Signal reflection may cause ringing. A typical signal ringing is shown in Figure 1.

So how does the signal ringing occur?

As mentioned earlier, if a change in impedance is felt during signal transmission, signal reflection will occur. This signal may be the signal sent by the driver, or it may be the reflected signal reflected from the far end. According to the reflection coefficient formula, when the signal feels that the impedance becomes smaller, negative reflection will occur, and the reflected negative voltage will cause the signal to undershoot. The signal is reflected multiple times between the driver and the remote load, and the result is signal ringing. The output impedance of most chips is very low. If the output impedance is less than the characteristic impedance of the PCB trace, signal ringing will inevitably occur if there is no source termination.

The process of signal ringing can be intuitively explained by the bounce diagram. Assuming that the output impedance of the drive end is 10 ohms, and the characteristic impedance of the PCB trace is 50 ohms (can be adjusted by changing the width of the PCB trace, the thickness of the dielectric between the PCB trace and the inner reference plane), for the convenience of analysis, suppose the remote end is open , That is, the far-end impedance is infinite. The drive end transmits a 3.3V voltage signal. Let’s follow the signal and run through this transmission line once to see what happened. For the convenience of analysis, the influence of parasitic capacitance and parasitic inductance of the transmission line is ignored, and only resistive loads are considered. Figure 2 is a schematic diagram of reflection.

The first reflection: the signal is sent out from the chip, after 10 ohm output impedance and 50 ohm PCB characteristic impedance, the signal actually added to the PCB trace is the voltage at point A 3.3*50/(10+50)=2.75 V. Transmission to the remote point B, because point B is open, the impedance is infinite, and the reflection coefficient is 1, that is, all the signals are reflected, and the reflected signal is also 2.75V. At this time, the measured voltage at point B is 2.75+2.75=5.5V.

Second reflection: the 2.75V reflected voltage returns to point A, the impedance changes from 50 ohms to 10 ohms, negative reflection occurs, the reflected voltage at point A is -1.83V, the voltage reaches point B, and the reflection occurs again, and the reflected voltage is -1.83 V. At this time, the measured voltage at point B is 5.5-1.83-1.83=1.84V.

The third reflection: The -1.83V voltage reflected from point B reaches point A, and negative reflection occurs again, and the reflected voltage is 1.22V. When the voltage reaches point B, regular reflection occurs again, and the reflected voltage is 1.22V. At this time, the measured voltage at point B is 1.84+1.22+1.22=4.28V.

In this cycle, the reflected voltage bounces back and forth between point A and point B, causing the voltage at point B to be unstable. Observe the voltage at point B: 5.5V->1.84V->4.28V->……, it can be seen that the voltage at point B will fluctuate up and down, which is the signal ringing.

How does the signal ringing in the PCB circuit occur?

The root cause of signal ringing is caused by negative reflection, and the culprit is still impedance change, which is again impedance! When studying signal integrity issues, always pay attention to impedance issues.

The signal ringing at the load end will seriously interfere with the signal reception and cause logic errors, which must be reduced or eliminated. Therefore, impedance matching terminations must be performed for long transmission lines.