This article reviews passive components used in RF circuits. Like digital and low-frequency circuits, RF circuits include passive and active components. Passive RF circuits perform functions without external power, while active RF circuits require an external power supply.
Common Passive Components
Passive circuits include basic components such as resistors, capacitors, and inductors, as well as functional devices built from these components or from transmission-line structures. Typical functional devices include power dividers, directional couplers, circulators and isolators, filters and multiplexers, and antennas.
Resistors, Inductors and Capacitors
The three basic components in circuits are the resistor, inductor, and capacitor. They are critical in both digital and RF systems. In digital or low-frequency circuits, these components typically exhibit primarily resistive, inductive, or capacitive behavior. In RF circuits, however, each component has a specific operating frequency band; once the frequency exceeds that band, the component characteristics change.
RF resistors are usually modeled with distributed parasitic elements. Pay attention to lead inductance and lead capacitance, which determine the frequency response. As frequency increases, capacitive behavior can become more pronounced, and inductive behavior can also emerge. At a certain frequency, lead inductance and lead capacitance form a resonance; beyond that resonance point, inductive behavior typically dominates.
Inductors and capacitors show similar shifts in behavior with frequency. In RF circuit design, confirm the operating frequency range of chosen components to avoid unexpected performance issues.
Power Dividers
Power dividers split signals into two or more paths, either equally or unequally. Power dividers are widely used in RF systems, particularly in antenna feed networks. The Wilkinson power divider is a common topology; cascading stages can achieve 1-to-N splitting.
Directional Couplers
Directional couplers allow controlled sampling or routing of RF power with defined coupling and phase relationships. A directional coupler is a four-port device typically labeled as input, output, coupled port, and isolated port. It is used when a signal needs to be distributed with specific phase and amplitude relationships.
Circulators and Isolators
Circulators are non-reciprocal devices that are often used as isolators. An isolator provides unidirectional transmission of electromagnetic power: forward power is delivered to the load with low loss while reflected power from the load is heavily attenuated. This protects the source from load variations. Key performance requirements for isolators include low forward insertion loss (typically not exceeding 0.5 to 1 dB), high reverse isolation (typically greater than 20 to 30 dB), low voltage standing wave ratio (VSWR, typically not greater than 1.10 to 1.25), sufficient bandwidth, and specified power handling and operating temperature ranges.
Filters and Multiplexers
Filters act as channel cleaners in RF systems. They are essential for image rejection and for suppressing spurious signals and harmonics. Filter technologies range from dielectric waveguide filters to surface acoustic wave (SAW) and bulk acoustic wave (BAW) filters, depending on the application and frequency range.
Antennas
Antennas are a critical element in any wireless system. Key antenna parameters include radiation pattern, gain, impedance matching, bandwidth, and polarization. Antenna selection and feed network design have a major impact on system performance.
Other passive RF components include attenuators, phase shifters, and various passive RF switches and networks.
