Radio frequency (RF) and microwave circuits in applications such as radar, magnetic resonance imaging (MRI), communication systems, and medical electronic devices require precise, stable, and low loss application specific passive components. These requirements are particularly applicable to inductors, as they must have a stable inductance value and be minimally affected by temperature and frequency. They must also minimize losses and avoid self resonant frequencies (SRFs) within the application bandwidth to maintain signal integrity.
This article reviews the requirements and characteristics of inductors in RF applications. Then Knowles' high-Q ceramic core inductors were introduced, which designers can use to meet the most demanding RF application requirements.
Inductors in RF circuits
Inductors are passive reactive components that counteract changes in current by storing energy in a magnetic field. They are composed of a coil with a relatively simple structure, but the coil generates several parasitic elements. The equivalent circuit of an inductor includes lead inductance and capacitance, coil resistance, and turn to turn capacitance. (Figure 1).
Equivalent circuit diagram of inductor
Figure 1: The equivalent circuit of an inductor includes parasitic components such as inductance, capacitance, and resistance. (Image source: Art Pini)
Inductors have various functions in RF circuits, ranging from simple chokes used to isolate AC and DC signal components to fine-tuning devices used in energy storage circuits and filters. They operate at radio and microwave frequencies, and to minimize reflection and standing waves, components with smaller parasitic capacitance and inductance must be used in their design. In such applications, frequency dependent effects such as skin effect and radiation must also be considered. RF inductors cannot tolerate losses when processing small signals and require high quality factor (Q) and low equivalent series resistance (ESR). Therefore, inductor specifications not only include inductance, tolerance, and rated power, but also include some RF specific requirements, mainly Q value, SRF, and ESR.