the observed line of the eye diagram will become blurred.
If inter-code crosstalk is also present, the “eyes” will open even smaller.
Generally, the wider the eye of the eye diagram, the higher the eye height of the eye diagram, the better the signal quality.
Signal simulation can be done to obtain the eye diagram, and then based on the eye diagram to determine the quality of the signal, if the eye diagram is not good, you can adjust the hardware design or PCB design, so that the eye diagram of the eye height to become high, to ensure that the signal quality of the products produced.
