1. Feedthrough Capacitor

The feedthrough capacitor is a kind of capacitor, it is a three-terminal capacitor, the role is to eliminate high frequency. Unlike ordinary three-terminal capacitors, the feedthrough capacitor is directly installed on the metal panel, so that the grounding inductance is smaller, and the impact on the lead inductance is almost negligible, therefore, the feedthrough capacitor has a good filtering effect, and the better the electromagnetic interference suppression effect is, especially for electronic devices with high operating frequencies.

2. What Is Filtering?

We all know: the higher the frequency of voltage or current, the easier it is to produce radiation, in addition to changing the circuit board, increasing the necessary magnetic ring, in fact, there are filters, many times, reduce the interference caused by radiation sometimes add the corresponding filter, so that the high-frequency interference signal can play a great attenuation role. For ordinary interference filters, the effective filtering frequency range is several kHz to tens of MHz, while the effective filtering frequency range of RF interference filters is from several kHz to more than GHz.

Because ordinary capacitors are not ideal capacitors, high-frequency noise can not be effectively filtered, which is because: (1) the inductance of the capacitor lead causes capacitor resonance, which presents a large impedance to the high-frequency signal, and weakens the bypass effect of the high-frequency signal; (2)The parasitic capacitance between the wires makes the high-frequency signal coupling, reducing the filtering effect.

3. How to Filter the Feedthrough Capacitor?

Using the feedthrough capacitor as the bypass capacitor can make the high-frequency filtering effect very good, the feedthrough capacitor has a very small parasitic inductance, the bypass impedance is very small, and because of the isolation installation mode, the high-frequency coupling between the input and output terminals is eliminated. The feedthrough capacitor can form a variety of RF filters suitable for high frequency applications, which are also called "feed-through filters". Because of the coaxiality of the tubular feedthrough capacitor, even at 10GHz frequency, there is no obvious self-resonance phenomenon.

The medium of the feedthrough capacitor is ceramic medium, and the capacity of the ceramic capacitor will change with the change of the ambient temperature, which will affect the filter cutoff rate. Therefore, the selection of appropriate ceramic medium is particularly important for the feedthrough capacitor. Since the feedthrough capacitor shell is another electrode of the capacitor and is connected with the "ground", the high-frequency electromagnetic interference signal is short-circuited to the "ground" when it passes through the central conductor, and the electromagnetic interference is eliminated, which is the principle that the feedthrough capacitor can filter out noise.

Insertion loss is the main performance index of filter, which determines the performance of filter.

4. Forms of Feed-through Filter?

Feed-through filters, sometimes called RF filters. This filter shape structure through the core capacitor is similar, but will add ferrite core, this filter in order to meet the requirements of different occasions, this filter generally has five forms: C filter, LC filter, π filter, T filter, double T filter.

(1) Type C filter

The Type C filter is composed of three terminal capacitors or feed through capacitors, which is suitable for suppressing high frequency signals. It is a low self-sensing device to avoid high-frequency noise interference to the ground, low cost, suitable for high impedance source and high load occasions.

(2) LC Filter

An LC filter is a feed-through filter consisting of inductors and capacitors, typically used in circuits with low impedance sources and high impedance loads, and vice versa. Note: The inductors should be oriented towards low impedance sources.

(3) π-type Filter

The pi-type filter is composed of two capacitive elements and an inductor element between the two capacitive elements, and exhibits low impedance to both the impedance source and the load. π-type filters provide better high-frequency filtering performance than C-type and LC type structures.

(4) T-type Filter

The T-type filter consists of two inductors and one capacitor. The circuit structure is characterized by a high impedance input from either end. It has similar structure to PI filter, but it is not as widely used as PI filter, and can be used in the field of switching.

(5) Double T Filter

The double T filter is composed of two T-type filters, which is a high-performance filter with strict circuit requirements.

It is worth noting that the simple core structure is composed of an internal and external electrode and ceramic composed of one (type C) or two capacitors (type π). The capacity of this capacitor can be from 10pF, and the operating voltage can be up to 2000VDC. The following figure is a type C feed through filter, which is plug-in, but for electronic products when the installation space is small, the use of welded feed through filter is particularly important.

5. Application Situation

The feedthrough capacitors and various filters are mainly used for filtering signals, data lines and AC power lines, telecommunications equipment, microwave filters, industrial computer, composite circuit filter components. For example, adding a core capacitor to the high-speed data bus of the computer CPU can effectively suppress noise, and adding a core capacitor to the transmitter has a great effect on suppressing interference.

6. Installation Precautions

Due to the special structure of the feedthrough capacitor and feed-through filter, in order to ensure the normal operation of the filter, the shell of the filter must be sufficiently connected to the ground of the panel to provide an effective path for interference. As a result:

1) Through the core filter is a disc multi-layer or tubular ceramic capacitor. Like other ceramic items, they can be damaged by sudden temperature changes, mechanical vibrations and excessive voltages. When installing the feedthrough filter to the plate surface, welding the guide pins of the filter and shaping the filter, all kinds of stresses must be carefully reduced to a small size.

2)For the installation of threaded through-through filters, the installation rotation force recommended relative to the housing should be used when mounting to the partition or panel. Otherwise, due to the deformation of the shell can cause damage to the capacitor inside.

Kete Microwave Electronics Co., Ltd. specializes in designing and manufacturing state of the art RF/Microwave passive components solution. The primary products lines include ferrite components (RF Isolator, RF Circulator), High Power Resistive Components (RF Resistor, Coaxial Fixed Attenuators & Termination), Feedthrough Capacitor (EMI RF Filter), Microwave Power Divider and RF Filter.