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Beyond Design: How to Handle the Dreaded Danglers, Part 1
September 14, 2016 | Barry Olney, In-Circuit Design Pty LtdEstimated reading time: 2 minutes
 
                                                                    Dangling via stubs can distort signals passing through your interconnect, and decrease the usable bandwidth of the signal. A via stub acts as a transmission line antenna, and has a resonant frequency determined by the quarter wavelength of the structure. At this frequency, the transmitted signal is greatly attenuated, by up to 3dB. For low-frequency signals, this is not much of an issue because these signals are significantly lower than the resonant frequency of the via stub.
However, for higher-frequency signals (>1GHz), which are becoming more common as performance specifications are increased, this issue becomes a problem because the signals are transmitted at frequencies near or at the resonant frequency of the via stub. Harmonic components that are odd multiples of the fundamental frequency can also be highly attenuated.
The conventional solution to this problem is to back-drill (or control-depth drill) the vias to bore out the via stub barrels, so that the via stubs are reduced in length, if not completely removed.
If the via is short, compared to the signal rise time, then it acts mostly as excess shunt capacitance. The entire length of the via contributes to the capacitance, while only the section where the signal current actually flows makes up the inductance. However, a long via stub can develop resonance that exacerbates the effects of its capacitance. I should point out that it is fine to have a plated through-hole (PTH) via, providing the signal goes in at one end and out at the other, using the entire length of the barrel.
When a via’s stub length is equal to a quarter wavelength of the signal frequency, the signal travels from the trace to the end of the stub and then bounces off the open circuit end-point and back to the trace for a total distance of a half wavelength. This half wavelength travel has the effect of shifting the phase of the signal by 180 degrees, creating resonance in the via stub. The phase-shifted, reflected signal has a maximum value at a time when the signal has a minimum value, and vice-versa.
The Nyquist frequency of a discrete signal is defined as a half of the sampling rate of the signal and will have a strong frequency component at this frequency. In addition, the signal can have strong power spectrum harmonic components at frequencies greater than the Nyquist frequency typically up to the 5th harmonic. The resonant frequency of the via stub is inversely proportional to the dielectric constant of the material, surrounding the via, with a wavelength of four times the length of the unused portion of the via.
To read this entire article, which appeared in the August 2016 issue of The PCB Design Magazine, click here.
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Designers Notebook: Power and Ground Distribution Basics
10/29/2025 | Vern Solberg -- Column: Designer's NotebookThe principal objectives to be established during the planning stage are to define the interrelationship between all component elements and confirm that there is sufficient surface area for placement, the space needed to ensure efficient circuit interconnect, and to accommodate adequate power and ground distribution.
Beyond Design: The Fundamental Structure of Spectral Integrity
10/21/2025 | Barry Olney -- Column: Beyond DesignImpedance can be characterized in both the time and frequency domains. In the time domain, it influences how electromagnetic energy propagates through interconnects, affecting signal integrity and waveform fidelity. In the frequency domain, AC impedance determines how well the network can suppress noise and deliver clean power at a range of frequencies. AC impedance shapes how power rails respond to transient loads.
Elementary Mr. Watson: Chasing Checkmarks, Not Signal Integrity
10/01/2025 | John Watson -- Column: Elementary, Mr. WatsonFor the September 2025 issue of Design007 Magazine on signal integrity, I explored how the PCB is similar to a military obstacle course: walls that sap energy like impedance mismatches, barbed wire that cuts like crosstalk, and mud pits that drag a signal down like attenuation. The takeaway was clear that a PCB is not a flat drawing; it's an electromagnetic ecosystem filled with hazards that test every signal that dares to cross it. The real danger lies not in the obstacles themselves, but in the fact that many designers never see them.
Signal Integrity: A Game of Margins
09/25/2025 | Andy Shaughnessy, Design007 MagazineAs the founder of Wild River Technology, Al Neves deals with some of the most challenging aspects of signal integrity. Wild River’s engineers consult with high-tech companies that work at very high speeds and frequencies, often above 100G. Al is always a fun interview, so we asked him to share his thoughts on the current state of signal integrity engineering.
Understanding Signal Integrity, the Foundation of High-Speed Digital Design
09/25/2025 | Stephen V. Chavez, Siemens EDASignal integrity has become a critical factor in ensuring reliable performance in high-speed digital systems. As data rates continue to increase, engineers must understand the fundamental principles that govern how signals propagate through transmission lines and how to mitigate common issues that can degrade signal quality.

 
                                     
                                     
                                     
                                     
                                             
                                             
                                             
                                             
                                             
                                     
                                             
                                             
                                             Fresh PCB Concepts: Resilience and Renewal in Domestic PCB Manufacturing
                                         Fresh PCB Concepts: Resilience and Renewal in Domestic PCB Manufacturing Designers Notebook: Power and Ground Distribution Basics
                                         Designers Notebook: Power and Ground Distribution Basics Elementary, Mr. Watson: Heat—The Hidden Villain of Power Electronics
                                         Elementary, Mr. Watson: Heat—The Hidden Villain of Power Electronics





 
                     
                 
                    