EMI Suppression Filters (EMC and Noise Suppression)Selecting Parts for Noise Suppression

Select EMI filter that are highly effective for removing noise at a carrier frequency causing interference.

If a normal-type ferrite bead is used, the carrier frequency impedance may be insufficient depending on the self-resonance frequency.
In that case, select a BLM-H series (for suppressing GHz band noise) or BLM-G series (for suppressing High-GHz band noise) ferrite bead with a high GHz band impedance.

Signals on the interface line to the LCD or camera module, which are likely to be a noise releasing source have become faster with an increase in data volume handled.
With this high-speeding of signals, the signal frequency and the noise frequency tend to come close to each other. To prevent degradation of the signal quality due to a loss of the harmonic component of signals by noise suppression parts, select EMI filter with a sharp impedance curve or insertion loss curve.

The measurement result of noise received by the main and sub antennas showed that noise is coupled with the antennas in the LTE communication band. Therefore, an EMI filter was used for the LCD data line. The part was selected from the following points.

We selected an LC composite-type EMI filter with a sharp insertion loss curve and a high selectivity of signal frequency and noise frequency because the frequency band at which noise must be removed is relatively low (700 MHz) that is close to the frequency band (up to 100 MHz) that must pass the signal frequency though the signal frequency is 12 MHz. The EMI filter is array-type NFA18SL series because of many signal lines. While checking the balance between the noise removing effect and the signal quality, we selected a cutoff frequency of 220 MHz.

We also selected a ferrite bead for the power line as a normal part for noise suppression because the power line can also be a noise releasing source. The ferrite bead was selected from the following points.

An inductor-type EMI filter can remove noise more effectively because the power line impedance is low. Considering the frequency at which noise must be removed, we selected the BLM15EG series for large current whose impedance characteristics cover the GHz band. We selected a filter that has rated current that covers the current value flowing in the power line.

We selected a ferrite bead that need not be connected to the ground because the frequency band 2 GHz at which noise must be removed has some margin to the signal frequency 24 MHz. But we selected the BLM15GA/GG series whose impedance characteristics cover up to several GHz because the frequency at which noise must be removed is high (2 GHz band). Among BLM15GA/GG series ferrite beads, we selected this time BLM15GA750SN1 because its impedance is the highest at 2 GHz and it maintains the 24-MHz clock waveform.

If the mounting space is restricted, the noise suppression effect lowers somewhat but BLM03HD331SN1 is also available.