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5
Circuit Board Design
Transmission Line Considerations
64
Stubs connecting signal transmission lines to the connector should be as
short as feasible. Longer stubs will cause more loading and reflections on
a transmission line. If the electrical length of a stub is less than 1/5 of the
signal rise time, it can be modeled as a lumped capacitance. Longer stubs
must be treated as transmission lines.
Example:
Assume you are using FR-4 PC board material with a dielectric constant of
~4.3 for inner-layer traces (stripline). For example, A 0.28 cm long stub in
an inner layer has a propagation delay of ~20 ps. Therefore, for a signal
with a rise time of 100 ps or greater, a 0.28 cm stub will behave like a
capacitor.
The trace capacitance per unit length will depend on the trace width and
the spacing to ground or power planes. If the trace is laid out to have a
characteristic impedance of 50 Ω it turns out that the capacitance per unit
length is ~1.2 pF/cm. Therefore the 0.28 cm stub in the previous example
would have an effective capacitance equal to ~0.34 pF.
This trace capacitance is in addition to the probe load model.
Keysight Technologies E5400-Pro Series Soft Touch Connectorless Probes User's Guide
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