Selection of RF Inductor

Inductance is one of the most important components in high-frequency analog circuits and signal processing for mobile phones, RFID, testing equipment, GPS, radar, Wi-Fi, satellite radio, and other applications. Generally, it can undertake several main functions including circuit tuning, impedance matching, high-pass and low-pass filters, and can also be used as RF throttling.

There are multiple options for an electronic engineer who chooses to use RF Inductor in the design. To simplify this selection, this article will discuss various types of inductor components and their common usage.

Usage of RF Inductor

Most electronic devices contain RF inductors. "The glass tubes implanted in the skin of our home animals contain an inductance to track animals," said Maria Del Mar w.arrubia, a R & D engineer at plemer, "each time a car is started, wireless communication is generated between two inductors, one inside the car and the other inside the key."

Figure 1 RF inductor is one of the basic components of High-Frequency Electronic Devices

However, just as such components are ubiquitous, RF inductance also has very specific purposes. In a resonant circuit, these components are usually used together with capacitors to select a specific frequency (such as an oscillating circuit or a voltage controlled oscillator ).

RF inductor can also be used for impedance matching to achieve impedance balancing of data transmission lines. This is required to ensure efficient data transmission between ICS.

When used as an RF throttling ring, inductance is connected in the circuit to act as an RF filter. In simple terms, RF Throttling is a low-pass filter that degrades a high frequency while a low frequency is unobstructed.

Q value

Q is the most important metric when discussing inductance performance. The Q value is an indicator for measuring the inductance performance. It is a non-dimensional parameter used to compare the oscillation frequency and Energy Loss Rate.

Deryl J. kimbro, Senior Product Manager at Murata, said: "The higher the Q value, the closer the inductance performance is to the ideal lossless inductance. That is to say, it has better selectivity in the resonant circuit ."

Another advantage of high Q value is low loss, which means less energy is consumed by the inductor. A low Q value may cause a wide bandwidth, and the resonant amplitude at and near the oscillation frequency is low.

Inductance Value

Apart from the Q factor, the true measurement of inductance is its Inductance Value. For audio and power applications, the Inductance Value is usually several Henrys, while for high-frequency applications, a much smaller inductance is usually within the range of one or more hens.

The inductance value depends on several factors, including the structure, core size, core material, and actual coil turns. There are both fixed inductance values and adjustable inductance values.

Other Specifications

The inductance value is not the only important value. DC resistance, current, and self-resonance frequency (SRF) are some more useful specifications provided in the RF inductor data sheet.

"Depending on the Application Scenario, each feature may be a key factor to consider and determine other features," Del Mar arrubia said. For example, if the element will be used in the tire pressure monitoring system, it is important to have the inductance stability within a wide temperature range, and this requirement will determine the choice of the core ."

Rated current

When selecting an inductor, the operating current should be lower than the rated current in the manual. If the current exceeds the rated current, the product may be damaged.

DC resistance (DCR)

Kimbro said that the DC resistance (DCR) is closely associated with the rated current. Based on the coil resistance, the DC resistance is equal to the loss of the inductance. If the winding diameter increases, the DC resistance decreases and the rated current increases. The large winding diameter reduces the loss and improves the current processing capability.

Doug Lillie, product marketing manager at Vishay's inductance department, said: "DC resistance will limit the DC current that the device can transmit without heat or saturation (sharp reduction in the induction coefficient."

Self-resonance frequency (SRF)

Each turns in the inductance can be regarded as a capacitor plate. The overall effect of the capacitance between turns and between the coil and the core can be expressed by a single capacitor in parallel with the inductance, it is called a distributed capacitor (CD ). The resonance frequency of this parallel structure is called the self-resonance frequency (SRF ).

Lillie said: "At this frequency, the inductance looks like a pure resistance with an impedance. If the frequency exceeds the self-resonance frequency, the parallel structure will become the main factor in the Resistance ."

Laminated Chip Inductor

Laminated chip inductor is made of ceramic material structure through the integration process. The structure of ceramic material can provide good performance at high frequencies, while the laminated chip process provides a variety of Inductance values.

The Inductance Value Range of laminated devices is wider than that of thin films or empty-core coils, but less than the Inductance Value range or rated current of wirewound components. Due to its excellent electrical characteristics, especially its low cost, laminated chip technology is becoming increasingly popular.
Thin Film Inductor

Thin-Film inductor is produced by means of a lithography process that produces very precise coil patterns on the ceramic substrate to meet harsh inductance tolerances. The ceramic substrate makes these inductors an ideal component for RF Applications. However, the thin film inductor can transmit less current, and the Inductance Value range is limited.

Winding Inductance

Winding Inductance is usually used in low frequency applications. Winding Inductance is made by winding copper wires around the ceramic (alumina) core.

Due to its structure and material, the Winding Inductance can provide excellent electrical properties. The horizontal winding structure makes the tolerances small and stray capacitance small, while the copper wire makes the DC resistance small, which increases the quality factor performance and rated current.

Conical Inductor

The conical inductor is applicable to broadband and high-frequency applications. Its structure can increase the bandwidth of the coil. The actual size of the conical inductor is small, which is usually made up of thin wires. Therefore, the stray capacitance is small.

In ultra-wideband bias-T devices, the conical inductor also provides DC offset extraction or injection paths, which can isolate the power supply from the active device.

Core Selection

High-frequency devices usually use hollow or inert (ceramic) cores. They provide better thermal performance than magnetic cores, but their inductance values are limited.

If devices usually use cores. The core is not saturated, but it cannot provide a large Inductance Value like the ferrite core. Low-frequency devices usually use ferrite cores. Do not use ferrite cores as much as possible because they will be saturated at a small IDC value and will be affected by temperature (△l/△t ).

Manufacturers are also developing and using newer ferrite such as amorphous and nano-crystalline materials.