How Ferrites Are Specified for RF Performance

Ferrites are frequency dependent components used to attenuate unwanted high frequency RF signals that can cause failure of emission and immunity compliance tests. There is an endless variety of form factors for ferrites, including toroidal shaped cores, clamp-on cable shells, surface mount beads, leaded through hole components for installation on printed circuit boards, and other custom shapes and sizes for use in many different types of applications.

Choosing a Ferrite Suppression Component

Successfully choosing a ferrite suppression component involves knowing two critical items.

First, determine the frequency range that needs to be attenuated. This frequency may be obtained after having performed pre-compliance testing or perhaps discovered unexpectedly during full-compliance, run for record types of compliance testing. The former situation allows for successful integration of the ferrite component into the design prior to release to production of the end product. The latter situation usually involves adding the ferrite suppression device as an after-thought, usually attaching a clamp-on type of ferrite device to external cables to suppress the RF signal and obtain passing results.

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Ferrites are manufactured using different compositions of materials. Each mixture provides a different suppression capability over certain bands of frequencies. For example, mixture A might provide the most RF suppression from the lower frequency range of 100 kHz to 30 MHz (useful if the end-product is failing conducted emissions or immunity tests), mixture B might provide its best RF suppression from 1 to 300 MHz (where the noise is likely to emanate from cables attached to the end-product), mixture C might work best over the mid-range frequency band of 25 to 300 MHz, and high frequency material D might work best over the higher frequency range of 200 to 1000 MHz.

Pro Tip: Be sure to consult the manufacturer’s data sheets for which mixture works best over the frequency range that needs attenuating. If there is more than one noise source, perhaps one at the lower frequency band and one at the upper, then the use of two different types of ferrites is likely required.

Second, determine the cable size for any signals that may be radiating from the cables attached to the end-product. Ferrite suppression cores are available in a wide variety of internal diameters that fit almost any possible cable size. Impedance is proportional to the square of the number of turns and the core geometry. Impedance, and therefore the suppression available, increases substantially as the number of turns through the core increases. The dimensions of the core also play a vital role in the amount of RF suppression available. Doubling the length or height of the core doubles the impedance. Larger cores provide more impedance and thereby more suppression than smaller ones.

Predicting the Amount of Attenuation from a Ferrite Core

Predicting the amount of attenuation from a ferrite core is possible if the source and load impedance of the offending signal (identified as ZS and ZL, respectively), along with the core’s impedance (available from manufacturer’s datasheet or catalog, identified as ZSC) is known. This attenuation is predicated by taking 20 times the log10 of (ZS + ZL + ZSC) divided by (ZS + ZL).

To utilize this formula in practice you will need to refer to the ferrite manufacturer’s website, datasheet, or catalog, knowing what particular frequency you need to suppress and the cable dimension. From there, select the appropriate material and size of ferrite choke required. Let’s assume that you have a troublesome emission emanating from the power cable at 27 MHz and it is 5 dB over the emissions limit. Looking through the ferrite manufacturer’s website, you determine that mixture C might work best over this frequency and for 2 turns of cable through the core that ZSC = 250Ω. You might also know or have reason to suspect that the offending circuit has ZS = ZL = 50Ω. The predicated attenuation is then equal to 20 x log10 (50 + 50 + 250)/100 » 10.9 dB. The particular ferrite chosen in this example appears to have about 5 dB more attenuation than is required to successfully suppress the unwanted emission to the limit line plus provide another 5 dB for margin, allowing the end-product to pass with ease.

The next step in the process is obtaining samples of the ferrite identified above and repeating testing using the same exact setup (same EUT, same power supply voltage, same cable configuration, etc.) to confirm it works.

The above assumes you have a good understanding of both the source and load impedances of the offending circuit. If the assumption is incorrect then the predicated attenuation will be in error. That’s why it is important to test the chosen ferrite in the actual end-product configuration that the failure was originally observed on.

Pro-Tip: If you’re unsure of the actual source and load impedances of the offending circuit then repeat the above calculation using several different impedance values and select more than one ferrite component that might work. Select impedance values on both the low and high-end of what you think might be possible. Obtain samples of several different cores based on your calculations of possible attenuation, then repeat testing with each different ferrite selected, noting which gives the best attenuation in the actual end-product application. Use that component on all production going forward.

Summary

• Ferrites are frequency dependent components used to attenuate unwanted high frequency RF signals.
• Many different configurations and material types are available.
• Choosing the correct ferrite is dependent upon the frequency of the offending signal and the size of the cable that it must attach to.
• The attenuation possible for a ferrite is dependent upon the material it is made from, its size and core geometry, number of turns through the core, the source and load impedance of the offending circuit, and the core’s impedance.
• Predicting the amount of attenuation that any particular ferrite can provide involves a review of the ferrite’s data sheet and plugging numbers into a simple formula.

“Predicting the Performance of Ferrite Suppression Cores,” Fair-Rite Products Corp. YouTube Channel, November 24, 2020.

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