What exactly is a radio frequency inductor?

RF inductors have a variety of uses and are available in a variety of construction types to meet the performance needs of a specific application.Matching, resonators, and chokes are common uses for inductors in RF circuits.Matching involves eliminating impedance mismatches and minimizing reflections and losses in lines between circuit blocks, such as antennas and radio frequency blocks or intermediate frequency (IF) blocks.Resonance is used in synthesizers and oscillator circuits to tune the circuit and set the desired frequency.

When used as chokes, inductors can be placed in the power supply lines of functional components, such as RF blocks or IF blocks, with the purpose of attenuating high-frequency AC currents.The bias tee allows DC current to bias active devices such as diodes.The DC bias current and AC/RF signal are superimposed together and output from the AC+DC output port.

Prejudice tee diagram

RF inductor specifications

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

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

The saturation current is the direct current that 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 is related to physical characteristics.

Self-resonant frequency (SRF) refers to the frequency beyond which the sensor 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 terminal electrodes or between turns of a wound 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 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, which provides better signal blocking.

Q-factor is a dimensionless parameter that describes the underdamping of an oscillator or resonator.It is approximately defined as the ratio of the initial energy stored in the resonant cavity to the energy lost within one arc 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 when the inductor is used as part of an LC cell (oscillator) circuit or in narrow bandpass applications.High Q can also reduce insertion loss and minimize power consumption.All frequency-dependent real and imaginary losses are included in the measurement of Q, including inductance, capacitance, the skin effect of conductors [1] and core losses of magnetic materials.

How specs weigh

The physical RF inductor is a non-ideal device, including parasitic resistance, inductance and capacitance, which are non-linear and affect performance, thus requiring trade-offs between various performance specifications.For example:

Higher currents require larger wires to keep losses and temperature rise to a minimum.While larger wires reduce DCR and increase Q, they come at the expense of larger part size and possibly lower SRF.In terms of rated current, wirewound inductors are superior to multilayer inductors of the same size and inductance value.For multilayer inductors with the same size and inductance, the Q value of wirewound inductors is much higher.

Using a lower number of turns ferrite core inductor results in higher current capacity and lower DCR.However, ferrites may introduce new limitations, such as inductance changes with temperature, looser tolerances, lower Q, and reduced saturation current ratings.Ferrite inductors with open magnetic structure will not saturate even at full rated current.

Selection of RF Inductor Structure

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

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

Ferrite or core chip inductors are wirewound RF chokes used to provide isolation and broadband filtering without the need for core saturation.They provide 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 film or air core, but cannot match the inductance range or current rating of wirewounds.

Air-core inductors are wirewound RF chokes that provide isolation and broadband filtering without the need for core saturation.They provide the highest inductance and lowest DCR at a given EIA size.

Tapered and broadband inductors have high impedance over a wide bandwidth.Tapered inductors are suitable for ultra-wideband bias tees up to 100GHz.In broadband bias 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 bias tees and can be used in communications platforms and RF test setups up to 100 GHz

RFID and NFC transponder sensors are specialized devices that provide high sensitivity and long read range in transponder tags and NFC/RFID antennas.They can be optimized for applications such as tire pressure monitoring that require 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 capabilities.Once a specification is developed, a large number of construction options must be sorted before arriving at the optimal component for a particular application.

Prejudice tee diagram

RF inductor specifications

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

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

The saturation current is the direct current that 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 is related to physical characteristics.

Self-resonant frequency (SRF) refers to the frequency beyond which the sensor 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 terminal electrodes or between turns of a wound 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 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, which provides better signal blocking.

Q-factor is a dimensionless parameter that describes the underdamping of an oscillator or resonator.It is approximately defined as the ratio of the initial energy stored in the resonant cavity to the energy lost within one arc 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 when the inductor is used as part of an LC cell (oscillator) circuit or in narrow bandpass applications.High Q can also reduce insertion loss and minimize power consumption.All frequency-dependent real and imaginary losses are included in the measurement of Q, including inductance, capacitance, the skin effect of conductors [1] and core losses of magnetic materials.

How specs weigh

The physical RF inductor is a non-ideal device, including parasitic resistance, inductance and capacitance, which are non-linear and affect performance, thus requiring trade-offs between various performance specifications.For example:

Higher currents require larger wires to keep losses and temperature rise to a minimum.While larger wires reduce DCR and increase Q, they come at the expense of larger part size and possibly lower SRF.In terms of rated current, wirewound inductors are superior to multilayer inductors of the same size and inductance value.For multilayer inductors with the same size and inductance, the Q value of wirewound inductors is much higher.

Using a lower number of turns ferrite core inductor results in higher current capacity and lower DCR.However, ferrites may introduce new limitations, such as inductance changes with temperature, looser tolerances, lower Q, and reduced saturation current ratings.Ferrite inductors with open magnetic structure will not saturate even at full rated current.