الرئيسية
A low-pass filter passes low frequencies fairly well, but attenuates 'high' frequencies. Therefore it is better called a high-cut filter or treble cut filter. Also the term hiss filter sometimes used. See also: high-pass filter and bandpass filter.
Low-pass filters are used to block unwanted high-frequency signals, whilst passing the lower frequencies. The low frequencies to be filtered out are relative to the unwanted higher frequencies and therefore do not have a definitive range. The frequencies that are cut vary from filter to filter. A low-pass filter is the opposite of a high-pass filter.
1 Examples of low-pass filters
A physical barrier acts as a low-pass filter for waves. When music is playing in another room, the low notes are easily heard, while the high notes are largely filtered out. Similarly, very loud music played in one car is heard as a low throbbing by occupants of other cars, because the closed vehicles (and air gap) function as a very low-pass filter.
Low-pass filters are also used in subwoofers and other types of loudspeaker, to block high pitches that they can't efficiently broadcast.
Radio transmitters use low-pass filters to block harmonic emissions which might cause interference with other communications.
DSL splitter s use low-pass and high-pass filters to separate DSL and POTS signals sharing the same pair of wires.
Low-pass filters also play a significant role in the sculpting of sound for electronic music as created by analogue synthesisers, for example the TB-303 created by the Roland corporation.
2 Types of low-pass filters
There are a great many different filter circuits, with different responses to changing frequency. The frequency response of a filter is generally represented using a Bode plot.
• A first-order filter, for example, will reduce the signal strength by half (-6 dBDB or db may stand for: DB (car), a French automobile maker Decibel dB Deutsche Bahn, the major German railway company Deutsche Bank Dominion Breweries, a major beer brewing company of New Zealand Dubnium Db , symbol for the chemical element A database Li) every time the frequency doubles (goes up one octaveIntervals : For the numerical computation software, see GNU Octave. In music, an octave (sometimes abbreviated 8ve or 8va is the interval between one musical note and another with half or double the frequency. For example, if one note is pitched at 400 Hz). The magnitude Bode plot for a first-order filter looks like a horizontal line below the cutoff frequencyIn telecommunication, the term cutoff frequency ''f has the following meanings: 1. The frequency either above which or below which the output of a circuit, such as a line, amplifier, or filter, is reduced to the specified level of 3dB 70. 1% of the refere, and a straight line approaching zero above the cutoff frequency. There is also a "knee curve" at the boundary between the two, which smoothly transitions between the two regions. (See RC circuitAn RC circuit or RC network consists of a resistor R and a capacitor C, either in series (a series RC circuit) or in parallel (a parallel RC circuit). A series RC circuit has the time constant ( tau) calculated with : When a voltage is applied to the circ.)
• A second-order filter (with a ButterworthButterworth can refer to: Butterworth filter, a fundamental electronic filter S. Butterworth, the inventor of the above filter A town in Province Wellesley, Penang, Malaysia George Butterworth, a composer A town in South Africa. response) will reduce the signal strength to one fourth its original level every time the frequency doubles (-12 dB per octave). The Bode plot for this type of filter resembles that of a first-order filter, except that it falls off more quickly. Third and higher order filters are defined similarly. (See RLC circuitAn RLC circuit is a kind of electrical circuit composed of a resistor (R), an inductor (L), and a capacitor (C). See RC circuit for the simpler case. A voltage source is also implied. It is called a second-order circuit or second-order filter as any volta.)
The meanings of 'low' and 'high'--i.e. the cutoff frequencyIn telecommunication, the term cutoff frequency ''f has the following meanings: 1. The frequency either above which or below which the output of a circuit, such as a line, amplifier, or filter, is reduced to the specified level of 3dB 70. 1% of the refere--depend on the characteristics of the filter. (The term "low-pass filter" merely refers to the shape of the filter's response. A high-pass filter could be built that cuts off at a lower frequency than any low-pass filter. It is their responses that set them apart.) A physical barrier acts as a filter at audio frequencies (between about 20 and 20000 Hz). Electronic circuits can be devised for any desired frequency range, right up through microwave frequencies (above 1000 MHz) and higher.
One simple electrical circuit that will serve as a low-pass filter consists of a resistor in series with a load, and a capacitor in parallel with the load. The capacitor exhibits reactance, and blocks low-frequency signals, causing them to go through the load instead. At higher frequencies the reactance drops, and the capacitor effectively functions as a short circuit. The break frequency, also called the turnover frequency (in radians per second), is determined by the choice of resistance and capacitance, fc = 1/RC.
Another type of electrical circuit is an active low-pass filter.
In this example, the cutoff frequency (in Hertz) is defined as:
The gain in the passband is −R2/R1, and the stopband drops off at −6 dB per octave, as it is a first-order filter.
