In the case of PCB routing, it is often the case that when a route passes through a certain area, the area has a limited space and has to use finer lines. After passing through this area, the lines are restored to their original width. The change in trace width can cause impedance changes, so reflections occur and have an effect on the signal. Then what circumstances can ignore this effect, and under what circumstances we must consider its impact?

There are three factors related to this effect: the magnitude of the impedance change, the rise time of the signal, and the delay of the signal on the narrow line.

First discuss the size of the impedance change. Many circuits are designed to reflect noise less than 5% of the voltage swing (which is related to the noise budget on the signal), according to the reflection coefficient formula:

ρ=(Z2-Z1)/(Z2+Z1) =△Z /(△Z+2Z1)≤5%

Can calculate the impedance of the rate of change required: △Z/Z1≤10%

You may know that the typical impedance of the circuit board is +/- 10%, the root cause is here.

If the impedance change occurs only once, for example, the line width changes from 8mil to 6mil, it keeps the 6mil width. To achieve the noise requirement of the mutated signal does not exceed 5% of the voltage swing, the impedance change must be less than 10%. This is sometimes difficult to do, to FR4 sheet on the microstrip line as an example, we calculate it. If the line width is 8mil, the thickness between the line and the reference plane is 4mil and the characteristic impedance is 46.5 ohms. Line impedance changes to 6mil after the characteristic impedance becomes 54.2 ohms, the impedance change rate reached 20%. The amplitude of the reflected signal must be exceeded. As for the impact on the signal, but also the signal rise time and the drive to the reflection point of the signal delay. But at least it is a potential problem. Fortunately, this time through the impedance matching termination to solve the problem.

If the impedance change occurs twice, for example, the line width changes from 8mil to 6mil, pulls out 2cm and then changes back to 8mil. Then in the 2cm long 6mil wide lines at both ends of the reflection will occur, once the impedance becomes larger Reflection, then the impedance becomes smaller, negative reflection occurs. If the two reflex intervals are short enough, the two reflections are likely to cancel each other, thereby reducing the impact. Assuming that the transmission signal is 1V, the first positive reflection is reflected by 0.2V, 1.2V continues to move forward, the second reflection has -0.2 * 1.2 = 0.24v is reflected back. Assuming that the 6mil line length is very short, the two reflections occur almost simultaneously, then the total reflection voltage is only 0.04V, less than 5% of this noise budget requirement. Therefore, whether this reflection affects the signal, how much impact, and impedance changes at the delay and signal rise time. Research and experiments show that the reflected signal will not cause problems as long as the delay at the impedance change is less than 20% of the signal rise time. If the signal rise time is 1ns, then the delay at the impedance change is less than 0.2ns corresponds to 1.2 inches, the reflection will not cause problems. In other words, for the case of this case, 6mil wide line length as long as less than 3cm will not be a problem.

When the PCB line width changes, according to the actual situation carefully analyzed, whether the impact. Need to focus on the parameters from the three: how much change in impedance, signal rise time is how much, line width changes in the neck-shaped part of how long. According to the above method roughly estimated, the appropriate set aside a margin. If possible, try to reduce the neck length.

It should be noted that the actual PCB processing, the parameters can not be as accurate as the theory, the theory can provide guidance for our design, but can not copy the copy, can not dogma, after all, this is a practice of science. The estimated value should be revised according to the actual situation, and then applied to the design.