LC FiltersBasic knowledge about LC filters

A filter receives something and extracts the desired portion while removing the undesired portion. This process is called filtering.
High-frequency refers to waveforms, such as electrical waves or sound waves, with high frequencies.
A high-frequency filter allows signals at desired frequencies to pass through while blocking signals at unwanted frequencies. Such filters are sometimes referred to as RF filters.

LC filters are components comprising inductors (L) and capacitors (C). They block signals at unwanted frequencies while allowing signals at desired frequencies to pass through.
Murata LC filters support a wide range of frequencies from 300 ＭHz to 40 GHz.

LC filters have a multilayer structure consisting of low temperature co-fired ceramic (LTCC) with a low temperature coefficient and copper printed coils. The multilayer structure of the inductors and capacitors makes it possible to achieve greater compactness.
The filter waveform can be altered by changing the coil pattern or the capacitor pattern.

LTCC stands for low-temperature co-fired ceramic.
The characteristic feature of LTCC is that it is fired at a relatively low temperature of around 900°C, rather than the usual temperature of 1,500°C or higher that results in the formation of glass components intermixed with the ceramic material. This allows use of copper or silver, which have low conductor resistance, for the internal wiring.

Murata’s use of copper for the internal electrodes in LTCC suppresses ion migration^* and provides low-loss electrical characteristics, making it possible to commercialize superior LC filters.

• *Ion migration: A phenomenon where, when voltage is applied in a highly humid environment, the anode metal in the wiring pattern can become ionized, causing ions to migrate to the cathode, and then the same thing takes place with the cathode metal. There is a danger that a short circuit may result.

The example below shows a circuit diagram of a low-power wide-area (LPWA) network.
The figure on the left shows the use of 10 inductors (L) and capacitors (C) to configure an 868 ＭHz low-pass filters + balun.
The figure on the right shows the use of a single LC filter to produce an equivalent result. LC filters provide the following four benefits.

LC filters can be classified into four types based on the frequencies they allow to pass through.

This type of filter allows low frequencies to pass through. Frequencies lower than the cutoff frequencies can pass through, hence the name low-pass filter.

This type of filter allows high frequencies to pass through. Frequencies higher than the cutoff frequencies can pass through, hence the name high-pass filter.

This type of filter allows frequencies within a specific range to pass through. Only frequencies within the specific range can pass through, hence the name band-pass filter.

This type of filter blocks frequencies within a specific range, hence the name band-elimination filter.

Some terms often used in connection with LC filters are explained below.

Insertion loss is sometimes abbreviated as IL.
Insertion loss is the amount of loss of signal power in the desired frequency band, expressed in decibels (dB), that occurs when the signal passes through the product. If the insertion loss characteristics are good, the filter has little loss of signal power and can be said to have low insertion loss. If the insertion loss characteristics are poor, there is significant loss of signal power as it passes through the product and in serious cases the signal may not be able to pass through at all.
For this reason, products with good insertion loss characteristics are superior.

Attenuation is sometimes abbreviated as ATT.
Attenuation is the amount of reduction of signal power in the undesired frequency band, expressed in decibels (dB), that occurs when the signal passes through the product. If the attenuation characteristics are good, the amount of reduction of signal power caused by the filter is large, and it can be said to have high attenuation. If the attenuation characteristics are poor, noise, etc., in the undesired frequency band can pass through the product.
When the design focuses on insertion loss, there is a tendency for attenuation characteristics to suffer. For this reason, filter developers strive to maintain a balance between insertion loss and attenuation characteristics.

Return loss is sometimes abbreviated as RL.
Return loss is the amount of reflection of the signal, expressed in decibels (dB), that occurs when it is input to the product. If the return loss characteristics are good, the amount of reflection of the desired signal band by the filter is small, allowing more of the signal to pass through the filter without being reflected.
Return loss and insertion loss are related characteristics, and products with good insertion loss characteristics tend also to have good return loss characteristics.

Bandwidth is sometimes abbreviated as BW.
Bandwidth is the frequency band of the signal that passes through the filter, expressed in hertz (Hz). One thousand hertz is 1 kHz, one million hertz is 1 MHz, and one billion hertz is 1 GHz.
A frequency of 1 Hz represents an AC signal with a waveform that alternates between positive and negative once per second. A frequency of 1 MHz represents a waveform that alternates between positive and negative one million times per second.

Murata LC filters employ co-fired ceramic (CC) technology.
Co-fired ceramic (CC) is produced using a technique in which multiple (up to three) ceramic materials are fired at the same time. This technology makes it possible to realize high-performance filters.
High-epsilon (high-ε) dielectric material is used for the capacitor layers and low-epsilon (low-ε) dielectric material for the inductor layers. This technology makes possible more compact filters with low insertion loss, meeting the requirements of applications with increasingly high functionality.
In addition, these filters have a lower profile, making them ideal as internal module components for product developers.