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What is a stub on PCB and what is it used for? Stubs exist with PCB traces and also vias, why?

quantum231
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1 Answers1

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They're used for impedance matching and filtering.

The image from Wikipedia below shows different stripline features used to create various elements etched into a PCB with descriptions below.

enter image description here

Some simple planar filter structures are shown in the first column. The second column shows the open-wire equivalent circuit for these structures. The third column is a semi-lumped element approximation where the elements marked K or J are impedance or admittance transformers respectively. The fourth column shows a lumped-element approximation making the further assumption that the impedance transformers are λ/4 transformers.

(a) A short-circuit stub in parallel with the main line.

(b) An open-circuit stub in parallel with the main line.

(c) A short-circuit line coupled to the main line.

(d) Coupled short-circuited lines.

(e) Coupled open-circuited lines.

Stripline vias, (the little silver connections on the ends), represent a strap through the board making connection with the ground plane underneath

If you're familiar with transmission lines then you'll recognize the purpose for these elements. If you're not familiar, it's a scary yet fascinating topic I recommend learning more about. It's the weird stuff about electricity. Know your enemy.

There is a bit more explanation in a previous answer I gave on your previous question on via reflections.

Community
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Samuel
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    Heh heh, know your enemy +1 – Andy aka Nov 06 '13 at 22:58
  • Thank you for your answer Samuel. I was getting confused because I saw stub on a PCB trace connected to a trace point for use with oscilloscope probe. I also saw stub that we get in a through hole via. Seeing it at different places made me confused what it is and what it meant. I have been exposed to tranmission line theory and do find it quite fascinating. It is sad that yet it is not taught to a good in depth level at universities. – quantum231 Nov 07 '13 at 08:40
  • @quantum231 I had a required course entirely about transmission lines in my undergrad at Oregon State University. It was quite comprehensive. – Samuel Nov 15 '13 at 20:18
  • There's also the unintended PCB stubs to care for, e.g. vias with unused sections that act like stubs. See this link: http://www.polarinstruments.com/support/si/AP8166.html – Guillermo Prandi Jun 28 '14 at 13:22