“RF inductors are versatile and are available in a variety of construction types to meet the performance requirements of specific applications. Matching, resonators, and chokes are common uses of inductors in RF circuits. Matching includes eliminating impedance mismatches and minimizing line reflections and losses between circuit blocks such as an antenna and a radio or intermediate frequency (IF) block. Resonance is used in synthesizers and oscillator circuits to tune the circuit and set the desired frequency.

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RF inductors are versatile and are available in a variety of construction types to meet the performance requirements of specific applications. Matching, resonators, and chokes are common uses of inductors in RF circuits. Matching includes eliminating impedance mismatches and minimizing line reflections and losses between circuit blocks such as an antenna and a radio or intermediate frequency (IF) block. Resonance is used in synthesizers and oscillator circuits to tune the circuit and set the desired frequency.

When used as choke coils, inductors can be placed in the power lines of functional components, such as RF blocks or IF blocks, in order to attenuate high-frequency AC currents. Bias tees allow DC current to bias active devices such as diodes. The DC bias current and the AC/RF signal are superimposed and output from the AC+DC output port.

Bias tee schematic

RF Inductor Specifications

Inductance is the property of an electrical conductor that resists changes in the current flowing through it. It is the ratio of the induced voltage to the rate of change of the current that produces the induced voltage, measured in Henry (H). RF inductors typically have inductance ratings in the range of 0.5 nanohenry (nH) or less, to hundreds of nanohenries. Inductance depends on construction, core size, core material, and number of turns. Inductors are available with fixed or variable inductance values.

The DC current rating (DCR) is related to the DC resistance in amperes. DCR determines the amount of current an inductor can handle without overheating or saturating. This is an important metric when considering the thermal performance of the inductor. Power loss increases with current and DC resistance, which causes an increase in inductor temperature. The temperature rating of an inductor is usually a specific ambient temperature, and the rise in temperature is due to the current flowing through the inductor. For example, a part rated at 125°C ambient temperature and a 15°C rise due to full rated current (Irms or Idc) will have a maximum temperature of approximately 140°C.

Saturation current is direct current, which reduces the inductance to a certain value. The inductance drops because the core can only contain a certain amount of flux density. The saturation current is related to the magnetic properties of the inductor. DCR describes the maximum DC current an inductor can pass, and it is related to physical properties.

Self-resonant frequency (SRF) is the frequency above which the inductor stops working. Generally speaking, due to the influence of parasitic capacitance, the larger the inductance, the lower the self-resonant frequency (SRF), and vice versa. Inductors have low distributed capacitance between electrodes at both ends or between turns of a wire conductor, and the inductance of the device resonates with the distributed capacitance at the SRF. In SRF, the inductor acts as a resistor with impedance. At higher frequencies, distributed capacitance dominates.

When selecting inductors in high frequency circuits and modules, it is not enough to consider only the required inductance; the SRF should be at least 10 times higher than the operating frequency. For choke applications, SRF is the frequency at which the impedance reaches its maximum value, which provides better signal blocking.

The Q factor is a dimensionless parameter that describes the underdamped condition of an oscillator or resonator. It is approximately defined as the ratio of the initial energy stored in the cavity to the energy lost within one radian of the oscillation period. The Q factor can also be defined as the ratio of the center frequency of the resonator to its bandwidth when driven by oscillation.

High Q results in narrow bandwidth, which is important for inductors as part of an LC cell (oscillator) circuit or in narrow pass applications. High Q also reduces insertion loss and minimizes power dissipation.All frequency-dependent real and imaginary losses are included in the measurement of Q, including the skin effect of inductance, capacitance, conductors[1]and core losses of magnetic materials.

How the specs weigh in

A physical RF inductor is a non-ideal device including parasitic resistances, inductances, and capacitances, which are nonlinear and affect performance, requiring trade-offs between various performance specifications. E.g:

Higher currents require larger wires, this is to keep losses and temperature rise to a minimum. While larger wires reduce DCR and increase Q, it comes at the expense of larger part size and possibly lower SRF. In terms of current rating, wirewound inductors outperform multilayer inductors of the same size inductance value. For multilayer inductors of the same size and inductance, the Q value of wirewound inductors is much higher.

Higher current capacity and lower DCR can be achieved by using lower turns ferrite core inductors. However, ferrites may introduce new limitations such as inductance variation with temperature, looser tolerances, lower Q, and reduced saturation current ratings. Ferrite inductors with open magnetic structures do not saturate even at full rated current.

Selection of RF Inductor Structure

There are several manufacturing methods available today to mitigate the effects of various parasitics and optimize the RF inductor characteristics to meet the needs of specific applications.

Ceramic core chip inductors are used for narrowband filtering in RF and microwave frequency communication equipment. They offer very high Q values ​​and can reduce inductor tolerances down to 1%.

Ferrite or iron core chip inductors are wirewound RF choke coils used to provide isolation and broadband filtering without the need for core saturation. They offer the highest inductance and lowest DCR for a given EIA size.

Multilayer chip inductors provide low DCR, high Q and high temperature operation. The ceramic material structure enables high performance at high frequencies, and the multilayer process provides a wide range of inductance values. Multilayer devices can provide a wider inductance range than thin-film or air core, but cannot match the inductance range or current rating of wirewounds.

Air core inductors are wire wound RF choke coils that provide isolation and broadband filtering without the need for core saturation. They offer the highest inductance and lowest DCR at a given EIA size.

Tapered and broadband inductors have high impedance over a wide bandwidth. Tapered inductors for ultra-wideband bias tees up to 100GHz. In wideband biasing applications, a single tapered inductor can replace multiple cascaded narrowband inductors.

Broadband tapered RF inductors are suitable for applications ranging from test instrumentation to microwave circuit design.These broadband inductors work well in biased tees and can be used in communication platforms and RF test setups up to 100 GHz

RFID and NFC transponder sensors are specialized devices that provide high sensitivity and long read distances in transponder tags and NFC/RFID antennas. They can be optimized for applications such as tire pressure monitoring requiring high performance in harsh mechanical environments and high temperature operating environments.

Inductors are an important part of the RF/Microwave signal chain. Categorizing them can be difficult and requires understanding the various properties. Once a specification has been developed, a large number of construction options must be sorted before reaching the optimum assembly for a particular application.

[1] The skin effect generally refers to the skin effect. When there is alternating current or alternating electromagnetic field in the conductor, the current distribution inside the conductor is uneven, and the current is concentrated in the “skin” part of the conductor, that is to say, the current is concentrated in the thin layer on the outer surface of the conductor. The closer to the conductor surface, the greater the current density. , the current inside the conductor is actually smaller.