Maximum Via Stub Length Calculator

A via stub functions as a resonant circuit at a specified resonance frequency during which it can store maximum energy. The length of the via stub should always be within the acceptable range to avoid signal integrity issues in your design. If the stub length increases beyond the maximum allowable length, it will store most of its incoming energy at the resonance frequency. This will affect the functionality of the components operating within the via stub resonant frequency range. Hence knowing the maximum via stub length and its resonant frequency greatly helps you to design an efficient PCB.

Sierra Circuits’ Maximum Via Stub Length Calculator helps you to determine the optimum stub length and its resonant frequency.

Features of the tool

• Calculates via stub resonant frequency and maximum allowable via stub length based on dielectric constant and maximum data transfer rate/fastest signal rise time/maximum frequency content/3dB bandwidth
• Facilitates reverse calculation i.e, it computes maximum data transfer rate, fastest signal rise time, maximum frequency content, and 3dB bandwidth for the desired stub length and dielectric constant
• The units of the inputs and outputs can be changed using the respective dropdowns
• A brief description of each input and output parameter can be viewed by clicking on the help button

How to use the Maximum Via Stub Length Calculator

The input parameters for this tool are dielectric constant (mandatory) and any one among the following:

• Maximum data transfer rate (DTR)
• Fastest signal rise time (tr)
• Maximum frequency content (Fmax)
• 3dB bandwidth (BW)

You can pick the second input parameter by selecting the checkbox right next to the field that you wish to enter. For instance, if you select tr as the second input, the tool will compute DTR, Fmax, and BW, along with maximum stub length and its resonant frequency.

For example, if you choose the dielectric constant as 3 and fastest signal rise time (second input) as 2 ns and hit calculate next to fastest signal rise time, then the tool computes via stub resonant frequency, maximum via stub length, maximum data transfer rate, maximum frequency content, and 3dB bandwidth (the respective values are shown in the figure above).

You can change the units of these input and output parameters using the respective drop-downs.

This tool also enables you to perform the reverse calculation. i.e., if you key in the desired stub length and dielectric constant, it computes the rest of the aforementioned parameters.

Knowing the optimum stub length and its resonant frequency is one of the important design prerequisites as it directly impacts the loss profile of your circuit board. This calculator allows you to compute these important parameters in real-time.