How Chinese Inductor Manufacturers Design RF Inductors?

Component selection should be a necessary step for engineers to design. Choosing the right inductor can help the RF receiver process signals more efficiently and suppress more peak noise better, but when choosing an inductor, multiple parameters need to be considered comprehensively. So how to correctly choose the appropriate RF inductor?

The selection of RF inductors involves such key parameters as: mounting method (surface mount or in-line), inductance value, current rating, DC resistance (DCR), self-resonant frequency (SRF), quality factor and temperature rating . In applications, inductors are often pursued to be small in size, but the size of an inductor in a given application is often limited by the laws of physics. Inductance value and current rating are the main determinants of its size, after which other parameters can be optimized.

Key 1. Factors that determine the inductance value

If the inductor is used as a simple single-element (first stage) high-frequency choke, it should be selected according to the frequency of the peak noise that needs to be suppressed. At the inductor's self-resonant frequency (SRF), the series impedance will be at its maximum. Therefore, to choose a simple RF choke, look for an inductor with an SRF close to the desired choke frequency.

For higher-order filters, the inductance value of each component must be calculated based on the filter's cutoff frequency (low-pass and high-pass filters) or bandwidth (pass filter). Commercial circuit simulation software such as SPICE, AWR's MicrowaveOffice, and Agilent's Genesys or ADS are usually used for these calculations.

Key 2. The current requirement determines the DC resistance

Current rating and DCR are closely related. In most cases, if all other parameters are held equal, a larger size product will need to be selected to lower the DCR.

Key 3, The self-resonant frequency that allows the inductor to work

The calculation formula of SRF is:

In choke applications, the SRF is most effective at blocking the frequency of the signal. At frequencies below SRF, impedance increases with frequency. At SRF, the impedance reaches a maximum. At frequencies above SRF, the impedance decreases with decreasing frequency.

For higher order filter or impedance matching applications, it is more important to have a flatter inductance curve (constant inductance vs. frequency) near the required frequency. This requires selecting an inductor with an SRF much higher than the design frequency. As a rule of thumb, choose an inductor with an SRF 10 times (10×) higher than the operating frequency. In general, the choice of inductor value usually determines the SRF and vice versa. The higher the inductance value, the lower the SRF will be due to the increased winding capacitance. Key four, the relationship between inductance and impedance and frequency

As shown in Figure 1, the inductance and impedance rise sharply near the self-resonant frequency (SRF) of the inductor. For choke applications, choose an inductor with an SRF equal to or close to the attenuation frequency. For other applications, the SRF should be at least 10 times higher than the operating frequency.  

Key 4. When is the Q value more important?

High Q results in a narrow bandwidth, which is important for applications where the inductor is used in an LC (oscillating) loop or a narrow bandpass. Please refer to Figure 2. The high Q also produces low insertion loss, which minimizes power loss.

The formula for calculating the Q value of an inductor is:

All frequency-dependent real and imaginary losses are included in the Q calculation, including inductance, capacitance, skin effect of conductors, and core losses of magnetic materials. 

Key 5, How to choose the temperature rating

Power loss increases with current and DC resistance, causing the component to heat up. Inductors are usually rated for a certain ambient temperature and the temperature rise above ambient due to current flowing through the inductor. For example, a component rated at an ambient temperature of 125°C and a temperature rise of 15°C due to full rated current (Irms or Idc) will have a maximum temperature of approximately 140°C. You just need to confirm that your application's ambient temperature and current draw do not exceed the inductor's rating.

FKS COILS offers two RF inductors: ceramic and ferrite. Ceramic inductors include wirewound, thin film, and multilayer junction versions. To learn more about our RF inductors, please contact us directly.