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A filter is a device used to remove unwanted part of the signal or to extract useful parts of the signal, such as of the components lying within a certain frequency range. There are two types of filters; namely, Active filter and passive filter.
Passive filters
Single pole types
The combinations of resistors, inductors and capacitors in RC, RL, LC and RLC combination is collectively known as passive filters.
LOW PASS CIRUIT
: A low-pass filter is a circuit offering easy passage to low-frequency signals and obstructing passage to high-frequency signals.
There are two basic kinds of circuits used to achieve the purpose, which are as follows:
1. Inductor low-pass circuit: In this type of circuit, the inductor`s impedance increases with increasing frequency. Thus, high impedance in series tends to block high-frequency signals from getting to the load.
2. Capacitor low-pass filter: In this type of circuit, the capacitor`s impedance decreases with increasing frequency. This low impedance in parallel with the load resistance tends to short out high-frequency signals, dropping most of the voltage gets across series resistor R1.
In short, the Inductors used in such circuit blocks high-frequency signals and conducts low-frequency signals, while capacitors do the reverse. Therefore, a filter in which the signal passes through an inductor, or in which a capacitor provides a path to earth therefore presents less attenuation to low-frequency signals than high-frequency signals and is a low-pass filter.
Why capacitive filters are preferred over inductive filters?
Capacitive filter designs are usually preferred over inductive circuits due to their ability to offer little resistive effects than inductors, which makes them almost 100% "purer" reactive components than inductors. But, Inductors, at the same time exhibit significant dissipative (resistor-like) effects, both in the long lengths of wire used to make them, and in the magnetic losses of the core material.
Also, Capacitors are less expensive, and doesn`t generate and/or receive interference from other components via mutual electric or magnetic fields with other components leading to "Coupling effects" -which inductors tend to exhibit.
Where Inductive low-pass filter scores over capacitive low-pass filters?
In AC-DC power supplies, when AC is converted (rectified) into DC, passing only the pure DC component, AC "ripple" waveform is created. Thus, in order to remove such unwanted ripples the inductive low-pass filter is preferred. The primary reason for this is the requirement of low filter resistance for the output of such a power supply. A capacitive low-pass filter requires an extra resistance in series with the source, whereas the inductive low-pass filter does not. In case of a high-current circuit like a DC power supply where additional series resistance is undesirable, the inductive low-pass filter is the right choice. On the other hand, if low weight and compact size are higher priorities than low internal supply resistance in a power supply design, the capacitive low-pass filter should be preferred.
Cut-off frequency: The frequency above which the output voltage falls below 70.7% of the input voltage. In a simple capacitive/resistive low-pass filter, it is the frequency at which capacitive reactance in ohms equals resistance in ohms. In a simple capacitive low-pass filter (one resistor, one capacitor), the cutoff frequency is given as:
F(cut-off)=1/2i-iRC
HIGH PASS FILTER
CAPACITIVE HIGH PASS FILTER
A high-pass filter`s task is just the opposite of a low-pass filter: to offer easy passage of a high-frequency signal and difficult passage to a low-frequency signal. As one might expect, the inductive and capacitive versions of the high-pass filter are just the opposite of their respective low-pass filter designs:
The capacitor`s impedance increases with decreasing frequency. This high impedance in series tends to block low-frequency signals from getting to load.
INDUCTIVE HIGH PASS FILTER
The inductor`s impedance decreases with decreasing frequency. This low impedance in parallel tends to short out low-frequency signals from getting to the load resistor. As a consequence, most of the voltage gets dropped across series resistor R1.
BAND PASS FILTER
In certain cases, it becomes necessary to filter to filter particular band or frequencies are from a wider range of mixed signals. Hence, filter circuits can be designed by combining the properties of low-pass and high-pass into a single filter. The result is called a band-pass filter. Creating a bandpass filter from a low-pass and high-pass filter can be illustrated using block diagrams:
What emerges from the series combination of these two filter circuits will only allow passage of those frequencies that are neither too high nor too low.
BAND STOP FILTER
This type of circuits are also called band-elimination, band-reject, or notch filters, this kind of filter passes all frequencies above and below a particular range set by the component values. This kind of circuits is made by connecting the two filter sections in parallel with each other instead of in series.
RESONANT FILTERS
Resonant filters are constructed by combination of L and C circuit. Although combinations of L and C will tend to resonate, this property can be exploited in designing band-pass and band-stop filter circuits.
Series LC circuits give minimum impedance at resonance, while parallel LC ("tank") circuits give maximum impedance at their resonant frequency. With this, we have two basic strategies for designing either band-pass or band-stop filters.
Multipole types
Measurement of Second order filters is done by their quality or "Q" factor.
Note:
A filter is said to have a high Q if it selects or rejects a narrow range of frequencies compared with its centre frequency. Q is defined as center frequency/3 dB bandwidth.
Active filters
Generally, Active filter comprises of combination of passive and active (amplifying) components. Operational amplifiers are frequently used in active filter designs. These can have high Q, and achieve resonance without the use of inductors. However, their upper frequency limit is limited by the bandwidth of the amplifiers used.
Digital filters
In digital filters, the signal is sampled and an analog to digital converter turns the signal into a stream of numbers and used as per requirement.
In addition to these kinds of filters, other types of filters available are Quartz filters and piezoelectrics, SAW filters, Garnet filters and Atomic filters.
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