-
- News
- Books
Featured Books
- design007 Magazine
Latest Issues
Current IssueCreating a Culture of Collaboration
PCB designers could learn quite a bit from NASA and the private companies that develop spacecraft: Every one of these vehicles is a testament to the value of collaboration among disparate stakeholders. Without a collaborative culture, the rocket might never get off the ground.
Breaking High-speed Material Constraints
Do you need specialty materials for your high-speed designs? Maybe not. Improvements in resins mean designers of high-speed boards can sometimes use traditional laminate systems. Learn more in this issue.
Level Up Your Design Skills
This month, our contributors discuss the PCB design classes available at IPC APEX EXPO 2024. As they explain, these courses cover everything from the basics of design through avoiding over-constraining high-speed boards, and so much more!
- Articles
- Columns
Search Console
- Links
- Events
||| MENU - design007 Magazine
Estimated reading time: 1 minute
Mythbusting: There are No One-way Trips!
One of the greatest myths in PCB design is that we only have to route signal traces from pin-to-pin to make a complete connection. And, that ensuring these traces have matched delay is the only timing issue we need to consider. However, current is not a one way trip--it must complete the circuit back to the source to provide the round-trip current loop. This misconception comes from the fact that we only draw the pin-to-pin connections on the schematic and ground the chips at one point.
Current always flows in a loop. However, it does not go down to the end of the trace, to the load, and then begin to make its way back to the source. But rather, the outbound pulse charges the local parasitic capacitance as it propagates down the transmission line and returns to the driver. As the pulse progresses down the line, current returns to the source as the wave front moves until it finally reaches the load. If the return path is disrupted and does not flow directly beneath the trace, the loop area and hence delay are extended. This generally results in increased emissions of radiation.
In a previous column, The Dumping Ground, I discussed why the ground plane is not a dumping ground for unwanted signals. Most PCB designers think that the ground only serves to make the routing easier, allowing the designer to ground anything, anywhere without having to run multiple tracks. Generally, a component requiring a ground connect is just grounded at any point on the board creating the connection. But this does not consider the return current path which is just as important as the actual trace routing for high-speed design.
In a DC circuit, the return current takes the path of least resistance. But at high speeds, the return current takes the path of least inductance which just happens to be the reference plane (either ground or power) directly above or below the trace.
Read the full column here.
Editor's Note: This column originally appeared in the April 2014 issue of The PCB Design Magazine.
More Columns from Beyond Design
Beyond Design: Dielectric Material Selection GuideBeyond Design: The Art of Presenting PCB Design Courses
Beyond Design: Embedded Capacitance Material
Beyond Design: Return Path Optimization
Beyond Design: Just a Matter of Time
Beyond Design: Design Success with IPC Standards
Beyond Design: Integrating AI Into PCB Design Flow
Beyond Design: Standing Waves in Multilayer PCB Plane Cavities