Solving RF Isolation Problems with RF Inductors

Many consumer products communicate with each other over broadband networks. From televisions to fiber-optic transmission networks, the bandwidth of data communications is increasing, and the integrity of RF signals has become a major design issue.

This application note illustrates how different inductors can be used as RF isolation in a number of circuits, from narrower bandwidth applications such as portable devices to broadband network data distribution. The various inductors for these applications are described and discussed below.

Why Choose an Inductor?

By its nature, an inductor is a low-pass filter (XL = ωL).

At high frequencies, an inductor becomes a high-impedance element that can be used for RF isolation. High frequencies are blocked from passing through the inductor, but DC current and low-frequency signals are allowed to pass. Without this isolation, antenna efficiency is reduced, signal loss can occur, and RF noise can interfere with other components in the circuit.

The peak impedance of an inductor occurs at its natural resonant frequency (fres = 1/ωLC). The magnitude of the peak impedance is related to the quality factor (Q) of the inductor. High-Q, low-loss inductors (such as Coilcraft's 0603CS) have high peak impedance, while higher-loss inductors (such as Coilcraft's 0603LS) have lower peak impedance (see Figure 1). By changing the winding method of the coil or the materials used in the construction, multiple resonances can be adjusted to produce high impedance over a wide bandwidth. Coilcraft's conical inductors fall into this category. Applications When narrow frequency bands need to be suppressed, such as small signal low-frequency transmission lines in telephones, high-Q ceramic or air-core inductors such as 0603CS can be used, as shown in Figure 1. On the other hand, for audio lines inside telephones, it is important to limit low-frequency losses, and ferrite products such as 0603LS are ideal. When the peak impedance is low, the DC resistance is only one-third or less of the corresponding ceramic inductor.