Anything with a radio frequency that is higher than 2 GHz is considered to be a microwave PCB. The radio frequency at which they function is the key distinction between RF circuit boards and Microwave PCBs. RF boards operating at a frequency greater than 2GHz are categorized as microwave PCBs.
Any application that calls for receiving and transmitting radio signals uses RF boards and microwave PCBs for communication signals. Radar stations and cell phones are two examples of common applications.
In particular, RF transmissions are susceptible to noise. Thus, the possibility of signal ripple or reflection is common. The need to adjust the impedance value along each of the circuit’s traces makes impedance a particularly important factor in this class of circuits.
Additionally, it’s important to limit the power losses brought on by signal reflections by designing appropriate paths for return current, which, as frequency rises, prefers to take the routes with the lowest inductance.
Crosstalk, which is the energy transfer between adjacent traces brought on by inductive or capacitive couplings, becomes more important as the performance and density of the components rise.
RF PCB design requires taking into account material properties like the dissipation factor and dielectric constant. When compared to materials designed for high frequencies, FR-4 has a larger dissipation factor, resulting in significant insertion losses that rise with frequency.
Additionally, as the frequency rises, the FR-4’s dielectric constant might rise by up to 10%, causing impedance fluctuations throughout the PCB traces.
Best Practices for RF PCB Design
Material Selection
Although materials like FR-4, which are frequently used in PCB fabrication, are very affordable, they are not the best option for high-frequency RF applications due to the non-uniformity of the dielectric constant and a lower tangent angle.
Specific materials are used for RF PCB design. materials include ceramic, PTFE, hydrocarbons, and several kinds of glass fiber. The PFE and PTFE materials, which are members of the fluoropolymer family, enhance the base material’s chemical resistance and possess anti-adhesion, smoothness, and exceptional heat resistance qualities.
Even at temperatures of 200 °C, they are resistant. The ideal option is PTFE with fiberglass if money is not an issue and quality is more essential than cost.
The ceramic-coated PTFE is used because it is cheaper and requires a simplified production method. Although Rogers materials are more expensive, they allow for power losses to be reduced by about 50%, ensuring great performance even over 20GHz and a low dielectric constant value that is stable and reproducible as the frequency changes.
The most common methodology is to use various materials that fulfill the requirements for electrical performance, thermal properties, and cost because RF PCBs are often multi-layered.
For instance, the exterior layers can be made from high Rogers laminates, whereas the inside layers can be made of less expensive epoxy glass laminates.
Transmission Lines
RF transmission lines for RF PCBs must have their impedance value rigorously managed to prevent power losses and guarantee signal integrity. The characteristic impedance in microstrip transmission lines is determined by the trace width, layer thickness, and type of dielectric.
Strip lines are used on the inner layers, and microstrips are used on the outer layers. In contrast, coplanar waveguides offer the highest level of isolation, particularly when it comes to RF signals that cross very close traces.
Understanding the importance of the knowledge of the precise value of the dielectric constant of each layer, one of the many tools readily available online may be used to compute the characteristic impedance and, in turn, the width of the trace.
Impedance and Inductance
One strategy that is typically applied by designers is to set a benchmark impedance value, usually 50 ohms, and then only RF components that have this particular impedance are selected.
Given that it is prominently used, this value makes impedance matching easier, enabling each PCB trace to be given the proper width. However, inductance should be maintained as low as possible because this can significantly affect the RF PCB design.
All of that is accomplished by equipping each RF component with a suitable ground connection via multiple through holes and ground planes that are sizable and gap-free. High-frequency elements and traces need to be placed close to ground planes.
Routing
The first rule relates to the curvature and angles of a trace. It is ideal to generate an arc with a curvature radius that is more than 3 times the trace width when an RF transmission line needs to change direction due to routing necessities.
This makes it so that the characteristic impedance is guaranteed to remain unchanged throughout the curvilinear segment. If this isn’t realizable while keeping in mind that right angles must be replaced by two 45° degrees, draw a suitable angle.
It is recommended to install at least two through holes for each crossing of a transmission line when it must cross through two or more layers to reduce the consequent discrepancy in inductance.
When utilizing a via pair with the maximal diameter value compatible with the trace width for the holes, the inductance variation can be lowered by 50 percent. When crossed by sensitive signals, it is very important to have the traces linking the RF components as short as possible, properly spaced away, and oriented orthogonally on the next layers.
The four-layer multilayer construct is the best option for the stackup. Although more expensive than a double-layer approach, the outcomes are far superior and are simple to duplicate. High frequencies cannot tolerate the discontinuities of the ground planes; therefore, continuous ground planes must be introduced beneath the RF signal traces.