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Signal Reflection and Distortion in PCBs

Key Takeaways

  • PCB signals undergo signal integrity issues such as signal reflections, signal distortions, crosstalk, coupling, and ground bounce.

  • Signal reflections result from impedance mismatches and discontinuities. 

  • Signal distortions in the form of signal losses are common in long PCB traces.

 Signal distortion in a PCB

Signal distortion in a PCB is a major signal integrity issue

In this era of high-speed electronics, delivering information securely and reliably is not too difficult. A quick click on your computer or smartphone can send critical information from one end of the world to another in mere seconds. However, certain critical flaws in the internal PCBs of computers and phones can cause signal distortions, leading to communication failures. Signal distortion in PCBs is a major signal integrity issue that makes electronic gadgets unreliable.

Signal Integrity Issues in PCBs

Advancements in technology have allowed designers to develop miniaturized and high-speed electronics. PCB signals undergo signal integrity issues such as signal reflections, signal distortions, crosstalk, coupling, and ground bounce.

Let's dive deeper into two types of signal integrity issues–signal reflection and signal distortion. 

Signal Reflection

High power density and high-speed, miniaturized PCBs increase the routing density of circuits. With increased routing density and frequency, the edge turnover time of the signal decreases. When the interconnection delay is 1.1 times or greater than the signal turnover time, the signal experiences distortion in the form of reflection and crosstalk. 

When the impedance of the PCB traces seen by the signals remains unchanged, the signals experience no reflections. Signal reflections result from impedance mismatches and discontinuities. They primarily create ringing in PCB interconnects. At each reflection, the transient response of the signal is damped oscillations, which correspond to the ringing phenomenon. The aftereffects of signal reflections are signal distortions, overshoots, and undershoots. 

Signal Distortion

As aforementioned, signal distortions can be an end result of signal reflection due to an impedance mismatch. The signal distortions caused by impedance mismatches increase drastically with faster signal rise times. The improper termination of leads (for example, impedance discontinuity) is one of the main reasons for signal reflections, and the reflected signals can be detrimental to signal integrity.

Reflected signals can vary logic conditions and introduce numerous signal distortions in a PCB. The distorted signals are more sensitive to noise, and this increases the chances for PCB design failure from signal distortion.

Signal distortion can result from dispersion in the PCB substrate. Dispersion causes different frequency components on the PCB board to travel at different speeds and experience different levels of distortion. The velocity difference between various frequency components spreads and distorts the signal, so when the signal reaches the receiver end, it looks stretched. This stretching increases as the PCB interconnects get longer.

Another signal distortion effect is the increase in insertion loss in PCB interconnects, especially at higher frequencies. PCB traces are more like low-pass filters due to conductor losses and dielectric losses. At high frequencies, the signal undergoes scattering and absorption in PCB interconnects or traces, and increases the conductor losses and dielectric losses. As the PCB interconnect length increases, the signal losses get bigger. The larger the signal loss, the higher the signal distortion.

Signal distortions in the form of signal losses are common in long PCB traces. The signals propagating through lengthy PCB traces are subject to conductor losses and dielectric losses. Material resistance is a major cause of conductor losses, and also creates a temperature rise due to heat generation. Alumina, silicon, polytetrafluoroethylene, and FR4 (epoxy laminate material) are commonly used dielectric materials in PCBs, and the loss tangent or dissipation factor of these materials plays a role in reducing the dielectric loss. Lower dielectric loss in PCBs infers that the dissipation factor is lower as well. 

Types of Signal Distortions in PCBs

The signal distortions in high-speed PCBs are classified into two types:

  1. Linear signal distortion - The discontinuities in material properties and the dispersion in PCB substrate are some of the causes of signal distortion.

  2. Non-linear signal distortion - Non-linear signal distortions are commonly seen in non-linear circuits. Circuits handling two or more frequency components are more prone to non-linear distortions. Harmonic distortion is an example of non-linear distortion. 

Irrespective of the type, signal distortion degrades the reliability as well as the signal integrity of circuit boards. Signal distortions and signal reflections are critical when considering signal integrity in a PCB. The dependency of signal reflections on signal distortions can make signal integrity issues even worse. The signal integrity issues caused by distortion in PCBs must be addressed to ensure the reliable operation of circuits as well as safe and secure communication.

Cadence software can help you in designing reliable PCBs with minimal signal integrity issues. Cadence provides circuit design software to design boards for mission-critical applications with a high level of signal integrity.

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