PCB layout design layout is good or bad will directly affect the performance of the whole system, most of the high speed design theory to after the final layout can be realized and verified. Therefore, the wiring in high speed PCB design is crucial. The following will be aimed at the actual wiring may be encountered in some cases, analysis of its rationality, and give some of the more optimal route of the line.

　　The main difference from the right angle line, walk the line, three aspects to elaborate the serpentine.

　　1 right angle walk line

　　Rectangular wire is generally PCB wiring requirements as far as possible to avoid, almost become gauge wiring standards of good and bad one, then rectangular wire what will have much impact on signal transmission. In principle, the right angle alignment will change the width of the transmission line, impedance discontinuities. In fact, not only is the right angle line, a line angle, acute angle may cause the change of impedance.

　　The influence of the right angle to the signal is mainly reflected in three aspects:

　　The corner can be equivalent to the transmission line of capacitive load, slow down the rise time;

　　Two is the impedance discontinuity can cause signal reflection;

　　Three is the right angled tip of the EMI.

　　The parasitic capacitance of the right angle of the transmission line can be calculated by the following formula:

　　C=61W (Er) 1/2/Z0

　　In the type and C refers to the corner of the equivalent capacitance (unit: PF) and W wire width (unit: inch), epsilon r refers to the dielectric constant of the medium, the characteristic impedance Z0 is transmission line. For example, for a 4Mils 50 ohm transmission line (epsilon R is 4.3), caused by a right angle electric capacity about 0.0101pF, and can estimate the resulting rise time variation:

　　T10-90%=2.2*C*Z0/2 = 2.2*0.0101*50/2 = 0.556ps

　　It can be seen from the calculation that the capacitance effect caused by the right angle is very small.

　　Due to the right angle go line width increases, the impedance will decrease, so will produce a certain signal reflection phenomenon, we can according to the mentioned in the chapter on transmission line impedance calculation formula to calculate the linewidth increases the equivalent impedance, then according to the empirical formula calculated reflection coefficient:

　　P = (Zs-Z0) / (Zs+Z0)

　　The impedance variation caused by the common right angle walk line is between 7%-20%, and the reflection coefficient is about 0.1. And from below, you can see that in time the inward W/2 length of transmission line impedance changes to a minimum, then after a period of W/2 returned to normal impedance, the occurrence of impedance change of the time is very short, often within 10ps so quickly and small changes of general signal pass transmission is almost negligible of.

　　Many people have this understanding of the right angle, think the tip is easy to transmit or receive electromagnetic waves, resulting in EMI, which has become one of the reasons many people think that can not be a right angle to the line. However, many of the actual test results show that the right angle to the line does not produce a very obvious than straight line EMI. Perhaps the current instrument performance, the test level has restricted the accuracy of the test, but at least a problem, the right angle of the radiation has been less than the measurement error of the instrument itself.

　　Generally speaking, the right angles to the line is not imagined so terrible. At least in the application below 3 GHz, which produces any such as capacitors, reflection, EMI effect in TDR test is almost not manifest, the focus of high speed PCB design engineer should be put in the layout, the power / ground design, design line, holes and other aspects. , of course, although walk the right angle line bring influence is not very serious, but is not to say that we can go right angled line. Attention to detail is outstanding engineers from each essential basic qualities, and with the rapid development of digital circuit, PCB engineer to deal with the signal frequency will continue to improve, to above 10GHz RF design field, these small rectangular may become the focus of high-speed problems.