Digital system designers may be familiar with some RF components and wiring styles, but there is more to RF circuit design. RF circuits can include integrated circuits, discrete semiconductors, and printed RF components, which work together to produce desired functions. RF circuit design involves combining all these elements to build the whole system and create PCB layout.
The RF circuit is not as intuitive as the typical circuit diagram, and sometimes the diagram may violate the basic electrical design rules. However, due to the propagation characteristics of electromagnetic field, the behavior of circuits operating at RF frequency is very different from that of typical integrated circuits operating at DC or digital frequency band. Whether you are designing a wireless communication system or only need to design a transmission line with a specific impedance, please pay attention to these basic knowledge of microwave engineering.
People often joke that the radio frequency (RF) design of integrated circuits and PCBs is something you only need to know to pass the college qualification examination. However, today’s many special products will need to use mixed-signal components, integrated wireless communication modules, or support radar and other high-frequency applications. RF design is now back in the mainstream. Designers who are unfamiliar with RF design should read this guide to improve their skills.
Introduction to RF Circuit Design
RF circuit aims to imitate standard circuit elements and some simple integrated circuits by using printed components on circuit boards to construct structures. RF circuits may seem a little strange because they don’t always use off-the-shelf components. On the contrary, RF circuits can provide the required functions on the circuit board by using printed traces on the PCB and some additional components.
Printed radio frequency circuit
The printed part of the RF circuit board will use copper traces to construct circuit elements. The arrangement of microstrip lines, capacitor or inductor elements and semiconductors in RF circuits may not seem intuitive, but they make use of the propagation behavior in electromagnetic fields to produce the required electrical behavior. With regard to the RF circuit design and the electrical behavior of the RF circuit on the PCB, some important conceptual points need to be remembered:
Passivity: All printed RF circuits are passive unless active ready-made components are added in the design. However, the active RF components constructed entirely by printed traces have been studied.
Linearity: The RF circuit composed of printed traces is always linear, which means that the voltage and current are related by a linear function (straight line on the graph). Only when nonlinear semiconductor elements (such as diodes) are added to the circuits will these circuits become nonlinear.
Propagation: All RF circuits use waves to propagate. This means that the input impedance needs to be used when determining how to match the impedance around the circuit and how to create interfaces between different parts of the RF circuit.
Signal integrity: RF signal integrity depends on electromagnetic shielding and isolation, because RF signals need to be as noiseless as possible. Many unique shielding structures and layout techniques have been designed to help provide the required shielding and isolation in RF systems.
Active RF circuit
Active RF circuits can include anything from oscillators to drive amplifiers, ADCs, and transceivers.In addition to the print trace, these components can be used to provide additional functionality.Many radar modules, wireless systems, amplifiers and telecommunications components will use active components and passive circuits to route RF signals and provide the required signal propagation behavior.Signal sampling, manipulation and processing are performed by active components, which also provide interfaces back to the digital system.
Layout planning
Just like high-speed digital PCB, successful RF circuit design depends on building a PCB stack that can support RF circuits. The stack design should make the RF components have the required characteristic impedance, although the impedance of the system will be a more complex function of the RF circuit layout and wiring. In addition, the relevant frequency at which your circuit board works will determine how the laminate should be built, what type of printed circuit you may need, and which RF components you can use. RFIC design follows many of the same concepts as RF PCB design, and mastering these concepts will help you succeed in any field of RF design.
Rf circuit board material
FR4 material is suitable for RF transmission lines and interconnects operating at WiFi frequency (~6 GHz). In addition to these frequencies, RF engineers also suggest using alternative materials to support RF signal propagation and printed RF circuit board design. Standard FR4 laminate uses resin-filled glass fiber braid to fix components, but if the manufacturing procedure is not specified correctly, these fiber braiding effects in some materials may cause signal and power integrity problems.
The alternative material system uses PTFE-based laminate and adhesive layer materials to bond the PTFE layer to the next layer in the PCB stack. The loss tangent of these materials is lower than that of FR4 materials, so the signal can propagate further without attenuation, and it is still within an acceptable range. These laminates should form a substrate that supports high-frequency RF transmission lines (such as 77 GHz radar) or low-frequency extremely long interconnects (such as 6 GHz WiFi). The following table summarizes some important material characteristics of common RF PCB materials.
