Characteristic Impedance

NI Digital Waveform Generator/Analyzer Help

Edition Date: June 2013

Part Number: 370520P-01

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The characteristic impedance of a transmission line largely influences the transient response of a signal passing through it. The physical properties of the transmission line materials determine its characteristic impedance. For example, the dielectric of the insulators and the cross-sectional geometry of a cable determine its capacitance. Likewise, the inductance of the cable is a function of the length and the properties of the dielectric. The characteristic impedance is a function of both this inductance and capacitance. Manufacturers of cables provide the specification for the characteristic impedance of that cable, along with how it behaves over environmental extremes.

It is critical to match the characteristic impedance to the source impedance. If the impedances do not match, the resulting signal at the load is greatly distorted in both time and amplitude. Any time you disrupt the geometry described above, it results in impedance mismatches and signal reflections. For example, at the interfaces or boundaries between the cables and the devices, you should use connectors designed to maintain this characteristic impedance (coaxial connectors). Screw terminals, tees in the transmission line, or wire stubs are not recommended.

Caution  Failure to use the connectors designed for these cables may result in impedance mismatches.

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