RF pcb design guidelines

RF pcb design guidelines

How to determine RF PCB substrate materials?

As an early stage in circuit design, PCB substrate material selection plays such a key role in RF/Microwave PCB design that optimal substrate material contributes to excellent performance and high reliability of end products. When considering substrate material in conformity with your PCB design, some aspects have to be focused like relative permittivity, loss tangent, thickness, environment etc. The following content will detail their significance and ideal selection approaches will be displayed.

• Relative Permittivity

Relative permittivity refers to the ratio between dielectric constant and vacuum permittivity. Relative permittivity of substrate materials applied for RF/Microwave PCB design must be sufficiently high to meet demands of space and weight. Other applications such as high-speed interconnect, however, call for extremely low relative permittivity to produce high-impedance circuits with acceptable line width and impedance tolerances.

In advance of final substrate materials determination, some parameters have to be confirmed including line width for a certain range of board thickness, wavelength of circuit working frequency and approximate dimensions of leading components. A sketch of circuit board diagram has to be drawn in order to establish acceptable maximum and minimum relative permittivity.

Moreover, relative permittivity deviation provided by substrate material manufacturer has to be low enough to make electric performance within a tolerance range.

A few decades ago, there wasn’t much demand for RF and microwave circuits. They were difficult to design into the architectures of the time, and so costly that only mil/aero projects could afford them. But today RF circuitry is crammed into a large variety of commercial products. Most of these are handheld wireless devices for medical, industrial, and communications applications, plus applications in a variety of fields are migrating from desktop models to become portable communications units. Not only is RF becoming more ubiquitous, but microwave circuitry as well, both capturing very high frequencies (VHF) and ultra-high frequencies (UHF).

printed circuit boards (PCBs) now encompass much more than pure digital or mixed-signal technologies, and the PCB layout designer faces many more challenges when designing sub-assemblies with high frequency RF and microwave.

The RF frequency range is typically from 500 MHz to 2 GHz, and designs above 100 MHz are considered RF. The microwave frequency range is anything above 2 GHz. There’s a 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, at any point in time an RF signal can be at any voltage and current level between minimum and maximum limits.