PCB transmission lines present signal integrity challenges, requiring thoughtful planning to create. Below, we will help you understand what they are, what they do, and their design guidelines. Let’s begin with their brief definition.
What is a Transmission Line in PCB?
A PCB transmission line consists of a high-speed signal conductor and its associated return path. The signal path is usually a trace, while the return path is typically a ground plane. A dielectric material separates the two conductors.
The trace is specially designed for high-speed signals, with controlled impedance, width, and length. Controlling the parameters prevents signal distortion or loss by guiding the waves that the signals form.
Many circuit boards today include communication components and digital circuits. These features necessitate special attention to trace design. Let’s see why below, including the properties that define such traces.
When PCB Trace Becomes a Transmission Line
A circuit board signal trace becomes a transmission line if it acts as a waveguide. That means it must carry high-speed or high-frequency signals. These are signals with a rise time of 5 nanoseconds or less, or when the trace length exceeds 1/10th of the signal’s wavelength.
At higher frequencies, signals travelling in a conductor are not just currents. They create waves, making the PCB traces more challenging to design. Longer trace lengths also contribute to the problem.
Properties that characterize these conductors include resistance, capacitance, and inductance. These properties, in turn, are influenced by various factors, such as trace geometry and the base material or dielectric.
Resource: https://www.researchgate.net
Types of PCB Transmission Lines
There are 3 types of transmission lines for printed circuit boards: microstrip, stripline, and coplanar waveguide. They mainly differ in their configuration, or in the arrangement of the trace and the return path.
Microstrip
A microstrip is a signal trace paired with a single ground plane or return path. Most microstrips are traces on the PCB’s outer layer. That said, it can also be a trace in the inner layers.
The microstrip transmission line is easier to place. This property makes it common in simpler circuit board layouts. However, it offers limited protection, making it less ideal for high-frequency applications.
Stripline
A stripline is a signal trace sandwiched between two reference planes. Based on the arrangement, most striplines are traces embedded in the substrate.
A stripline offers better EMI shielding since it sits between two reference/ground planes. The configuration makes it suitable for highly sensitive circuits.
Coplanar Waveguide
The coplanar waveguide is a less common transmission line. It has the trace and return path on the same layer, with the trace in the middle of the two return paths and the dielectric material beneath.
Coplanar transmission lines, although less popular, are ideal for designs with space constraints. They are also well-suited for most high-frequency circuits.
Applications of PCB Transmission Lines
Signal transmission lines on PCBs operate at high frequencies. They also support faster data transfer rates. The following are their key application areas on a printed circuit board.
RF and Microwave Circuits
High-frequency signals make circuit board interconnects act as transmission lines. They are standard on radio-frequency (RF) boards, as well as those using shorter wavelengths in the microwave range.
High-frequency applications in electronics are diverse. Examples include communication technologies like radar, Wi-Fi, and cellular or mobile phones.
High Speed Digital Connections
High-speed digital interfaces, such as those of processors and memories, also create transmission lines. Most modern circuit boards are this type, making the digital signal paths more common.
High-speed signals are seen in PCBs of computers and other systems with data lines. Connections include USB, DDR, and similar interfaces or circuits.
Signal Quality Issues Related to Transmission Lines
Transmission line problems cause signal quality issues. They often result from wrong material choices, poor board layout, and improper terminations. Transmission line effects include the following.
Attenuation
Attenuation is the reduction in signal strength caused by resistance, skin effects, and absorption by the PCB dielectric. It can lead to data errors. The problem is more pronounced at higher frequencies.
Signal Reflections and Ringing
Signal reflections result from impedance mismatches, causing ringing, undershoots, or overshoots. Causes include impedance and ground issues that result in logic errors and EMI in high-speed circuits.
Signal Interference
Signal interference occurs when signal paths are too close together, causing coupling and crosstalk. It can also result from external electromagnetic waves hitting signal lines and causing distortion.
Resource: https://electronics.stackexchange.com
PCB Transmission Line Design Guidelines
The importance of transmission lines in PCBs cannot be overstated. They ensure proper signal transfer between the transmitter and the receiver when designed correctly. Here are the rules circuit board designers must adhere to.
1. Choose low-loss dielectrics when designing a circuit board with signal traces. It improves signal quality, especially at higher frequencies.
2. Place the traces directly next to their return paths. It protects the signals from ground issues, ensuring signal integrity.
3. Ensure a specified and uniform width along the trace to control or match impedance. Controlled-impedance lines experience fewer reflections, thereby improving signal quality.
4. Route the signal paths on solid ground planes. Gaps disrupt the return path, leading to unwanted signal issues.
5. Use ground vias (stitching vias) for layer transitions. Placing them next to signal paths reduces quality issues, such as noise and even loss.
6. Use correct terminations to prevent reflections and other problems that affect signal quality: series termination at the driver and parallel termination at the receiver.
7. Keep differential pairs close together. Use the 3W rule to maintain sufficient spacing between the pair and adjacent signals to prevent crosstalk.
8. Use advanced design programs to create the PCB and its transmission lines. They have the required tools, such as impedance calculators, to make the entire process smooth and more accurate.
Conclusion
As circuit boards become more advanced and high-speed, their traces transform into more than simple conductors. They act as waveguides and require more effort to design. Well-designed paths minimize signal attenuation and interference.
