What is controlled impedance?
What is Controlled Impedance? Essentially, controlled impedance is the matching of substrate material properties with trace dimensions and locations to ensure the impedance of a trace’s signal is within a certain percentage of a specific value. Controlled impedance boards provide repeatable high frequency performance.
How do you specify controlled impedance?
To specify impedance, designers typically need to specify values for a few different parameters:
- Target impedance.
- Trace width.
- Trace height.
- Layer or Layers the impedance trace or traces are on.
- Spacing between copper components on controlled traces (for coplanar or differential calculations)
Why do transmission lines have a uniform impedance?
A transmission line should have a uniform characteristic impedance. Any variation or discontinuities to the impedance causes signal reflection and distortion. This phenomenon is equally true for PCB traces and transmission lines. The reason for this is that the physical wavelength of a high-frequency signal is very short.
How long does a trace need to be to match the impedance?
With the most stringent rule to keep trace length less than one-sixth (1/6) of the electrical length of the rise time of the signal’s edge, then signal with a 1 ns rise time would need termination for traces longer than 1 inch. There are various ways to impedance match.
How are RF traces routed to prevent transmission line effects?
With higher frequency RF circuits, traces carrying your analog signals will need to be quite short in order to prevent transmission line effects. The separation between lines should be as large as possible, and they should not be routed close together over long distances.
What are the guidelines for routing RF signals?
RF PCB Routing Guidelines: RF Signals and Your Layer Stack Routing your RF board to ensure signal integrity is just as much about designing the right layer stack as it is about laying traces. You can suppress transmission line effects in your signal lines with the right layer stack in your PCB.