RF and microwave circuits were rarely used on conventional PCBs in earlier days. But now, they are commonplace given the fact OEMs heavily rely on these high frequency devices for newer portable communications products. Consequently, the PCB layout designer must concentrate on a variety of issues when designing PCBs with high- frequency RF and microwave devices.
The RF frequency range is typically from 500 MHz to 2 GHz-bandwidth. PCB 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, Fig. 1. Unlike digital signals associated with one voltage or one current, RF and microwave signals operate on a frequency.
Considering there are variations in digital, analog, and RF/microwave circuitry and PCB designs, it’s worth knowing some of the key aspects associated with RF and microwave. For more detail on this particular subject, read our article in EE Times/embedded.com.
Also, here are some things to know and tips on getting a better handle on RF and microwave PCB design for your next PCB project.
- The PCB designer must have a solid understanding of RF and microwave circuitry and clearly know to follow certain guidelines and rules.
- Impedance matching is critical for RF. Transmission line impedances of 50Ω are typical. If that is the case the PCB designer must keep it at 50Ω — 50Ω out from the driver, 50Ω during transmission, and 50Ω in to the receiver.
- Return loss caused by signal reflection or ringing must be minimized.
- Cross talk becomes more important and knowing how to deal with it becomes critical for RF and microwave PCB design.
- High-speed signals should be routed as far apart as possible. Distance from center to center should be at least four times the trace width for these signals.
- It’s highly important to consider laminate properties like dissipation factor and dielectric constant or Dk value and its variations within the dielectric.
Aside from signal reflection or ringing, there are the skin effect and dielectric losses of a signal. The skin effect loss is on the trace of a signal. It’s important to know that skin effect is a measure of frequency. At high frequencies, electrons tend to flow on the outer surfaces of the conductor, also known as “skin effect.” On the trace, itself, there is an extremely small area that is 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 since it is also a measure of frequency. Like the skin effect loss, dielectric loss 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. The effect is amplified at high frequencies.