What is a Filter?

What is a Filter?

Webster dictionary defines a filter as a device that passes electronic signals at certain frequencies or frequency ranges while preventing the passage of others.

Filter circuits are used in a wide variety of applications. The following are a few examples:

·         Modems and speech processing use band pass filters in the audio frequency range (0 kHz to 20 kHz).

·         Audio circuits use filters for bass and treble control.

·         Telephone central offices use high frequency band pass filters (several hundred MHz) for channel selection.

·         System power supplies use band rejection filters to suppress the 60 Hz line frequency.

·         Anti-aliasing low-pass filters, as well as low-pass noise filters, are used in the signal conditioning stage.

Types of Filters

·         There are five basic filter types. Four of them can be included in one category and the fifth is its own type. They are as follows:

·         Frequency selective circuits.

·         Low-pass filters ideally pass all frequencies within the band pass and reject frequencies outside the band.

·         High-pass filters ideally have a pass band between a low and high cut off frequency and reject frequencies outside the band.

·         Band-pass filters ideally allow a narrow band of frequencies to pass and reject all others.

·         Notch filters ideally reject only a specific, and often very narrow, band of frequencies and pass all others.

·         Time-delay filters or all-pass filters pass all frequencies equally in amplitude but change the phase of the input signals depending upon their frequency.

 

Passive filters

Passive implementations of linear filters are based on combinations of resistors (R), inductors (L) and capacitors (C). These types are collectively known as passive filters, because they do not depend upon an external power supply and/or they do not contain active components such as transistors. Inductors block high-frequency signals and conduct low-frequency signals, while capacitors do the reverse. A filter in which the signal passes through an inductor, or in which a capacitor provides a path to ground, presents less attenuation to low-frequency signals than high-frequency signals and is therefore a low-pass filter. If the signal passes through a capacitor, or has a path to ground through an inductor, then the filter presents less attenuation to high-frequency signals than low-frequency signals and therefore is a high-pass filter. Resistors on their own have no frequency-selective properties, but are added to inductors and capacitors to determine the time-constants of the circuit, and therefore the frequencies to which it responds.

The inductors and capacitors are the reactive elements of the filter. The number of elements determines the order of the filter. In this context, an LC tuned circuit being used in a band-pass or band-stop filter is considered a single element even though it consists of two components.

At high frequencies (above about 100 megahertz), sometimes the inductors consist of single loops or strips of sheet metal, and the capacitors consist of adjacent strips of metal. These inductive or capacitive pieces of metal are called stubs.

A low-pass electronic filter realized by an RC circuit

The simplest passive filters, RC and RL filters, include only one reactive element, except hybrid LC filter which is characterized by inductance and capacitance integrated in one element.^[1]

L filter

An L filter consists of two reactive elements, one in series and one in parallel.

T and π filters

Main article: Capacitor-input filter

 

Low-pass π filter

 

High-pass T filter

Three-element filters can have a 'T' or 'π' topology and in geometries, a low-pass, high-pass, band-pass, or band characteristic is possible. The components can be chosen symmetric or not, depending on the required frequency characteristics. The high-pass T filter in the illustration has a very low impedance at high frequencies, and a very high impedance at low frequencies. That means that it can be inserted in a transmission line, resulting in the high frequencies being passed and low frequencies being reflected. Likewise, for the illustrated low-pass π filter, the circuit can be connected to a transmission line, transmitting low frequencies and reflecting high frequencies. Using m-derived filter sections with correct termination impedances, the input impedance can be reasonably constant in the pass band.

Low pass & High-Pass T & Pi Filters                                          The majority of L & C filters consist of one or more cascaded basic L-C sections used in applications where pass-band ripple and frequencies of infinite attenuation in the stop-bands are of no interest. 

E.g., simple power supply smoothing filters and harmonic suppression filters following RF power amplifiers this program assists with design of such simple filters without the operating inconveniences inherent in programs intended for more sophisticated designs.

The four possible basic sections are computed simultaneously: Low-pass Pi, Low pass T, High-pass Pi and High-pass T. (T sections cannot be cascaded with Pi.)

Input data is filter cut-off frequency, terminating impedance Ro, the number of basic sections in cascade and a multiplier which sets the frequency at which the overall filter insertion-loss is to be computed.

Output data is total LuH and CpF per section. As with other networks and lines,

Ro = Sqrt (L/C).

Actual L and C component values for the required cut-off frequency are shown on the four circuit diagrams. Insertion loss is defined as that when the filter is inserted between generator and load, both of resistance 

 

PI-TYPE FILTERS                                                                                pi type filter use both capacitive and inductive filters connected in a pi-type configuration. Because of the combination of filtering devices, the ability of the pi filter to remove ripple voltage is superior to that of either the capacitance or inductance filter.

 

 

VOLTAGE REGULATORS

 Are circuits designed to maintain the output of power supplies at constant amplitude despite variations of the ac source voltage or changes of the resistance of the load. This is done by creating a voltage divider of a resistive element in the regulator and the resistance of the load. Regulation is achieved by varying the resistance of the resistive element in the regulator.

 

 

A SERIES REGULATOR

 Uses a variable resistance in series with the load. Regulation is achieved by varying this resistance either to increase or decrease the voltage drop across the resistive element of the regulator. Characteristically, the resistance of the variable resistance moves in the same direction as the load. When the resistance of the load increases, the variable resistance of the regulator increases; when load resistance decreases, the variable resistance of the regulator decreases

 

 

Applications

Low pass filters are used in a wide number of applications. Particularly in radio frequency applications, low pass filters are made in their LC form using inductors and capacitors. Typically they may be used to filter out unwanted signals that may be present in a band above the wanted pass band. In this way, this form of filter only accepts signals below the cut-off frequency.

Low pass filters using LC components, i.e. inductors and capacitors are arranged in ether a pi or T network. For the pi section filter, each section has one series component and either side a component to ground. The T network low pass filter has one component to ground and either side there is a series in line component. In the case of a low pass filter the series component or components are inductors whereas the components to ground are capacitors.

LC Pi and T section low pass filters

There is a variety of different filter variants that can be used dependent upon the requirements in terms of in band ripple, rate at which final roll off is achieved, etc. The type used here is the constant-k and this produces some manageable equations:

L     =     Zo / (pi x Fc) Henries

 

C     =     1 / (Zo x pi x Fc) Farads

 

Fc     =    1 / (pi x square root ( L x C) Hz

 

Where
Zo = characteristic impedance in ohms
C = Capacitance in Farads
L = Inductance in Henries
Fc = Cutoff frequency in Hertz

In order to provide a greater slope or roll off, it is possible to cascade several low pass filter sections. When this is done the filter elements from adjacent sections may be combined. For example if two T section filters are cascaded and each T section has a 1 uH inductor in each leg of the T, these may be combined in the adjoining sections and a 2 uH inductor used.

The choice of components for any filter, and in this case for a low pass filter is important. Close tolerance components should be used to ensure that the required performance is obtained. It is also necessary to check on the temperature stability to ensure that the filter components do not vary significantly with temperature, thereby altering the performance.

Care must be taken with the layout of the filter. This should be undertaken not just for the pass band frequencies, but more importantly for the frequencies in the stop band that may be well in excess of the cut off frequency of the low pass filter. Capacitive and inductive coupling are the main elements that cause the filter performance to be degraded. Accordingly the input and output of the filter should be kept apart. Short leads and tracks should be used, components from adjacent filter sections should be spaced apart. Screens used where required, and good quality connectors and coaxial cable used at the input and output if applicable.