Table of Contents
Advertisement
4
Circuit Board Design
Transmission Line Considerations
24
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 (microstrip). 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 ohms, 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 U4205A Direct Connect 34-Channel Mictor Probe
Advertisement
Table of Contents
