Key Parameters
for Selecting
RF Inductors
Understanding RF Inductor Specifications
RF inductor selection involves these key parameters:
mounting (surface mount or through-hole), inductancevalue, current rating, DC resistance (DCR), self-resonant frequency (SRF), Q factor, and temperature rating.
While small size is typically desired, the laws of physics
limit how small an inductor can be for a given application.
Inductance value and current rating are the chief determinants of size; other parameters can then be optimized.

What determines my required inductancevalue?
If the inductor is to be used as a simple one-element(1st order) high-frequency choke, the selection is basedon the frequency of peak noise to be choked out. At theself-resonant frequency (SRF) of an inductor, the series
impedance is at its maximum. So for a simple RF choke
the selection is based on finding an inductor whose SRF
is near the frequency where choking is needed.
For higher order filters, the inductance value for eachelement must be calculated based on the filter cutoff frequency (for low-pass and high-pass filters) or bandwidth(for bandpass filters). These calculations are frequently
performed using commercial circuit simulation programs,
such as SPICE, AWR’s Microwave Office, and Agilent’s
Genesys or ADS.
For tuned circuits or impedance matching, a tightinductance tolerance is desirable. As shown in Table 1,
wirewound inductors typically achieve tighter tolerances
than multilayer or thick film type inductors.

How does my current requirement affect myselection?
Higher current requires larger wire or more strands of thesame wire size to keep losses and temperature rise to aminimum. Larger wire lowers the DCR and increases theQ, but at the expense of larger part size and possibly lower
SRF. The current ratings shown in Table 1 indicate thatwirewound inductors are superior to multilayer inductorsof the same size and inductance value.
Higher current capacity and lower DCR can be achievedby using a ferrite core inductor with a lower turn count. Ferrite, however, may introduce new limitations such as larger
variation of inductance with temperature, looser tolerances,lower Q, and reduced saturation current ratings. Ferrite
inductors with open magnetic structures, like Coilcraft’s
LS series, will not saturate, even at full rated current.

It looks like my current requirement
determines the DC resistance.
Current ratings and DCR are closely related. In most cases,
if all other parameters are held equal, a larger part size isneeded to decrease the DCR.

How high must the Self-Resonant
Frequency be for the inductor to work?

12 LCFor choke applications, the SRF is the frequency thatprovides the best signal blocking. At frequencies below
the SRF, impedance increases with frequency. At the SRF,

SRF is defined by the equation: SRF = f0 =TABLE 1
Inductance(nH)

Inductancetolerance

Q at 900 M Hz

Current rating(mA)

Wirewound (Coilcraft 0402DC-2N8XGL)

2.82%

782100Multilayer (TDK MLK1005S2N7ST)

2.7

11%

31500

Inductance(nH)

Inductancetolerance

Q at 900 M Hz

Current rating(mA)

Wirewound (Coilcraft 0402DC-68NXGL)

682%

54400

Multilayer (TDK MLK1005S68NJT)

68

5%

20150

Document 671-C

Revised 01/07/21

impedance is at its maximum. At frequencies above theSRF, impedance decreases with frequency.
For higher order filter or impedance matching applications,
it is more important to have a relatively flat inductancecurve (constant inductance vs. frequency) near therequired frequency. This suggests selecting an inductor
with an SRF well above the design frequency. A rule ofthumb is to select an inductor with an SRF that is a decade(10×) higher than the operating frequency. In general, thechoice of inductance value typically determines the SRF
and vice versa. The higher the inductance value, the lower
the SRF, due to increased winding capacitance.

Inductance and Impedance vs. Frequency
Inductance and impedance rise sharply near an inductor’s
self resonant frequency (SRF) as shown in Figure 1. For
choking applications, choose an inductor whose SRF is ator near the frequency to be attenuated. For other applications, the SRF should be at least 10 times higher than theoperating frequency.

50000

1000

37500
Inductance025000– 100012500– 2000

Impedance
0500

1000

1500

2000

Impedance (Ohms)

Inductance (nH)

SRF
2000

02500

Frequency (MHz)Figure 1. Inductance and Impedance for a 100 nH
Wirewound Inductor

When is Q factor important?
A high Q value results in a narrow bandwidth, which isimportant if the inductor is to be used as part of an LC
tank (oscillator) circuit or in narrow bandpass applications.
Refer to Figure 2. High Q also leads to low insertion loss,
minimizing power consumption.

Figure 2. High Q Gives Narrow Bandwidth andLow Insertion Loss

Im[Z]
Re[Z]
All frequency dependent real and imaginary losses areincluded in the measurement of Q, including inductance,
capacitance, skin effect of the conductor and core losses
from the magnetic material. As indicated in Table 1, wirewound inductors have much higher Q values than multilayer
inductors of the same size and value.

Q factor of an inductor is calculated from: Q =How do I select the temperature rating I need?
Power loss increases with current and DC resistance,
causing the component’s temperature to rise. Inductors
are typically rated for a specific ambient temperature anda temperature rise above ambient due to current throughthe inductor. For example, a part rated for 125°C ambient and a 15°C rise due to the full current rating (Irmsor Idc) will have approximately a 140°C maximum parttemperature. You only need to verify that your application
ambient temperature and current draw do not exceed theinductor ratings.

How can I quickly search for inductors thathave all of the characteristics I need?
Comparing data sheets of inductor manufacturers canbe time consuming. Coilcraft’s RF Inductor/ChokeFinder and Analyzer tool allows selection of inductors
based on six different parameters. By entering just theinductance value and operating frequency, a list of all
inductors that meet your requirements is presented.

Document 671-CRevised 01/07/21