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An Overview of RF Circuit Design Basics

Published Date
Author Cadence PCB Solutions

Key Takeaways

  • The Internet of Things (IoT), vehicular radar, high precision positioning sensors, wearable electronics, and implanted devices are a few examples where RF and high-frequency energy is utilized.

  • RF circuit design goals are different from baseband or analog circuits.

  • At high frequencies, PTFE or thermoset polymer materials are preferred. 

  RF circuit

RF circuits enable mobile communication

RF circuit layouts appear to violate basic electrical design rules, so if you aren’t familiar with RF designs, it may seem like these PCBs should be discarded. In RF circuits, signals are conducted as well as radiated, and their propagating nature makes RF circuits appear and act differently from ordinary AC and DC circuits. As most electronics today employ mixed-signals and wireless communication technologies, it is important to have a good understanding of RF designs.

The Basics of RF Circuit Design 

There is a proliferation of RF and mmWave circuits in present-day electronics. These circuits enable key applications in our daily life such as mobile communications, wireless connectivity, low-power communications, industrial automation, medical instrumentation, and automobile systems. The Internet of Things (IoT), vehicular radar, high precision positioning sensors, wearable electronics, and implanted devices are a few examples where RF and high-frequency energy is utilized. As a design engineer, you may develop various innovative mixed-signal circuits where you incorporate DC circuits, baseband circuits, RF circuits, mmWave circuits, etc. To design such complex PCBs with RF sub-circuits, you need a good understanding of microwave engineering.

RF Circuit Design Goals

To simplify, one can say an RF circuit is a special type of analog circuit operating at RF frequencies that enable signal propagation. However, RF circuit design goals are different from baseband or analog circuits. For example, consider a simple RF circuit with a transmitter and receiver. The transmitter and receiver handle both baseband and RF signals. The transmitter input and receiver output belong to baseband frequency, whereas the transmitter output and receiver input correspond to RF signals.

The design goals of the baseband and RF section of the transmitter are entirely different from each other. The baseband section of the transmitter aims to achieve a specified power to transmit, whereas the RF section focuses on not interfering with the transceivers operating on adjacent channels. Similarly, the goals of the receiver baseband and RF sections are the recovery of small signals and rejecting the interferences from undesired channels, respectively. It is critical that designers adhere to baseband and RF circuit goals during the design process.

In RF circuit design, the steady-state operation of a circuit is one of the main goals that designers need to set. Under the steady-state, circuit parameters such as signal fidelity, distortion, noise, and interference are evaluated to gauge the performance of an RF circuit. Hence, RF circuit design should focus on improving signal fidelity and mitigating noise, distortion, and interference issues. The difference in the behavior of RF passive, active components, and substrate material makes a difference in circuit performance, and considerations need to be taken to mitigate such effects. 

Common RF Circuit Design Considerations

As we have discussed above, RF circuit behavior differs from analog or digital circuits. The high operating frequency of RF circuits requires designers to take a few things into consideration during the design process, including:

  • PCB material selection must be given significance, as it influences the performance of the circuit. At high frequencies, PTFE or thermoset polymer materials are preferred over FR4 material. 

  • The RF circuit PCB stackup should be designed so that it provides the desired impedance characteristics to the RF components.

  • Consider the high-frequency behavior and parasitic influence when designing RF circuits involving passive components. 

  • In RF circuits, the crystals used should be accurate and stable compared to those used in digital circuits. 

  • While designing baluns and antennas, the characteristics and requirements of the RF system under design should be considered. 

  • It is possible to give additional functionalities to RF active components, such as low-noise amplifiers and oscillators, by using printed traces. 

Since wireless communication technologies are incorporated in every electronics product, a designer must have a clear grasp of these RF circuit design basics to develop high-performance boards. Cadence’s suite of design and analysis tools offers a mixed-signal design environment that supports analog and RF circuit development.

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