Reference Designer

Signal Integrity TUTORIAL

Impedance

We will call the high speed traces the Transmission Line. Impedance is the resistance that a transmission line presents to the flow of energy along the transmission line.

Each transmission line has a Capacitance per unit length, inductance per unit length and resistance per unit length. These values depend upon the geometry.

The Characteristic impedance of a Transmission line is given by

Zo = sqrt (Lo/Co)

Where, Lo is the inductance per unit length and Co is the Capacitance per unit length.

Characteristics Impedance for PCB designers

As PCB designers we will often be required to maintain a given characteristic impedance for some traces. The next two pages will show formulas for calculating Characterisic impedances of two most common configurations - microstrip and stripline.

We must also know that if we increase the trace width, the capacitance of the trace will increase ( Remember High School formula ?). This will lower the characteristic impedance. You must bear this in mind while designing.

Also notice that if you decrease the separation between the trace and the power plane, the capacitance with increase and therefore the impedance will decrease.

By varying the trace width and the separation between the trace and the power plane we acheive the required characteristics impedance. The third factor that it depends upon is the relative permittivity, which depends upon the material and is usually between 3.5 to 4.7.

As PCB designer you will be required to create the stack up. You can use the formulas in the following two pages to calculate and prepare the stackup requirements.