In PCB layout design and wiring, it is required to avoid the situation of right angle wiring as far as possible, which has become one of the standards to measure the quality of wiring, so how much impact will the right angle wiring have on signal transmission?
In principle, the right angle wiring designed by layout will change the line width of transmission line and cause discontinuity of impedance.In fact, not only right angle wiring, but also sharp angle wiring may cause impedance changes.
The influence of right angle wiring on signal is mainly reflected in three aspects
Firstly, the corner can be equivalent to the capacitive load on the transmission line to slow down the rise time;
Second, the impedance discontinuity will cause the signal reflection;
The third is EMI produced by right angle tip
The parasitic capacitance caused by the right angle of the transmission line can be calculated by the following empirical formula: C = 61W (ER) 1 / 2 / Z0, where C is the equivalent capacitance of the corner (unit: PF), W is the width of the line (unit: inch),εR is the dielectric constant of the medium and Z0 is the characteristic impedance of the transmission line.
As the line width of PCB design right angle wiring increases, the impedance at this point will decrease, so there will be some signal reflection phenomenon. We can calculate the equivalent impedance after the line width is increased according to the impedance calculation formula mentioned in the transmission line chapter, and then calculate the reflection coefficient according to the empirical formulaρ=(zs-z0) / (ZS + Z0), the impedance change caused by right angle wiring is 7% – 20%, so the maximum reflection coefficient is about 0.1.