writer: G
May 04, 2018

Right-angle alignment is generally a situation that PCB wiring needs to avoid as much as possible, and it is almost one of the standards to measure the quality of the wiring. How much influence does the right-angle wiring have on signal transmission? In principle, a right-angled trace will cause the linewidth of the transmission line to change, resulting in an impedance discontinuity. In fact, not only the right-angle traces, but also the angles of sharp angles and acute angles may cause impedance changes. The influence of the right-angle trace on the signal is mainly reflected in three aspects: First, the corner can be equivalent to the capacitive load on the transmission line, slowing the rise time; secondly, the impedance discontinuity will cause the reflection of the signal; and the third is the right-angle tip. EMI.

The parasitic capacitance due to the right angle of the transmission line can be calculated by the following empirical formula: C=61W(Er)1/2/Z0 In the above equation, C is the equivalent capacitance of the corner (unit: pF). The width of the line (unit: inch), εr refers to the dielectric constant of the medium, and Z0 is the characteristic impedance of the transmission line. For example, for a 4-Mils 50-ohm transmission line (<εr is 4.3), a right-angle brings about 0.0101 pF of capacitance, and the resulting rise time variation can be estimated: T10-90%=2.2 *C*Z0/2 = 2.2*0.0101*50/2 = 0.556ps

It can be seen from the calculation that the capacitive effect brought by the right-angle trace is extremely small. As the line width of the right angle trace increases, the impedance at this point will decrease, and a certain signal reflection phenomenon will occur. We can calculate the equivalent impedance after the line width is increased according to the impedance calculation formula mentioned in the transmission line section, and then The reflection coefficient is calculated according to an empirical formula: ρ=(Zs-Z0)/(Zs+Z0). Generally, the impedance change caused by a right-angled trace is between 7% and 20%, and thus the maximum reflection coefficient is about 0.1. Furthermore, it can be seen that the impedance of the transmission line changes to the minimum during the length of the W/2 line, and then returns to the normal impedance after the W/2 time, and the entire impedance change takes a very short time, often within 10 ps. Fast and minor changes are almost negligible for general signal transmission.

Many people have such an understanding of right-angle alignment that the tip is easy to emit or receive electromagnetic waves, resulting in EMI, which has become one of the reasons why many people think that it is not possible to line at right angles. However, the results of many practical tests show that the right-angle traces do not produce significant EMI compared to straight lines. Perhaps the current instrument performance and test level limit the accuracy of the test, but it at least illustrates a problem that the radiation of the right-angle trace is less than the measurement error of the instrument itself.

In general, right-angle alignments are not as frightening as they are. At least in applications below GHz, any effects such as capacitance, reflection, and EMI that are produced in the TDR test can hardly be reflected. High-speed PCB design engineers should focus on layout, power/ground design, and trace design. Other aspects such as vias. Of course, although the influence brought by the right-angle alignment is not very serious, it does not mean that we can all follow the right-angled line. Attention to detail is an essential quality for every outstanding engineer. Moreover, with the rapid development of digital circuits, PCB The frequency of signals processed by engineers will also continue to increase, and in the area of RF design above 10 GHz, these small right angles may become the focus of high-speed problems.