Noise Suppression Filter Guide

Basics of Noise Countermeasures Lesson 12: How to Make the Best Use of LC Compound-type EMI filters

In this lesson we will talk about what you need to know to effectively use LC compound-type EMI filters.
Previously, LC compound-type EMI filters were mainly used as noise countermeasures for analog video lines. Recently, however, use is increasing in single-ended digital bus lines such as the flat cables used to connect from the main board to the LCD panel of mobile phones. For this reason, most LC compound-type EMI filters are array types that include four circuits on a single chip.

 
Figure 1: Example of a 4-element array-type LC compound-type filter: NFA18SL series

1. π-type or T-type

LC compound-type filters have various configurations, such as C-L-C configuration π types, L-C-L configuration T-types, and C-L or L-C configuration L-types. The impedance of the circuits connected before and after the filter provides a hint as to which configuration to choose. Although there are exceptions, the capacitors in LC filters are low-impedance capacitors that function to bypass high frequencies to ground to suppress noise, so the effects increase when the areas next to the capacitor are high-impedance.
On the other hand, inductors have high impedance and function to reject high frequencies, so the effects decrease when connected next to high-impedance areas, and increase when connected next to low-impedance areas. Figure 2 summarizes this relationship, and shows the filter configurations with high noise-suppression effects for each input and output impedance level condition. This chart shows that the effects change when the input and output of an asymmetrical filter such as an L-type filter are inverted, so care should be taken for use.

 
Figure 2: Highly effective filter configurations for different input and output impedance combinations

2. Filters with multiple resonance frequencies

The introduction mentioned that, recently, LC compound-type filters are increasingly used as noise countermeasures for mobile phones. However, a characteristic of noise countermeasures for mobile phones is that countermeasures are needed not only to suppress noise leaking to the outside of the set, but also to suppress noise interference within the set. 

Mobile phones perform large amounts of digital processing, and the resulting noise may lower the reception sensitivity or cause other problems if it interferes with the reception radio wave of the mobile phone. When considering countermeasures for noise leaking to the outside of the set, the devices affected by the noise are relatively distant, but noise interference within the mobile phone requires stronger noise countermeasures due to the short distances involved. 

Normally, the frequencies that require countermeasures against this noise interference are the mobile phone carrier frequency band from around 700 MHz to 2 GHz. However, when the mobile phone is equipped with a TV tuner, countermeasures must also be taken for frequencies around 500 MHz. Normal LC-type filters have a single clearly evident self-resonance frequency, but this type of filter attenuates noise over only a narrow band, and does not provide sufficient performance when countermeasures must be taken for a wide range as described above. 

Therefore, LC compound-type filters with multiple resonance frequencies are also provided. Figure 3 shows an example of a filter designed with self-resonance frequencies around 2 GHz and 500 MHz to provide noise suppression effects for both the TV frequency band and the mobile phone carrier frequency band. In this manner, noise can be effectively suppressed by selecting the filter in consideration of the frequency bands that require noise countermeasures and the insertion loss characteristics data. Please try different combinations to obtain the highest noise suppression effects for your set.

Figure 3: Example of an LC compound-type filter with two different self-resonance frequencies

Written by: Yasuhiro Mitsuya, Component Business Unit, Murata Manufacturing Co., Ltd.

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