What is Impеdance

Impedance, represented by the symbol Z, is a fundamental characteristic of circuits, especially PCBs. This is how the circuit acts when an alternative current (AC) passes. It is not simply a system, but a more complex combination of system (R) and system (X), which define the total system that the circuit opposed to the passage of AC.

While the system is based on the continuous flow of continuous current (DC), the impedance also takes into account the effects of reactance, when the current varies in the time, as per AC applications. This concept is crucial in the design of printed circuit boards, as the importance directly affects the distribution of the signal and the stability of the circuit, especially for high frequencies.

What is it caused by?

The impеdе in a PCB printed circuit board is not a static value but results from a complex mix of different factors. First of all, the geometry of the track has a crucial role, but the material of the substrate, with its own properties, is also an important factor.

To understand how the impеdance in these circuits is determined, it is necessary to consider the following element:

 

• Track Review:
  • Width and thickness: Wider tracks have a lower impеdance, but narrower tracks have a higher impеdance.
  • Distance between Traces: The proximity between parallel paths can affect the impеdance due to the electromagnetic interaction between them.

• Substrate Material:
  • The properties of the substrate, such as the dielectric constant and the dielectric loss, play a crucial role for the impedance of the circuit.
• Signal frequence:
  • The amount varies with the frequency of the signal that passes through the circuit. At different frequencies, the same circuit can have very differ behaviour.
• Dеsign:
  • Elements such as vias, connectors and other components can affect the impеdance. The arrangement and connection of these components within the PCB must be carefully designed.
• Temperature:
  • Changes in temperature can affect the properties of the materials used in the PCB, leading to changes in the impedance.

Understanding these factors is essential to designing printed circuit boards that operate within the desired parameters. Careful design that takes all of these factors into account is vital to ensure that the impedance is within the optimal values ​​for the circuit’s desired operation.

 

How is the Impеdance calculated?

Calculating impedance on a printed circuit board (PCB) requires a thorough understanding of how resistance and reactance combine to influence the circuit’s behaviour.

The general formula for impedance is Z = √(R² + X²), where R represents resistance and X represents reactance.

Resistance is relatively simple to calculate, as it depends on the materials used and the physical dimensions of the circuit. Reactance, on the other hand, is more complex, since it varies with the frequency of the signal and can take different forms, such as inductive or capacitive reactance, depending on the circuit configuration.

This dependence on frequency makes the calculation of impedance particularly critical in circuits that operate at high frequencies or that must handle a wide range of frequencies. For this reason, to accurately estimate the impedance, designers often use advanced simulation software that can model the behaviour of the circuit under different operating conditions, thus allowing them to optimize the design to achieve the desired impedance.

How to Control the Impеdance of PCB Circuits

Controlling impedance in a PCB is a challenge that requires precision and technical knowledge. A key aspect is the design: by modifying the width, thickness, and layout of PCB traces, it is possible to significantly influence impedance.

For example, in general, wider traces can reduce impedance, while narrower traces tend to increase it. The choice of substrate material is also crucial, as different materials have different dielectric constants, affecting the circuit’s reactance.

Furthermore, techniques such as controlled impedance are used to ensure that the impedance along the path remains constant, thus preserving signal integrity, especially in high-frequency applications.

A key component in impedance management is the prevention of crosstalk, an unwanted phenomenon in which a signal on one or more PCB traces interferes with another adjacent one. Crosstalk can significantly degrade signal quality and compromise circuit functionality.

Controlled impedance helps mitigate crosstalk by ensuring that each trace on the PCB has a constant resistance to signal passage. This way, the signal is less subject to distortion and interference.

Controlled impedance technology requires careful collaboration between PCB designers and manufacturers to ensure that specifications are met during the manufacturing process. Finally, verifying and measuring impedance using specific instrumentation is essential to ensure that the finished circuit satisfies the design requirements.