It is often a popular saying in the industry that “there are two kinds of designers, one that has encountered signal integrity issues and another that is about to encounter signal integrity issues.” SSD as a highly integrated high clock frequency hardware devices, the importance of signal integrity is self-evident. With this statement this article mainly to talk to you about some basic content of signal integrity.
What is signal integrity? In layman’s terms, the signal in the transmission process of the interconnect line, the interconnection line and other factors will be subject to the interaction of the signal waveform distortion of a phenomenon, when the signal can be said to be damaged in the transmission, become “incomplete”. Signal integrity does not have a unique normative definition, in a broad sense, refers to the signal in high-speed products caused by the interconnects all the problems.
Signal integrity plays an important role in high-speed digital systems. If signal integrity is faulty, it may cause the circuit to not work properly. The key electrical characteristic that affects signal integrity is the impedance of the interconnects, which is at the heart of the approach to signal integrity.
1.impedance
The impedance played by the current in the circuit is called impedance. The unit of impedance is the ohm, often expressed in Z. The expression is a complex number:
where the real part is the resistance and the imaginary part represents the reactance (capacitive and inductive). Why use complex numbers? Resistance represents the attenuation of the signal amplitude and reactance represents the change of the signal phase. The following are the impedances of resistor, capacitor and inductor respectively.
1.1 Characteristic resistance
Characteristic resistance is a concept related to the transmission line, the actual transmission process of the signal on the transmission line, will be affected by the parasitic parameters on the transmission line (such as parasitic inductance, parasitic resistance, parasitic capacitance), the characteristic impedance is a comprehensive transmission line scenario with these parasitic parameters synthesized impedance.
The following model is used to represent the unit length of the transmission line.
The impedance expression under this model is
In the actual PCB application the resistance part of the transmission line can be ignored, i.e., R and G in the above equation is 0. The general expression for the characteristic impedance of the PCB transmission line is
L is the inherent inductance per unit length of the transmission line, C is the inherent capacitance per unit length of the transmission line
Transmission line impedance in the PCB industry is usually referred to as the characteristic impedance of the transmission line impedance.
1.2 Impedance matching
Signal transmission on the transmission line will be subject to the transmission line impedance, any sudden change in impedance will cause signal reflection and distortion, excessive reflection and distortion will cause signal integrity problems. Impedance matching refers to the signal source or transmission line and the load to achieve a suitable match. Therefore, impedance matching is very important for a high-speed digital system. Impedance matching has two main functions, one is to adjust the load power, and the other is to suppress signal reflection.
It can be analogous to the flow of water in a pipe to signal transmission on a transmission line. The inconsistent thickness of the pipe will lead to the loss of kinetic energy of the water at the joints, and similarly, the mismatch of transmission line impedance will cause a serious waste of circuit energy.
2.Reflection
In the above mentioned the signal reflection due to impedance mismatch, in order to explain the signal reflection we can use light propagation in different media to image the introduction, light from the air to the surface of the water or glass, part of the light is reflected, another part of the light will be refracted into another medium, the following figure.
Similarly, the signal is the same, if the impedance of the transmission line is not consistent, where the impedance jumps, part of the energy will continue to be transmitted and part of the energy will be reflected back, as follows:
The expression of the reflection coefficient is
Reflection causes signal overshoot (Overshoot), ringing (Ringing), and edge retardation (hook-back phenomenon). Overshoot is the undamped state of ringing, and edge retardation is the over-damped state of ringing. The following figure shows the three manifestations of signal reflection.
2.1 Overshoot and downshoot
Overshoot means the first peak or valley value exceeds the set voltage value, and downshoot means the next valley or peak value. For rising edge, overshoot means the highest voltage; for falling edge, overshoot means the lowest voltage. As shown in the figure below.
Overshoot can cause the protection diode to work severely, leading to premature failure and, in severe cases, damage to the device. And excessive down-shoot can cause false clock or data errors, which may bring potential cumulative damage to the device and shorten its working life, thus affecting the long-term stability of the product. Generally the impedance of the transmitter side of the signal is low and the impedance of the receiver side of the signal is high. If the impedance of the transmitter side and the receiver side do not match, the transmitted signal will be reflected back and forth between the transmitter side and the receiver side, thus resulting in overshoot and undershoot of the signal reflection.
The general method for resolving overshoot is matching, or terminating ( TerminaTIon). The central idea of matching is to eliminate impedance glitches at the endpoints of the signal path.
The following figure shows a comparison of waveform plots without and with terminating resistors.
2.2 Ringing
The above subsection introduces overshoot and undershoot. If overshoot and undershoot are repeated, ringing will occur, and overshoot is often accompanied by ringing, or rather, overshoot is part of ringing. The first peak voltage generated by ringing is the overshoot. The reason why overshoot and ringing are separated is that their hazards are different. In addition to the hazards of overshoot, ringing also has its fluctuations that may exceed the threshold judgment voltage many times causing misjudgment, and will sharply increase power consumption, affecting device life. The following chart is the cause of ringing.
The following is the manifestation of the ringing waveform.
The root cause of the ringing phenomenon is caused by signal reflection, the essence of which is still impedance mismatch, so the solution to reduce or eliminate ringing is no different from dealing with overshoot and undershoot, which must be terminated with impedance matching.
In the actual application scenario, a variety of signal integrity problems will be encountered, typical problems are as follows: reflections, crosstalk, power/ground noise, timing, etc.. Among them, emission and crosstalk are the two main causes of signal integrity problems.
3.Crosstalk
In the above we introduced the signal reflection is due to the impedance of the transmission line caused by sudden changes, and crosstalk is the signal line between the mutual inductance and mutual capacitance caused by noise, is caused by two signal lines on the same PCB board and the ground plane, the principle is that at high frequencies between any two devices or wires on the PCB there is mutual capacitance and mutual inductance, when a device or a signal line on the signal changes, the change will be through mutual capacitance and mutual inductance coupled to other devices or signal lines, that is, crosstalk coupling. When the coupled signal or crosstalk signal is large enough, signal integrity problems occur on the signal line receiving the crosstalk signal.
A signal affected by the crosstalk of the interfering signal will be deformed, allowing its eye diagram to close. In engineering we want to be able to open the eye diagram as much as possible, because this way there is enough margin to ensure error-free transmission of data, on the contrary, if the eye diagram is closed will make the margin become small and the result is wrong.
There are many reasons for crosstalk, such as the length of the PCB wiring, spacing, lamination, the condition of the reference ground plane, termination methods, electrical characteristics of the driver/receiver, and other factors. The solution is also mainly for these factors mentioned above to deal with.
4.Signal Integrity Problem Solving Method
Before introducing how to solve signal integrity problems, first review the factors that cause signal integrity, the basic factors are impedance mismatch, then crosstalk, power integrity, timing, etc.. In the actual signal integrity analysis, the root cause of signal integrity needs to be described, for example
a.Signal rising edge is too short
b. Impedance matching is not reasonable
c. PCB structure design is not reasonable
d. Power integrity problems
… …
The following is a brief summary of common signal integrity problems, and lists the causes of the problem and the corresponding solutions.
