Skip navigation

tech drill down

back to list

Controlled Impedance

by Matt Stevenson

The general function of a PCB transmission line is to transmit an electrical signal from one component to another. The two requirements to make up a transmission line are, a signal trace, and a return path. The return path is often a ground plane located on a layer above or below the signal trace. The other characteristic of a transmission line is that the length of the line relative to the signal frequency be sufficiently long. If the line is at least ¼ of the signal wavelength, it is long enough to be a transmission line. Why would transmission lines be important in designing a PCB?

One of the answers is Impedance of that line. Impedance is sort of a measure of the resistance in Ohms that a circuit displays towards the current or voltage. In simple terms, the higher the Impedance the higher the input voltage must be to achieve a desired current in the circuit. Impedance differs from resistance in that the impedance is related to an AC circuit whereas resistance is related to a DC circuit.

For high speed (>100MHz) frequencies and noise-sensitive signals (video, graphic processing, telecommunication, etc.) controlling the characteristic impedance is important to maintaining signal integrity. Mismatches in the impedance of high-speed signals can cause signal reflections (essentially, a portion of the signal traveling in the opposite direction intended). The greater the difference in impedance the greater the amount of signal reflected in one portion of the circuit, which will cause the signal to be noisy and could even cause a disruption of the signal.

Several factors will influence the impedance of a circuit.

  • Cross-sectional area of the transmission line (height and width)
  • The height of the dielectric material between the transmission line and the reference (return) plane
  • The dielectric constant of the dielectric material
  • Spacing between the transmission lines (Differential pairs)
The following equation is used for single-ended microstrip structures is taken from IPC-2141. This is the simplest mathematical representation of impedance, but because of its simplicity, it can present up to a 7% error in calculations at high speeds but is very useful to understand the influences.
W = width of trace
T = Copper thickness
H = Dielectric Thickness
= Dielectric Constant (Dk)

These factors all come with a +/- tolerance during manufacturing and raw material creation. In order for a PCB to have the desired result of matching or controlled impedance for critical structures, it is imperative that the PCB manufacturer understand all of these factors. Collecting data across a variety of PCBs and then feedback their actual data into a predictive model will give the manufacturing a good starting point to achieve the desired outcome on new designs.

A good PCB manufacturer that has good process control, data collection, and understanding of the impedance influences will make a good partner for turning the high-speed signals into a reality, but it is also important that the design upfront takes into account these factors as well. Matching the material characteristics (Dk), targeted layer spacing and impedance needs to the design upfront will make the process go more smoothly.Even working with the PCB manufacturer up front to work on the variable parameters, stack up and material choices before a trace is laid down on your design.

There is no worse feeling than having to go back to a designer during quoting stages to say that your impedance is not possible with the parameters you have given, you are going to have to redesign or change materials. Planning upfront is the key!

Go Design, have fun and build something cool.

File Download

By downloading this file you are agreeing to receive email regarding software and product updates, industry news and discount codes from Sunstone Circuits. Privacy Policy