The importance of controlled impedance in a printed circuit board can depend on many variables, including the PCB’s unique characteristics and how the board will be used.
To understand the importance of precision when it comes to impedance requirements, let’s take a look at some of the specifics involved.
What Is Impedance?
Impedance is the sum of the resistance and reactance of an electrical circuit; it is measured in Ohms. Impedance is an alternating current characteristic in which signal frequency is an important element. The longer the trace or the higher the frequency, the more imperative it becomes to control the trace impedance.Signal frequency is a vital factor for traces which connect to components requiring two to three hundred MHz or more.
Many different trace configurations can be used in printed circuit boards to achieve controlled impedance. A PCB manufacturer can control the impedance through spacing and dimensions of the circuit board traces.
Levels of Impedance Control
Typically, printed circuit boards are available with three levels of impedance control service.
1) Impedance Control
– Most often used in high-end designs with a tight tolerance or unusual configuration. There are a number of different types of controlled impedance. The most common is referred to as characteristic impedance.
Other types include wave impedance, image impedance, and input impedance.
First, the board is manufactured and then tested via a Time Domain Reflectometry (TDR) test to determine if impedance specifications are met; changes can be made if needed. It will be necessary to work closely with your circuit board supplier on impedance control specifications to achieve the best results.
2) Impedance Watching
– This refers to compatibility in impedance. The impedance control trace will be indicated on the design. Width of the trace and height of the dielectric can be adjusted as needed. If necessary, a TDR test can be performed to check that impedance is correct. However, this does add some additional cost.3) No Impedance Control
– Because the impedance tolerances in the design are not tight, correct impedance can be achieved by conforming to standard specifications without impedance control. The printed circuit board manufacturer can provide accurate impedance without extra steps, which makes this the most cost-effective level.Importance of Accuracy
Printed circuit boards with controlled impedance require a meticulous level of accuracy in order to function correctly. PCB designers specify trace impedance and tolerance required, and work with the PCB manufacturer to confirm that specifications are met.