Stub (electronics)

In electronic signaling, a stub is a section of transmission line with a calculated length. It is used for impedance matching or to obtain an exact capacitance or inductance.

Because stubs take on reactive properties as a function of their electrical length, stubs are most common in UHF or microwave circuits where the line lengths are more manageable. Stubs are often used for antenna matching.

Smith charts can also be used to determine what length line to use to obtain a desired reactance.

hort circuited stubs

The general equation for the input impedance of a short circuited line is ::where j is the imaginary unit, $Z\_0$ is the characteristic impedance of the line, β is the phase constant of the line, and "l" is the physical length of the line.

Thus, depending on whether is positive or negative, the stub will be inductive or capacitive, respectively.

The Length of a stub to act as a capacitor is then given by:

::

The Length of a stub to act as an inductor is given by:

::

Open circuited stubs

For an open circuit, the general equation for input impedance is ::where j is the imaginary unit, $Z\_0$ is the characteristic impedance of the line, β is the phase constant of the line, and "l" is the physical length of the line.

It follows that whether is positive or negative, the stub will be capacitive or inductive, respectively.

The length of an open circuit stub to act as an Inductor:

::

The length of an open circuit stub to act as a capacitor:

::

Computer network stub

Electrical length = (Rise time) * (Speed of light) * (Cable propagation relative to light speed)

Maximum stub length = Electrical length * (1/6) [cite web|title=Application Bulletin AB-6 RS-485 Systems--Making Them Work|url=http://www.nve.com/Downloads/ab6.pdf 071115 nve.com]

This is relates to buses like SCSI, RS485, CAN-bus, Ethernet etc..

References