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Lightning Speed Laminates: Making Connection with Conductor Discontinuities

The title may be confusing for many technologists accustomed to dealing with electrical issues in traditional PCBs, but if you design PCBs that operate at microwave frequencies, it makes perfect sense. With microwave PCB design, it is not uncommon to have a conductor run come to a stop, followed by a space, followed by another conductor run, with the RF energy propagating through the discontinuity without the slightest problem. 

Not Your Traditional Design

In traditional PCB design world, this conductor-space-conductor configuration is called an “open,” and it is considered a reject that yields a dead circuit. But microwave PCB design is much different. How does RF energy propagate through conductor discontinuities in microwave PCBs?

When considering microwave technology, it is all about waves. Specifically, it is the electromagnetic (EM) wave that propagates on the PCB, and the wave properties are manipulated by the PCB design to get the desired circuit performance. To picture a simple example of an EM plane wave on a PCB, you can think of the wave in a cross-sectional view that looks like a sine wave. This sine wave will have different locations with high and low energy and ¼ of the sine wave is one of the maximum power points. A ½ sine wave is where the wave returns to zero and has no energy. These fractions of the sine wave are appropriately called ¼ wavelength and ½ wavelength, respectively.

At microwave frequencies, a resonator can be made by using a conductor with a physical length that is exactly ½ wavelength of the wave on the circuit. This ½ wavelength conductor will resonate, which is accomplished by a wave that bounces back and forth and sets up a standing wave. The standing wave generates a lot of energy and resonates at the frequency associated with that ½ wavelength.

The big question is this: How do you get the energy on the conductor that is acting like a ½ wavelength resonator? If you have a conductor that connects to the ½ wavelength resonator, that combination creates a much longer conductor, and it no longer works as a ½ wavelength resonator. This means that you cannot directly connect to the resonator conductor and you will need to “couple” energy to the resonator. This is done by using feed line conductors on both sides of the ½ wavelength conductor, which are physically very close to the resonator conductor; as a result, the energy on the feed lines will have electric fields that radiate onto the resonator.Read the full column here.Editor's Note: This column originally appeared in the December 2014 issue of The PCB Design Magazine.