Calculators / 06
Via Stub Resonance Calculator
The unused length of a via below the layer a signal leaves on is an open-circuited stub. At the frequency where it is a quarter wavelength long it looks like a short to the line, and energy that should pass through it is sent back instead. The fix is to keep that frequency well above everything the channel carries — by backdrilling, or by choosing layers that leave no stub.
Reading the numbers
- The notch falls as the stub grows: halve the length and it doubles.
- Loss starts to build well below the notch. Read |S21| at Nyquist to see what the stub already costs where the budget is quoted.
- The margin k is yours to set. Nothing in a specification fixes it; it depends on how much of the notch's shoulder the channel can afford.
Every value is in the address bar as you change it, so a link reproduces the calculation exactly.
Type values with SI prefixes — 100n, 2.2p, 16G — or
drag the sliders; a bare number keeps the prefix already shown.
The formulas
Go deeper — assumptions, how it is checked, and sources
Assumptions. The stub is lossless and has the line's impedance, so the notch is infinitely deep. A real stub has loss and a different impedance, which make the notch finite and shift its shoulder; the frequency is the part to trust. The effective Dk of a via barrel is not simply the laminate's, because the field around it is not uniform.
How it is checked. |S21| is 0 dB at DC and at twice the notch frequency, where the stub is a half wave; it vanishes at the notch; and the notch is exactly where the stub is a quarter wavelength.
Source. D. M. Pozar, Microwave Engineering, 4th ed., Wiley, 2012, §2.3 and §5.2.