I know, i know… calculation of PCB trace Impedance is quite a vexing issue of design engineers. When i started building embedded systems stuffs three years ago, I knew absolutely nothing about Impedance calculation. I kept searching for information and “how-to” guides on google, but they all required to study lots of equations and they were quite boring. I had been scratching my head like, “how could somebody calculate an impedance of nothing like a component, no inductors, no capacitors, no resistors, only copper above the PCB !!!”.

But now i have managed to learn very easy tools that calculate the PCB trace impedance. The impedance of PCB traces needs a very must attention to be paid, for better performance of RF signal flow. Usually datasheets says that keep the impedance 50 ohms. Today i am going to tell you how is this done, very simply follow the following steps.

  • Step 1: Google for AppCAD, download it and install it.
  • Step 2: Go for the Passive circuits tab and there after the Coplaner Waveguide Tab. The window of workspace will open as shown

Coplaner Waveguide

  • Step 3: Change the Length Units according to your comfort mils or mm, i prefer to choose mm
  • Step 4: Fill in the blocks required for the calculation of impedance.
    • “L”   is the Length of the trace. Changing the length doesn’t vary the impedance so its not to be worried with.
    • “H”  is the distance between layers of PCB in mm. Make sure its the distance between the simultaneous layer.
    • “W” is the width of the trace. keep it changing until your impedance is calculated 50 Ohms.
    • “G” is the Gap between the trace and the ground plane. As a rule of thumb try to keep the gap under three times the width of the PCB its recommended.
    • “T” is the thickness of the copper on the plane. make sure to input in millimeters.
    • “εr” is the Dielectric coefficient of the PCB material. Ask the details of your PCB from the manufacturer before you start your PCB design.
  • Step 5: Enter the Frequency of the signal intended to flow on the trace.
  • Step 6: Select the Standard PCB type you are using.
  • Step 7: Keep changing the “W” and “G” and hit the calculate Z0 or Impedance button until you get the desired output. Refer the picture below.

filled enteries


Finally design the PCB using these details and you’ll get a very good performance of RF signal flow.

Happy designing 🙂