Many times, a simple gain or attenuation amplifier is turned into a lowpass filter by adding the capacitor C. This decreases the frequency response at high frequencies and helps to avoid oscillation in the amplifier. For example, an audio amplifier can be made into a lowpass filter with cutoff frequency 100 kHz to reduce gain at frequencies which would otherwise oscillate. Since the audio band only goes up to 20 kHz, the frequencies of interest fall entirely in the passband, and the amplifier behaves the same way as far as audio is concerned.
A low-pass filter can also be realized by a digital filter
A high-pass filter passes 'high' frequencies fairly well, but attenuates 'low' frequencies. Therefore it is better called a low-cut filter or bass-cut filter. The term rumble filter is sometimes used. See also bandpass filter.
Hence it is useful as a filter to block any unwanted low frequency components of a complex signal whilst passing the higher frequencies. Of course, the meanings of 'low' and 'high' frequencies are relative. A high-pass filter is the opposite of a low-pass filter.
The simplest high-pass filter consists of a capacitor in series with the signal path in conjunction with a resistor in parallel with the signal path. The resistance times the capacitance (R×C) is the time constant; its reciprocal is the cutoff frequency, at which the output voltage is 70.7% of the input. Such a circuit would be used in combination with a tweeter and a loudspeaker.??
A band-pass filter is (usually) an electronic circuit that lets through frequencies between two other given frequencies. For example, an ideal bandpass filter would let through all signals above 30 hertz but below 100 Hz. All of the signal outside this range is attenuated or damped. See RLC circuit for basic theory regarding the frequencies passed.
It can be created by a combination of a low-pass filter and a high-pass filter. In practice, most bandpass filters are not ideal and do not attenuate frequencies just outside the desired frequency range completely. There is generally a smooth and quick decrease in transmitted frequency outside the band. This is known as the roll-off, and is usually expressed in dB per octave.
In the atmospheric sciences, for example, it is common to band-pass filter the data with a period range of, say 3 to 10 days, so that only cyclones remain as fluctuations in the data fields.
Between the lower cutoff frequency f1 and the upper cutoff frequency f2 of a frequency band there is the center frequency f0.
It is calculated as the geometric mean:
In electronics, a band-stop filter is a filter that attenuates, usually to very low levels, all frequencies between two non-zero, finite limits and passes all frequencies not within the limits. It is mostly called notch filter, which is the opposite of a band-pass filter.
Note: A band-stop filter may be designed to stop the specified band of frequencies but usually only attenuates them below some specified level.
Low-pass filters are used to block unwanted high-frequency signals, whilst passing the lower frequencies. The low frequencies to be filtered out are relative to the unwanted higher frequencies and therefore do not have a definitive range. The frequencies that are cut vary from filter to filter. A low-pass filter is the opposite of a high-pass filter.
1 Examples of low-pass filters
A physical barrier acts as a low-pass filter for waves. When music is playing in another room, the low notes are easily heard, while the high notes are largely filtered out. Similarly, very loud music played in one car is heard as a low throbbing by occupants of other cars, because the closed vehicles (and air gap) function as a very low-pass filter.
Low-pass filters are also used in subwoofers and other types of loudspeaker, to block high pitches that they can't efficiently broadcast.
Radio transmitters use low-pass filters to block harmonic emissions which might cause interference with other communications.
DSL splitter s use low-pass and high-pass filters to separate DSL and POTS signals sharing the same pair of wires.
Low-pass filters also play a significant role in the sculpting of sound for electronic music as created by analogue synthesisers, for example the TB-303 created by the Roland corporation.
2 Types of low-pass filters
There are a great many different filter circuits, with different responses to changing frequency. The frequency response of a filter is generally represented using a Bode plot.
• A first-order filter, for example, will reduce the signal strength by half (-6 dBDB or db may stand for: DB (car), a French automobile maker Decibel dB Deutsche Bahn, the major German railway company Deutsche Bank Dominion Breweries, a major beer brewing company of New Zealand Dubnium Db , symbol for the chemical element A database Li) every time the frequency doubles (goes up one octaveIntervals : For the numerical computation software, see GNU Octave. In music, an octave (sometimes abbreviated 8ve or 8va is the interval between one musical note and another with half or double the frequency. For example, if one note is pitched at 400 Hz). The magnitude Bode plot for a first-order filter looks like a horizontal line below the cutoff frequencyIn telecommunication, the term cutoff frequency ''f has the following meanings: 1. The frequency either above which or below which the output of a circuit, such as a line, amplifier, or filter, is reduced to the specified level of 3dB 70. 1% of the refere, and a straight line approaching zero above the cutoff frequency. There is also a "knee curve" at the boundary between the two, which smoothly transitions between the two regions. (See RC circuitAn RC circuit or RC network consists of a resistor R and a capacitor C, either in series (a series RC circuit) or in parallel (a parallel RC circuit). A series RC circuit has the time constant ( tau) calculated with : When a voltage is applied to the circ.)
