![]() ![]() There is another transmission line configuration where instead of a reference plane, an external signal trace has its return path facilitated by two ground traces on either side of it. As you can see below, a stripline offers alternative configurations, such as asymmetric and broadside-coupled, that a microstrip cannot.Ī diagram of the different types of transmission lines in a PCB layout Stripline: When the routed trace is between two reference planes for return paths, it is referred to as a stripline configuration.Typically, a microstrip configuration is used on exterior layers of the board, but it can also be embedded in an internal layer, as shown in the image below. Microstrip: A routed trace with a single adjacent reference plane for its return path is considered a microstrip configuration. ![]() There are two general categories of transmission lines that are used in the majority of PCB layouts: This ensures the line has the same impedance throughout its entire routed length and is referred to as controlled impedance routing. Designers determine the width of the trace by carefully calculating the copper weight of the trace with the dielectric material and the depth separating the two conductors. The two conductors are separated by dielectric material, and the usual construction is traces on a signal layer with an adjacent ground plane. Transmission lines in printed circuit board layouts are created using two conductors: a trace for the signal and its return path on a reference plane, which is usually ground. Traces on a high-speed design being routed over a ground plane layer Understanding the Types of Transmission Lines in PCBs Next, we’ll look at what a transmission line is in a circuit board layout. These conditions can lead to reflections, EMI, crosstalk, and a host of other signal integrity issues. The fast rise and fall times of the signal’s wave can create huge problems if the transmission line effects of the trace are ignored. The safe bet is that if you are working with a high-speed signal with a defined impedance value, you should always design a transmission line to the impedance requirements and not worry about the length of the trace. With digital signals having frequency components that vary, there isn’t just one wavelength to work with. Wavelength is also sometimes used as criteria for determining a trace's change in behavior, but signal integrity experts don’t always agree on how this should be calculated. ![]() When the signal’s rise time multiplied by its velocity roughly equals the length of the trace, then the trace will be prone to behaving like a transmission line. Trying to deduce when a trace stops acting like a simple conductor and starts behaving like a transmission line is not a simple task because there isn’t a set trace length that causes this change. In high-speed design, most long traces should be considered transmission lines. The delay of the high-speed signal traveling through the line and back is longer than its waveform rise and fall times, creating the potential for signal reflections. However, higher signal speeds coupled with longer traces will change the behavior of the trace into a transmission line. This is because short traces operating at lower frequencies do not create the conditions that cause signal integrity problems. PCB layout designers have routed traces for a long time without worrying about signal integrity issues with the lines. Transmission Lines in Printed Circuit Boards To do this, designers need to understand the types of transmission lines they will be working with and what layout techniques are best for their design. Instead, the priority is creating the most optimum trace routing for maximum signal performance and eliminating as many signal integrity problems as possible. No longer can traces be routed purely for aesthetics or cost reduction. Successful high-speed layout requires designers to understand the principles and techniques of working with high-speed transmission lines on their boards. At one time, high-speed signals were an occasional design challenge, but now they are becoming a normal part of the design process in PCB layout. To support these enhancements, circuit board designers are working with faster and faster signal speeds every day. The demand for the next best thing in electronics continually pushes circuit board technology to higher capabilities and performance levels. High-Speed PCB routing usually involves transmission line configurations Using your PCB CAD tools to help with the design of transmission lines. Understanding transmission lines and their different types in high-speed printed circuit boards. ![]()
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