Fresh PCB Concepts: Key Considerations for Reliability, Performance, and Compliance in PCBs
Elementary Mr. Watson: Navigating RF—A Glide Path Approach to Design Success
Designers Notebook: Layer Stackup Planning for RF Circuit Boards
Effective Characteristic Impedance
March 4, 2015 | Kirk Fabbri, KSPT Engineering ConsultingEstimated reading time: Less than a minute
In a typical interconnect, there lie multiple places where capacitance plays a factor in the signal integrity. This includes the driver and receiver output/input capacitance, as well as the packages, vias, and the transmission lines. Failing to optimize these parameters can often lead to unwanted reflections, excessive radiated and or conducted emissions, and sometimes failure of components and systems.
Reflections can occur anytime there is an impedance mismatch on the line. Sources of mismatches are plentiful and include trace width changes, vias, stubs, reference plane changes, and even the so-called fiber weave effect. In this case, a trace can encounter a different dielectric constant depending on whether it is routed over glass or the epoxy resin in the dielectric material.
In this investigation, it is the capacitive contribution of the different components that are of interest, and how they affect the characteristic impedance the driver sees.
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