RF circuits are designed to pass signals within a certain band. They use band pass filters to transmit signals in a so-called band of interest. The signal within a range of frequency passes through this band range, and the rest of the frequencies of the signal are filtered. A single band can be very narrow or very wide and carried upon a very high frequency carrier wave.
The RF frequency range is typically from 500 MHz to 2 GHz bandwidth, and designs above 100 MHz are considered RF. There’s considerable difference between RF circuits compared to typical digital and analog circuits.
In essence, RF signals are very high frequency analog signals. Therefore, unlike digital, RF signals can be at any voltage and current level between minimum and maximum point, at any point in time.
RF signals are one frequency or a band of frequencies on a very high frequency carrier. Unlike digital signals associated with one voltage or one current, RF signals operate on a frequency.
For more details on critical RF (and microwave) design guidelines, check out our EE Times article.
Here are some basic tips and hints to know to get a better grasp on RF PCB design.
- Make sure your PCB designer knows that RF is far more sensitive than high-speed digital signals.
- For RF, the higher the frequency, the smaller the tolerance. Thus, impedance matching is critical for RF. Your PCB designer must keep it at a 50 Ω, 50 Ω from the driver, 50 Ω during transmission, and 50 Ω into the driver.
- A return signal on its ways to the driver definitely finds a return path. It won’t be ideal and will cause reflection and ringing since it’s not longer an impedance – controlled signal, Fig. 1
- The PCB designer must keep crosstalk in mind when designing RF. System performance and PCB densities increase the crosstalk problem.
- High-speed signals should be routed as far apart as possible. Distance from center to center should at least be four times the trace width for these signals.
- Consider PCB laminate properties, such as dissipation factor and dielectric constant (Dk) value and its variation. This will help reduce the crosstalk to optimal levels and keep heat dissipation in check.
These and other important guidelines are becoming increasingly vital for today’s PCB applications. A majority is moving to new commercial, industrial, medical, and mil/aero products. The forms these products are taking include portable, mobile, wireless to include wearables and IoT devices. So, you’ll be seeing considerably more RF circuitry being implemented on near term small PCBs along with rigid-flex and flex circuitry for wearable and IoT products.