Issue link: https://iconnect007.uberflip.com/i/1457913
MARCH 2022 I DESIGN007 MAGAZINE 49 Other Benefits ere are other benefits to having a conve- nient ground on the outside signal layers. • Scope probing: Having ground available helps when probing with an oscilloscope as shown in Figure 11. is is especially true when probing any clock signal where you simply cannot use the long flying lead of the scope probe. • Fixing errors: Well, who hasn't designed a circuit that needed a resistor or capacitor "slapped" onto it to get it to work prop- erly? Having a ground nearby on the top and bottom layers makes this trivially easy. • Improved radiation performance: A properly placed copper pour on the signal layers does indeed reduce the radiation of the design in general by bending the electric field lines into the copper pour and not letting them be so far out into space. ink about it this way: If I create an antenna trace on a PCB and then I surround it with copper pours on just the sides, I will have made a sort of coaxial structure, and I have ruined that trace's effectiveness as a radiator because I have shorted out the electric field from the surrounding air. Everyone can probably reason that a coax cable is the best way to transfer signals without radiation, right? A properly designed copper pour does nearly the same thing to nearby traces. Note that you may have to adjust the final trace width to maintain the desired impedance levels with a close copper pour 7 . • Heatsinking: More copper means better heat sinking in general for the PCB as a whole, and the stitching vias thermally tie it all together. Possible Downsides of Adding Copper Pours There may also be disadvantages to this approach. • e possible increased cost of adding vias and then having to drill them • Takes time to add copper pours and the stitching vias correctly • Incorrectly done copper pours may indeed make the situation worse Conclusion It isn't a matter of, "Copper pours on a sig- nal layer make things worse." It's more like, "Poorly placed copper anywhere will ruin any design." Once you understand what is really going on, with the help of easy-to-use EM sim- ulation tools, you will improve your designs immensely. DESIGN007 References 1. "How to Make a Quick Turn PCB That Modern RF Parts Will Actually Fit On," and "Benefits of Copla- nar Waveguide Over Ground," by Steve Hageman. 2. For more examples of boards created with CPWG construction, just do a web image search for "Hittite Evaluation Board." Hittite (now part of Ana- log Devices) is famous for doing all their prototype boards using the CPWG technique. 3. The Sonnet EM simulator and a free lite version are available at www.sonnetsoftware.com. 4. To see examples of filters of this type, do an internet image search for edge-coupled filters. 5. "Propagation Times and Critical Length—How They Interrelate," by Douglas Brooks. 6. Another example of using the "quarter of a wavelength" rule of thumb, visit AnalogHome: Decoupling RF Circuits - Part 1. 7. There are numerous free calculators available that will do calculations for CPWG structures, or you can use Sonnet. Steve Hageman is the founder and lead engineer at Analog Home.