Features and How to Select RF Inductors | RF Inductor

Murata RF inductors

Inductors for high-frequency circuits are also called "RF Inductors," and as their name would indicate, they are used in the high-frequency band from tens of MHz to several GHz. As these products require a high Q (Quality factor) value, most have a non-magnetic core structure, and they are mainly used in the high-frequency circuits of mobile communications equipment, such as mobile phones, wireless LAN, and others.

Table 1 Examples of inductor applications in each circuit block of a mobile phone
Application	Location	Purpose
Matching	Lines between components in antenna and IF blocks, etc.	Eliminating impedance mismatch and minimizing reflection and loss
Resonance	Synthesizers and oscillation circuits	Securing the required frequencies
Choke	Power supply lines of functional components used in RF and IF blocks	Cutting AC currents such as high-frequency components

We offers a diverse lineup of inductor products for high-frequency circuits with three different structures: wire wound, multilayer and film.
The features and suitable applications of each structure are briefly described below.

Murata's Technology

1. 3 Manufacturing Technologies

2. High Q Wire Wound Inductor Technology

Realization of 0804 size

Miniaturization was realized by integrating the high precision wire wound technology, and the small size core molding technology with the common technologies of high frequency wire wound products.

3. Film Type Inductor Technology

High Precision Multilayer

Achieved High Q with larger inner diameters, and easier passage of magnetic flux, by high multilayer precision!

Fine Wiring

Achieved fine wiring with a narrow pitch! Achieved wiring of high linearity and dimensional accuracy!

Cross-section of electrode

Achieved low loss at high frequencies by rectangular and high aspect electrodes!

Difference in Characteristics by Each Structure

1. Features of the wire wound structure

The wire wound structure is formed by winding copper wire in a spiral shape around an alumina core. The coil can be formed using thicker wire than the multilayer and film structures, which provides the following features:

Low DC resistance is possible
Extremely high Q (Quality factor) value
Large currents can be supported

These features make the wire wound structure suitable for matching applications in antenna and PA circuits that require an extremely high Q factor, and resonance applications in IF circuits.

Applicable Murata products
LQW04AN_00/ LQW15AN_00/ LQW15AN_10/ LQW15AN_80/ LQW18AN_00/ LQW18AN_10Series

2. Features of the film structure

Film structure chip inductors have a multilayer structure, but the coil is formed with high accuracy over ceramic materials using Murata's original micro-processing technology.
Extremely accurate cores can be formed, providing the following features:

High-performance electrical characteristics can be realized even for compact chips such as 0603 size
Prepared fine lineup of close inductance steps and tight inductance tolerance.
High Q and high SRF

This makes film structure chip inductors suitable for RF circuit matching and resonance applications that require narrow tolerance and a high Q factor to support trends toward smaller and more lightweight mobile communication equipment.

Applicable Murata products
LQP03TN_02/ LQP02TN_02Series

3. Features of the multilayer structure

The multilayer structure is formed by layering ceramic materials and a coil conductor to create an integrated multilayer-type inductor. This enables a smaller size and lower cost compared to the wire wound structure.
While the Q factor is lower than that of the wire wound structure, the multilayer structure provides good overall balance between the L value tolerance, rated current, size, price, and other characteristics, enabling use in a wide range of applications.
The multilayer structure is suitable for various applications such as RF circuit matching, choke, and resonance for mobile communication equipment.

Applicable Murata products
LQG15HN_02/ LQG15HS_02/ LQG18HN_00Series

4. The differences in characteristics based on the structures

Q Factor

The frequency characteristics of the Q are shown in the graph by structure (wire wound, multilayer) of Murata's high frequency coil 1005 size. As shown in Figure 1, the feature of the wire wound type is the very high Q factor compared with the multilayer type.
The feature of the film type is also the Q factor that is higher than the multilayer method which is adopted by other companies in the same industry. Murata offers the film type in the small 0603 size and the 0402 size. (Figure 2)

Figure 1: Comparison of Q Characteristics between Multilayer LQG15 Series and Wire Wound LQW15 Series (both 2.7 nH)

Figure 2: Comparison of Q Characteristics between 0603 Size, LQP03TN Series and Multilayer Products of Other Companies (both 10 nH)

Step and Tolerance of Inductance

The tolerance between Murata's multilayer structure and film structure products of high frequency coils and L value lineup are shown in the following table. Compared with the multilayer type, the position accuracy of the film type is more accurate when forming the coil. Therefore, there is less variation in the L value, which allows for less tolerance and tighter steps.

5. Conclusion

	Wire wound type	Film type	Multilayer type
Structure
Features	Extremely good Q characteristics Low Rdc	Ultra-miniature size High Q value and small size Minimal tolerance in inductance, finely graded inductance levels	Line-up with wide range of inductance values
Uses and applications	Mainly RF matching circuits requiring Q value characteristics choke circuits that support high current levels Antenna matching circuits	Matching circuts of power amplifieras RF matching circuits that require small sizes, minimal tolerance in inductance and high Q	Matching circuits of RF units choke circuits

Effective use of Inductors

High-frequency coils are mainly used in cell phones, wireless LANs and other high-frequency circuits. Some typical examples of their uses and applications are described below.

1. Wire wound type

The wire wound inductors in the LQW series feature a high Q value. Inductors with high Q values are used in the matching circuits of RF units because their high Q values give them excellent attenuation characteristics inside the pass band of the filters. They are also frequently used in the matching applications of antennas for maintaining the transmission and reception sensitivity of the antennas. Furthermore, since they have low Rdc characteristics, they are also employed in choke circuits in which high current levels flow.

2. Film type

The film inductors in the LQP series have a different set of features, since micromachining of the coil patterns is enabled by forming the electrodes using a photolithography technique. The inductors can have smaller sizes and high Q characteristics, while at the same time the series offers a line-up of inductors with inductance values that both deviate minimally and are finely graded. The line-up consists of a wide range of the inductors in the 0603 size, which is becoming the mainstream, and in the 0402 size, which is the smallest in the industry, and both sizes support the trend toward miniaturized sizes. These inductors are used in the matching and resonance circuits of RF units that require miniaturized sizes, minimal tolerance in inductance and finely graded inductance levels. They are also used in choke circuits that demand miniaturized sizes and low Rdc.

3. Multilayer type

Multilayer inductors have the lowest Q value of the three structures. They feature a good overall balance in terms of the inductance value line-up, size and cost, and are used in the matching and resonance circuits of RF units and in all kinds of choke circuits.

For typical usage examples, please refer to"Application Examples"page.

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