Whenever you consider transmission lines, the first thing you will think of is cables. Although this isn’t our focus in this article, the PCB transmission lines act in a similar way. While the regular cables will help transfer electricity between two points, the PCB variants help in transmitting signals from and to the transmitter and receiver.
However if you are not familiar with the way it works, the integration of transmission lines into the PCB layout could be a challenging process. However, we will be providing you with all the important information before you make attempts to add then into your high-speed designs. We will be discussing the types of PCB transmission lines, their significance, and more.
What Do PCB Transmission Lines Mean?
The PCB transmission line is made up of linkages useful for transmitting and receiving signals from the board.
Also, they are made of two conductors, signal trace, as well as a return path. Dielectric material, such as PP, Coverlay, and fiberglass, make up this thickness that separates these conductors.
Furthermore, PCB transmission lines carry all the I/O signals. Signals are transferred from one location to another using a reference plane and signal trace within the single ended input/output connections. However differential I/O combines two traces using the reference plane to create the transmission lines.
Which are transmission lines and which ones are not?
Transmission lines won’t be necessary if the frequency of a signal on the electrical wires remains low enough to denote that the wire’s voltage at any specific time is constant over the whole length of the wire.
However, transmission lines should be used in this situation instead of just the usual connection when the traveling signal’s frequency is very high and you have a very long PCB trace length.
Whenever the interconnect is no less than 25 percent of the length of the signal’s wavelength passing through, you can refer to it as a transmission line.
Impedance-Controlled Transmission line
A transmission line‘s alternating current features a frequency high enough to exhibit characteristics of wave propagation. The main factor affecting how electrical signals travel down transmission lines is their impedance, which exists along their entire length assuming the line maintains a constant shape.
This line is referred to as controlled impedance lines. Distortions and signal reflections occur due to non-uniform impedance. To forecast the signal’s behavior at high frequencies, the transmission lines must have a regulated impedance.
To prevent any crosstalk, signal reflections, electromagnetic noise, and other problems which may affect the signal’s quality negatively and lead to mistakes, it is very important to consider the impacts of the transmission line.
While it isn’t necessary for the signals to feature a reference plane, it is required for the actual implementation of the differential traces on the printed circuit boards. Transmission lines having regulated and controlled impedance are essential for maintaining great signal integrity within a printed circuit board system. The reference planes, the signal traces, as well as the shape of these traces all affects the impedance.
The presence and lack of glass within a specific lamination weave may lead to impedance variation. However this hardly causes problems outside the high-speed connections.
What Kinds of Transmission Lines Are There?
Microstrips and striplines are the two main signal transmission lines that you can find on printed circuit boards. On the topmost layer of the board, these microstrip transmission lines are made up of a single uniform trace. Also, the trace usually runs parallel to the PCB’s ground plane.
What else? The traveling signal has a return route, which is provided by the ground plane. To prevent any contact between the ground and trace plane, a specific height of dielectric material would be helpful.
Contrary to this, stripline PCB transmission lines feature a single uniform trace within the inner layer of the circuit board. Additionally, parallel dielectric layers and conducting planes give a transmitted signal two return pathway. This separates the trace on either side.
Asides from the usual types, you can make use of a third category on your printed circuit board. However it’s quite uncommon. This return path and the signals trace are located on the layer of the board.
However, the signal trace, stays at the centre, with two neighboring ground planes also surrounding this trace. Additionally, the structure of the transmission line is where you may discover the dielectric substance. Coplanar structures are also an option for striplines and microstrips.
Microstrip
It consists of one uniform trace (which is designed for the signal). Also, it is parallel to the conducting ground planes on the top layer of the PCB and serves as the signal’s return path. A particular height of the PCB dielectric separates the trace from the ground planes.
Stripline
On the innermost layer of the PCB, there is a consistent trace known as a stripline that carries the signal. The parallel circuit board dielectric layer and the conducting plane are used to divide the trace on each of the sides. As a result, the two reference planes are its two return routes or paths.
Coplanar
In the coplanar waveguide setup, this conductor for the return path is on a similar layer within the printed circuit board as a signal trace. The three flat surfaces are referred to as “coplanar” because they are all on a single plane, with a signal trace in the center and the two adjacent exterior ground planes surrounding it. Underneath is where the printed circuit board dielectric is situated. Coplanar structures are possible for both striplines and microstrips.
Considerations for Transmission Line Design
There are a few aspects that must be taken into account when joining traces on the printed circuit board design. Below are some pointers to assist you with this.
The transmission lines comprise groups of at least two ports that serve as the receiving and generating ends of power. This is why, the majority of transmission lines are required to have conductors. Three conductors are required if your lines are three-phase. Coaxial cables or common PCB traces can also be used as lines for transmission.
When the circuit size is tiny and the wavelength is short, transmission lines operate differently. Additionally, the loop inductance all through the length of the conductor and the capacitance in-between both conductors will also affect how they function.
Although you could believe that transmission cables pass signals across various locations, there are other factors to take into account. Although that is the primary purpose, we advise carefully planning the design of the transmission line. Otherwise, the functionality of the PCB will be hampered by flaws in your transmission line and trace designs.
Importance of Transmission Lines in PCB
The transmission line in PCB plays an important role in reducing serial link loss and other associated expenses. This holds true for all high-frequency circuits since they are more likely to experience dielectric losses and signal interruptions. Therefore, during the process of design, it is essential to thoroughly consider and finalize the modeling and format of the transmission line modeling.
Thankfully, with great improvements in fabrication technology, modeling of integrated circuits, as well as higher operating frequencies, digital circuit designs may now be used for microwave and radio frequency connections. To properly connect the traces on the circuit boards, it is crucial to take into account some factors relating to the transmission line. Here are some guidelines that can be useful in this situation:
The transmission line effects in PCB is made up of a network of at least two ports which features both the power production end as well as the receiving end. Three conductors must be used for three-phase lines. Transmission lines might be made of regular PCB traces or coaxial cables. Because high-quality conductor materials or a sizable surface area within these conductors lower the resistance, the resistance needed within a PCB and the application must be taken into account. This is why there is an inverse relationship between these two variables and resistance. This holds true for lengthy circuit routes and broad wavelengths.
Fortunately, better solutions are becoming accessible as transmission lines are taken into account for uses other than resistive as well as low frequency ones.
Short wavelengths as well as small circuit sizes cause the transmission line to behave very differently. The transmission line’s loop inductance throughout its length and the capacitance between both conductors at a specific length both play a role.
Conclusion
Hope you understand what pcb transmission lines are. Impedance is a very important aspect you must consider during the construction of a PCB transmission line. Otherwise, if this impedance fails to match the length of the line, signal reflections would stop the entire process. Therefore, it is advisable that you estimate the PCB design’s physical properties to manage the impedance of the trace lines. This then enables the system to keep a constant value.
Do you have any further inquiries on this article? Please reach out to us immediately; we’ll be pleased to assist you the best way we can.
