Understanding PCB Transmission Lines: Design, Function, and Applications

Mar 12,2020

A regular transmission line is a cable of sorts used to transfer electricity from one point to the other or across multiple points, say from a substation to various distribution units in the area. So, it basically transmits electrical signals. On the same lines, a PCB transmission line is an RF interconnect used transfer signals to and from the transmitters and receivers on a printed circuit board. It comprises two conductors, wherein one is to trace the signal and the other to trace the return path. RF interconnects or PCB transmitters behave a lot differently than the regular electricity transmission line. This post focuses on the details of PCB transmission lines.

Significance of Transmission Lines in PCBs

Transmission lines are very important with regards to preventing serial link losses and the subsequent costs. This is especially true for high-frequency circuits as the likelihood of dielectric losses and signal disruptions is more in these cases. So, the transmission line format and modeling has to be carefully studied and finalized in the design phase. Fortunately, owing to the technological advancements in fabrication, integrated circuit modeling, and increased operating frequencies have enabled digital circuit designs to be implemented in RF and microwave connections. Here, it is important to consider certain transmission line parameters for accurately connecting traces in circuit boards. Here are a few pointers which may be helpful in this regard:

• A transmission line is basically a network of two or more ports with a power generation end and a receiving end. Hence, most transmission lines have two conductors at least.
• In case of three-phase lines, three conductors are required. Normal coaxial cables or PCB traces could be used as transmission lines.
• The resistance required in a PCB and its application needs to be considered because good conductor materials and large surface area on these conductors reduce the resistance. So, these two factors are inversely proportional to resistance. This is applicable for large wavelengths and long circuit paths. Fortunately, better options are available now with transmission lines being considered beyond just resistance and low frequency applications.
• In case of short wavelengths and small circuit sizes, the transmission line behaves quite differently. Capacitance between two conductors at a given length and loop inductance over the conductor length in the transmission line also have their influence.
• Resistance and conductance are applicable too, but they are resultant parameters. This is because conductance is the value derived based on dielectric losses between the conductors. Resistance is the result of physical properties of the materials used in conductors and the surface area available or the physical dimensions on a circuit board.
• Based all these characteristics such as conductance, capacitance, and so on, and their impact on high frequency signals, the behavior of the transmission line is decided.
• One may think that these lines simply transfer signals from one point to another. Although that is the core function of transmission lines, the design should be carefully done. Any loopholes in designing the traces and transmission lines will directly impact the functionality of the PCB.
• Using advanced fabrication techniques is key to a flawless design in case of high frequency applications and short traces being used as transmission lines. It is also crucial to use good quality dielectric materials. Also, with components and in general PCB sizes shrinking by the day, good fabrication and mounting techniques allow easy and correct placement of components on the board, and they can be accommodated in a small area.

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