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RF and Microwave Circuits on PCBs Takes Extra Design Savvy

Radio frequency or RF circuitry was unheard of on PCBs a decade or so ago. That was generally due to its high cost and limited application.

Fast forward to today and youíve got RF in virtually all portable devices like smartphones, other communications devices, and a variety of medical, consumer, industrial, and mil/aero applications.

So today with RF and microwave circuitry, the PCB designer must think way ahead of earlier design methodologies involving basic digital and mixed signal. RF frequency range is typically from 500 MHz to 2 GHz-bandwidth, and designs above 100 MHz are considered RF. The microwave frequency range is above 2 GHz.

There’s considerable difference between RF and microwave circuits versus typical digital and analog circuits. In essence, RF signals are very high frequency analog signals. Therefore, unlike digital, RF signal can be at any voltage and current level between minimum and maximum point, at any point in time. RF and microwave signals are one frequency or a band of frequencies on a very high frequency carrier, as shown in figure below. Unlike digital signals associated with one voltage or one current, RF and microwave signals operate on a frequency.

There’s more to know about RF and microwave circuitry for PCBs. Learn more by clicking on our EE Times/Embedded.com article.

Suffice it to say there are some key things you should know about. Here are some tips and hints to get you on the way to knowing more about RF and microwave on PCBs.

  • Foremost is the PCB designer must have the right mindset to successfully engage and complete an RF/microwave design.
  • Understand that impedance matching is extremely critical; keep in mind the higher the frequency, the smaller the tolerance becomes. For instance, the designer must keep it at 50O — 50O out from the driver, 50O during transmission, and 50O in to the receiver.
  • At very high microwave frequencies, the return signal takes a path of least inductance. There should not be any discontinuities in the plane underneath the signal from the driver to the receiver.
  • If a ground plane doesnít exist underneath a particular trace, the signal still finds its way to the driver on its return path. But it wonít be ideal.
  • Remember to accurately manage cross talk since it is a major issue in high frequency designs.
  • Route high-speed signals as far apart as possible. Best distance from center to center is four times the trace width for these signals.

It’s also important to keep in mind other signal losses. One is the ìskin effect.î On the trace, itself, there is an extremely small area used as a funnel to move electrons. This funnel causes heat on the trace, and some of that energy going through the trace is converted to heat and lost, thus creating the skin effect loss of a signal.

Dielectric loss is a companion to skin effect loss. Like the skin effect loss, it is created when electrons flow through a conductor. There is quiet energy in those electrons. They bounce back and forth with the electrons on the FR4 PCB substrate, for example. During this interaction, some of the energy from the electrons flowing through a conductor is then transferred to the electrons on the FR4.

Consequently, that energy is converted to heat and subsequently lost and dielectric loss is created. In such instances, it’s best to use polytetrafluoroethylene Teflon, also known in the industry as PTFE material.