Sunday, May 13, 2012

Co-Planar Waveguide over Ground - Techniques Update


I've been using a RF Layout Technique for over a decade now that is called “Co-Planar Waveguide over Ground”. The principle advantage over plain old Microstrip is that for a given substrate thickness you can achieve a narrower line width than you can with a comparable Microstrip line. This is great for RF work because we are now extensively using 0402 sized components in our high density circuits and a 0402 capacitor on a regular Microstrip line just doesn't fit well, unless the substrate thickness is very, very thin.

A few years ago I wrote a “Design Idea” for EDN titled: "Make a quick-turnaround PCB for RF parts" [1]. I have recently updated that article to include suitable line widths for Rogers 4350B material and the 0.050” Line widths that many RF modules use [1]. That article showed how to make a PCB that works well way above 10 GHz on those simple, overnight FR-4 protoboard processes!

Figure - A picture of a sample Co-Planar Waveguide over Ground (CPWG). This 50 ohm trace was fabricated on a very low cost 0.062" thick "overnight" protoboard. The CPWG line width was set to 0.032" and as can be seen 0603 and SOT parts fit very well on this construction. But CPWG has other advantages as well - read on to find out more.... 

But Co-Planar over Waveguide techniques have other benefits too. One of them is that there is less field spewing into the air around the RF trace (compared to Microstrip), this not only reduces crosstalk to other circuits (thereby increasing isolation at no extra cost) but it also makes the circuit less sensitive to other conductive objects around it. What other conductive object can you get near a RF trace you might ask?

Well shields are very conductive and we want our circuits to be as small as possible and this includes shield height as well. As shown in the next article, Co-Planar Waveguide over Ground layout techniques have great advantages in these situations also.

As a follow on article to the Co-Planar trace article I recently published a paper titled: "Benefits of Co-Planar Waveguide Over Ground" on the PCB Design 007 Website [2].

So go ahead and give Co-Planar Waveguide over Ground a try - your circuits will only work better with less variation to shields and other “things” nearby and your parts will fit better.

Steve Hageman
www.AnalogHome.com



[1] "Make a quick-turnaround PCB for RF parts", EDN, December 15, 2010 
Expanded version of that article is available here, 
http://www.analoghome.com/news/2011_8_29.html

[2] “Benefits of Coplanar Waveguide Over Ground”, PCB Design 007, March 21, 2012
http://www.pcbdesign007.com/pages/zone.cgi?a=82941&artpg=1&topic=0