What is a stub on PCB and what is it used for? Stubs exist with PCB traces and also vias, why?
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asked Nov 6, 2013 at 21:51
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\$\begingroup\$ It seems that a stub causes problems with signal integrity as well, how come? \$\endgroup\$– quantum231Nov 6, 2013 at 21:58
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1 Answer
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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.
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.
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\$\begingroup\$ 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. \$\endgroup\$ Nov 7, 2013 at 8:40
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\$\begingroup\$ @quantum231 I had a required course entirely about transmission lines in my undergrad at Oregon State University. It was quite comprehensive. \$\endgroup\$– SamuelNov 15, 2013 at 20:18
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\$\begingroup\$ There's also the unintended PCB stubs to care for, e.g. vias with unused sections that act like stubs. See this link: polarinstruments.com/support/si/AP8166.html \$\endgroup\$ Jun 28, 2014 at 13:22