PCB Tech - Signal integrity research: what is signal integrity?

If you find that the design experience accumulated in the previous low-speed era does not seem to work anymore, the same design used to be no problem, but now it can’t work, then congratulations, you have encountered the most core problem in hardware design: signal integrity . It is good for you to meet one day earlier.

In the past low-speed era, the signal rise time during level transitions was relatively long, usually several ns. The interconnection lines between devices will not affect the function of the circuit, and there is no need to care about signal integrity issues. But in today's high-speed era, with the increase of IC output switching speed, many are at the picosecond level. Regardless of the signal period, almost all designs have encountered signal integrity problems. In addition, the pursuit of low power consumption makes the core voltage lower and lower, and the 1.2v core voltage is already very common. Therefore, the noise margin that the system can tolerate is getting smaller and smaller, which also makes the signal integrity problem more prominent.

Broadly speaking, signal integrity refers to all the problems caused by interconnection lines in circuit design. It mainly studies how the electrical characteristic parameters of interconnection lines interact with the voltage and current waveforms of digital signals to affect product performance. Mainly manifested in the impact on timing, signal ringing, signal reflection, near-end crosstalk, far-end crosstalk, switching noise, non-monotonicity, ground bounce, power bounce, attenuation, capacitive load, electromagnetic radiation, electromagnetic interference, etc.

The root of the signal integrity problem lies in the reduction of the signal rise time. Even if the wiring topology does not change, if an IC chip with a small signal rise time is used, the existing design will be in a critical state or stop working.

Let's talk about several common signal integrity problems.

reflection:

Figure 1 shows the waveform distortion caused by signal reflections. It looks like ringing. Take out the circuit board you made and measure various signals, such as clock output or high-speed data line output, to see if there is such a waveform. If so, then you should have a perceptual understanding of the signal integrity problem. Yes, this is a signal integrity problem.

Many hardware engineers will connect a small resistor in series to the clock output signal. As for why, many of them are not clear about it. They will say that there are many mature designs and follow them. Maybe you know, but it is true that many people can't tell the function of this small resistor, including many hardware engineers with three or four years of experience. Are they surprised? But this is indeed true. I have encountered many. In fact, the function of this small resistor is to solve the signal reflection problem. And as the resistance increases, the ringing will disappear, but you will find that the rising edge of the signal is no longer so steep. This solution is called impedance matching. Oh, yes, you must pay attention to impedance matching. Impedance occupies an extremely important position in signal integrity problems.

Crosstalk:

If you are careful enough, you will find that sometimes for a certain signal line, there is no output signal in terms of function, but when measuring, there will be a regular waveform with a small amplitude, just like there is a signal output. At this time, you measure the signal line adjacent to it to see if there is a similar pattern! Yes, if the two signal lines are close together, it usually will. This is crosstalk. Of course, the waveform on the signal line affected by crosstalk is not necessarily similar to the waveform of the neighboring signal, nor does it necessarily have obvious rules, and it is more in the form of noise. Crosstalk has always been a headache in today's high-density circuit boards. Due to the small wiring space, the signals must be very close, so you have to face it. It can only be controlled but cannot be eliminated. For signal lines subject to crosstalk, interference from adjacent signals is equivalent to noise to him.

The magnitude of crosstalk is related to many factors on the circuit board, not just because of the distance between the two signal lines. Of course, distance is the easiest to control and the most commonly used method to solve crosstalk, but it is not the only method. This is also something that many engineers misunderstand. For more in-depth discussions, I will continue to introduce them in subsequent articles.

Track collapse:

Noise not only exists in the signal network, but also in the power distribution system. We know that there is inevitably an impedance in the path of current flowing between the power supply and the ground, unless you can make everything on the circuit board a superconductor. Then, when the current changes, a voltage drop will inevitably occur. Therefore, the voltage that is actually sent to the chip's power supply pins will decrease, sometimes greatly, just like the voltage suddenly collapses, which is the rail collapse. Track collapse can sometimes cause fatal problems, which may affect the function of your circuit board. The number of integrated gates of high-performance processors is increasing, the switching speed is getting faster and faster, more switching current is consumed in a shorter time, and the noise that can be tolerated becomes smaller and smaller. But at the same time, it is more and more difficult to control noise. Because of the demanding requirements of high-performance processors on the power supply system, it is becoming more and more difficult to build a lower impedance power distribution system. You may have noticed that it is impedance again. Understanding impedance is the key to understanding signal integrity issues.

Signal integrity issues involve a wide range of aspects, here is just a brief introduction to a few phenomena, I hope this article will give you a preliminary understanding of signal integrity. Signal integrity will be a required course for every hardware engineer. Contact one day earlier and benefit one day earlier.