RF design is sometimes called “black magic” by PCB designers and digital designers, and it’s unfortunate this continues to occur. The field is certainly not magic, but it seems that the basic physics involved in wave propagation was generally ignored by electronics designers for decades. Today, many more systems will implement an RF front-end, antenna, sensor, or unique materials, and it’s a PCB designer’s role to understand something about why these design decisions might be made.
In this article, our goal is to provide some of the most important learning goals to help designers be more successful in this important area of technology. While you don’t need to know how to design every RF printed circuit or build MMICs, you can certainly avoid respins and ensure RF signal integrity by understanding the design points outlined below.
Learning Goals in RF Design
Below we’ve outlined some of the basic points to understand in order to work in RF designs. These topics span from electromagnetic waves to antennas and materials, all of which are important for a PCB designer and a system/assembly designer.
Understand Electromagnetic Waves
The most important point to understand in RF design is the propagation of electromagnetic waves. This is something you would typically learn in a physics class. Wave propagation is defined in terms of three quantities:
- Wave speed (v)
- Frequency (f)
- Wavelength (λ)
The three quantities are related as follows: v = fλ.
Finally, electromagnetic waves combine an electrical field wave disturbance and a magnetic field wave disturbance point in orthogonal directions. The image below shows the direction of the electric and magnetic fields (y and x respectively) for a wave propagating along the z-axis.
The main point here is to understand the directions of the electric and magnetic fields as these are important for understanding antennas, printed circuits, and resonances. This simple picture helps explain many phenomena generally observed in electronics
Understand Dielectric Constant (Dk) and Loss
When a wave propagates in a material, the dielectric constant will determine how the wave propagates. The dielectric constant is a complex quantity and can be written as:
Dk = (Dk-Re) + i(Dk-Im)
The two values are often compared using the loss tangent (Df):
Df = (Dk-Im)/(Dk-Re)
The real part of the dielectric constant (Dk-Re) determines the wavelength and speed of an electromagnetic wave in comparison to the wavelength and speed in vacuum:
Wavelength: λ = (λ in vacuum)/(√Dk-real)
Speed: v = (v in vacuum)/(√Dk-real)
Loss during wave propagation is determined entirely by the imaginary part of the dielectric constant. The conventional wisdom is to use loss tangent (Df) from a material datasheet to quantify loss. However, this is incorrect and it causes people to erroneously overlook high Dk materials due to the belief that they increase losses in the system. Instead, select low loss materials based on Dk-Im and not based on Dk-Re.
Understand Materials Used in RF Designs
For integrated circuits, PCBs, and assemblies, certain materials are used to provide certain Dk targets and loss targets. The main material sets and where they are used are outlined below.
Make sure you match your system’s performance requirements to the materials being used. This generally starts by looking at the frequency in which you will be operating, as well as determining some loss targets that inform overall length of interconnects. For PCBs, the manufacturing process matters as well, and unreinforced PTFE materials can be unsuitable for use in very thin layers due to their difficulty in processing.
Antennas perform a simple function: they broadcast and receive electromagnetic waves. The important points to understand with antennas are their placement and construction in an electronic system. For example:
- Is a ground plane needed, or should ground be carved out below the antenna?
- What is the antenna impedance and how does a feedline match to the antenna?
- What is the radiation pattern (gain) of the antenna and will this meet operating requirements?
Full understanding of antenna design, placement, routing, and orientation will help a designer take full advantage of their RF system.
Understand Transmission Lines
Finally, the important structure used to route RF signals around a PCB is a transmission line. These components are printed on PCBs, or they are implemented as cables (for example, a coaxial cable) to form connections between components. Board, chip, and package designers should ensure they understand the main transmission line structures and how they determine wave propagation. An understanding of impedance will be the important metric here as this will determine all other signal integrity metrics needed to fully understand transmission lines.
Design teams working on advanced electronic products can get to market faster with higher quality by leveraging the complete set of system analysis tools from Cadence. Only Cadence offers a comprehensive set of circuit, IC, and PCB design tools for any application and any level of complexity. To learn more about interconnect simulation in high-speed systems, watch our webinar, In-Design EM Analysis for Microwave/RF Design and Verification Workflows.