• A second-order filter (with a ButterworthButterworth can refer to: Butterworth filter, a fundamental electronic filter S. Butterworth, the inventor of the above filter A town in Province Wellesley, Penang, Malaysia George Butterworth, a composer A town in South Africa. response) will reduce the signal strength to one fourth its original level every time the frequency doubles (-12 dB per octave). The Bode plot for this type of filter resembles that of a first-order filter, except that it falls off more quickly. Third and higher order filters are defined similarly. (See RLC circuitAn RLC circuit is a kind of electrical circuit composed of a resistor (R), an inductor (L), and a capacitor (C). See RC circuit for the simpler case. A voltage source is also implied. It is called a second-order circuit or second-order filter as any volta.)
The meanings of 'low' and 'high'--i.e. the cutoff frequencyIn telecommunication, the term cutoff frequency ''f has the following meanings: 1. The frequency either above which or below which the output of a circuit, such as a line, amplifier, or filter, is reduced to the specified level of 3dB 70. 1% of the refere--depend on the characteristics of the filter. (The term "low-pass filter" merely refers to the shape of the filter's response. A high-pass filter could be built that cuts off at a lower frequency than any low-pass filter. It is their responses that set them apart.) A physical barrier acts as a filter at audio frequencies (between about 20 and 20000 Hz). Electronic circuits can be devised for any desired frequency range, right up through microwave frequencies (above 1000 MHz) and higher.
One simple electrical circuit that will serve as a low-pass filter consists of a resistor in series with a load, and a capacitor in parallel with the load. The capacitor exhibits reactance, and blocks low-frequency signals, causing them to go through the load instead. At higher frequencies the reactance drops, and the capacitor effectively functions as a short circuit. The break frequency, also called the turnover frequency (in radians per second), is determined by the choice of resistance and capacitance, fc = 1/RC.
Another type of electrical circuit is an active low-pass filter.
In this example, the cutoff frequency (in Hertz) is defined as:
The gain in the passband is −R2/R1, and the stopband drops off at −6 dB per octave, as it is a first-order filter.
Many times, a simple gain or attenuation amplifier is turned into a lowpass filter by adding the capacitor C. This decreases the frequency response at high frequencies and helps to avoid oscillation in the amplifier. For example, an audio amplifier can be made into a lowpass filter with cutoff frequency 100 kHz to reduce gain at frequencies which would otherwise oscillate. Since the audio band only goes up to 20 kHz, the frequencies of interest fall entirely in the passband, and the amplifier behaves the same way as far as audio is concerned.
A low-pass filter can also be realized by a digital filter
A high-pass filter passes 'high' frequencies fairly well, but attenuates 'low' frequencies. Therefore it is better called a low-cut filter or bass-cut filter. The term rumble filter is sometimes used. See also bandpass filter.
Hence it is useful as a filter to block any unwanted low frequency components of a complex signal whilst passing the higher frequencies. Of course, the meanings of 'low' and 'high' frequencies are relative. A high-pass filter is the opposite of a low-pass filter.
The simplest high-pass filter consists of a capacitor in series with the signal path in conjunction with a resistor in parallel with the signal path. The resistance times the capacitance (R×C) is the time constant; its reciprocal is the cutoff frequency, at which the output voltage is 70.7% of the input. Such a circuit would be used in combination with a tweeter and a loudspeaker.??
A band-pass filter is (usually) an electronic circuit that lets through frequencies between two other given frequencies. For example, an ideal bandpass filter would let through all signals above 30 hertz but below 100 Hz. All of the signal outside this range is attenuated or damped. See RLC circuit for basic theory regarding the frequencies passed.
It can be created by a combination of a low-pass filter and a high-pass filter. In practice, most bandpass filters are not ideal and do not attenuate frequencies just outside the desired frequency range completely. There is generally a smooth and quick decrease in transmitted frequency outside the band. This is known as the roll-off, and is usually expressed in dB per octave.
In the atmospheric sciences, for example, it is common to band-pass filter the data with a period range of, say 3 to 10 days, so that only cyclones remain as fluctuations in the data fields.
Between the lower cutoff frequency f1 and the upper cutoff frequency f2 of a frequency band there is the center frequency f0.
It is calculated as the geometric mean:
In electronics, a band-stop filter is a filter that attenuates, usually to very low levels, all frequencies between two non-zero, finite limits and passes all frequencies not within the limits. It is mostly called notch filter, which is the opposite of a band-pass filter.
Note: A band-stop filter may be designed to stop the specified band of frequencies but usually only attenuates them below some specified level.