PCB laminated with radio frequency material
Once you have selected laminate and adhesive layer materials for your RF design, you can add them to your laminate. Although you can use RF materials to build the entire multilayer PCB stack, it is usually unnecessary and may be too expensive. One option is to build a hybrid laminate, in which the RF laminate is placed on the top layer to support RF transmission lines and circuits, and the inner layer is used to support the ground plane, digital signal routing and power supply. The facing layer can also support digital components that need to interface with the RF front end, any ADC or other components used to collect RF signals.
If you don’t need digital parts in your RF PCB layout, you can use 2-layer or 3-layer PCB with standard or near-standard thickness RF laminate. Once the PCB layer thickness and material system are determined, the impedance of RF traces needs to be determined.
Calculate RF trace impedance
After determining the lamination, you need to calculate the width of the conductor on the PCB to generate the required impedance (usually 50 ohms) in your RF circuit. Some formulas derived by using a technique called conformal mapping can relate the impedance of traces and their sizes. At present, the best resource for finding formulas to calculate trace impedance with complex dielectric constant is Brian C. Waddell’s transmission line design manual. However, these formulas can’t solve the specific width, so a numerical technique is needed to determine the width required for a transmission line to have a specific impedance.
For more complex arrangements, such as offset striplines or waveguides, a better choice is to use a stack design tool with an integrated field solver. These utilities can take into account the roughness of copper, the taper in the manufacturing process, the differential wiring arrangement, and the position of wiring between layers. They are also easy to use in your PCB design software.
Once you know the impedance of the interconnect, you still need to determine the impedance matching requirements by looking at the reflection simulation results or looking at the data sheet. For transmission lines used in printed radio frequency circuits, the input impedance of different transmission lines is used to determine the impedance matching of a given circuit. If you connect transmission lines and components in RF circuits, you need to include input impedance when designing RF components and impedance matching networks.
Common RF circuit design
It is very important to design PCB stacks before designing RF circuits (especially passive RF circuits), because they need to meet specific impedance targets to work properly. In addition, the printed radio frequency circuit uses the electromagnetic field on the transmission line to propagate, and the propagation behavior will depend on the dielectric function of the substrate material. Once these details are determined, you can start designing your RF circuit and selecting other components for your system.
The radio frequency circuit is designed by calculating the transmission line part for a specific structure on the PCB. Your transmission line design guides the propagating waves to the components, while also providing behaviors such as attenuation, amplification, filtering, resonance and emission (for example, as an antenna). Usually, impedance conversion is needed at the stub, the interface with the component and the antenna to overcome the impedance mismatch seen when the radio frequency signal propagates. Various printing structures that produce these functions are well known in many textbooks.
Some structures and components used in RF circuits and PCBs include:
And passive and active filters.
attenuator
circulator
amplifier
Radio frequency power distributor, distributor and combiner
aerial
resonator
Waveguide cavity
After adding other components, you need to create a circuit schematic before you can start the layout. The process of placing RF circuit in schematic diagram is the same as that used in digital system. Circuit simulation is also important in front-end RF engineering, because you need to evaluate the electrical function of the system before creating the PCB layout. This is usually performed in your design using SPICE simulation, and the printed elements in your circuit board are defined as transmission line objects in SPICE. The best schematic editor of will include transmission line objects to allow you to accurately simulate the electromagnetic behavior in the circuit board.
RF circuit layout tool
Once your RF circuit is designed and passed through the circuit simulation tool in your required frequency range, you can make physical layout. RF designers usually need to carefully design their RF interconnects by mechanical methods, while observing standard high-frequency design rules, such as minimizing the length of vias and traces. Any high-frequency circuit appearing on PCB needs to be designed to meet impedance targets and geometric tolerances, so your CAD tools need to be combined with your electrical design rules to ensure that these targets are met.
If you still have digital components that must be connected to RF circuits, you need to use the same toolset to place them in PCB layout. Careful placement and proper stack design will help to prevent interference with high-frequency circuits and RF signal collection. Native 3D design tools are also very helpful here, because some RF systems are multi-board systems, and the whole assembly needs to be checked before preparing for manufacturing.
When you need to build an advanced RF system while maintaining signal integrity, you need a complete set of circuit simulation tools, PCB routing and layout tools, and layer stack design tools to help you achieve impedance goals. Whether you need to design a low-noise amplifier for signal acquisition, an RF power amplifier for broadcast signals, or a complex interconnection with unique wiring and via structures, the best PCB layout tool will help you maintain flexibility when creating RF PCB layouts.
