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  Applications of Filters, Physics tutorial

Audio frequency uses of filters:

Audio filters are implemented both in analogue circuitry as analogue filters, and as computer software in digital filters. Their function mainly involves changing of harmonic content of audio signals. In audio terms; harmonic content of audio signals is timbre and is the estimate of subjective quality of sound.

Audio frequency filters are applied to frequencies commencing from direct current; 0 Hz to beyond 20 kilo Hertz and several kinds of filters exist and are utilized for audio frequency applications. These comprise graphic equalizers, synthesizers, sound effects, Compact Disk players and virtual reality systems.

Audio filters are developed to amplify, pass or attenuate some frequency ranges and common types comprise low-pass filters that pass through frequencies below their cutoff frequencies, and progressively attenuate frequencies above cutoff frequency, high-pass filters which do opposite by passing high frequencies above cutoff frequency, and increasingly attenuating frequencies below cutoff frequency.

Audio bandpass filters pass frequencies between their two cutoff frequencies while attenuating those outside range while band-reject filters attenuate frequencies between their two cutoff frequencies, while passing those outside 'reject' range. At last, audio all-pass filter passes all frequencies, but affects phase of any given sinusoidal component according to frequency.

Design of graphic equalizers and Compact Disk players depend on the set of objective criteria. These criteria comprise pass band, pass band attenuation, stop band, and stop band attenuation and they find out where pass bands are frequency ranges for which audio is attenuated less than the specified maximum, and stop bands which are frequency ranges for which audio should be attenuated by specified minimum.

Audio filter frequently give feedback loop that introduces resonance and attenuation. Some audio filters are developed to give gain and attenuation while in synthesizers and sound effects applications; filter audio aesthetic is of uttermost significance and should be estimated critically and subjectively.

Bandpass filters at radio frequencies:

Signals at radio frequencies often need that a range of frequencies within the band be filtering through circuit. This is a very common occurrence in high frequency applications like 20 metre band.

Above is a typical configuration of the radio frequency bandpass filter and below, frequency response curve. Topology above is that of T filter. Pair of series LC elements on upper arm of T are stagger tuned; that means one set is build to resonate slightly above while other slightly below the centre frequency that results in double peaked response with trough in between.

Being series resonant circuits, they permit desired frequencies within passband to pass suppressing slightly centre frequency. Parallel LC resonant circuit presents the high impedance to centre frequency while letting other frequencies around to bypass output of filter.

EMI and transient suppressor filters:

Electrical transient is rich in radio frequencies that are liable for electromagnetic interference. Electromagnetic disturbances influence electrical circuits through electromagnetic induction or electromagnetic radiation and disturbance may interrupt, obstruct, degrade or limit effective performance of the circuit. They can range from simple degradation of data to the total loss of data in computers.

Two major categories of transient and Electromagnetic Interference suppressors are suppressors which attenuate transients and thus prevent their propagation in sensitive circuit on one hand; and those that redirect transients away from sensitive loads and so limit residual voltages on the other.

Attenuating the transient means keeping it from propagating away from source or keeping it from impinging on the sensitive load. It can be achieved with filters inserted within the circuit.

If placed in series with the circuit, such filters are usually low-pass type and they attenuate high frequency transients while permitting signal or power frequencies to continue undisturbed.

Frequency components of the transient are many orders of magnitude above power frequency of the AC circuit and obvious solution is radio frequency interference is to install the low-pass filter between source of transients and load. This is generally composed of the capacitor placed across line. Impedance of capacitor forms the voltage divider with source impedance, resulting in attenuation of transient at high frequencies.

Inductive components in circuit may make unwanted resonances with this capacitance and series resistor is generally added to capacitor to make RC snubber circuit.

Amplitude modulated radio transmission is mainly susceptible to interference and different methods that enhance selectivity are utilized in radio receiver.

Wi-Fi digital radio systems use sophisticated error-correction methods while spread-spectrum and frequency-hopping are utilized with both analogue and digital signaling to decrease sensitivity to radio frequency and transient induced interferences.

Spectrum sweeping:

Swept-tuned spectrum analyzers sweep the voltage-controlled oscillator through the range of frequencies that are mixed with, and that down converts the portion of input signal spectrum to centre frequency of the band-pass filter.

The output of the oscillator can be mixed with the signal frequency to produce sum and difference of oscillator and signal frequency.

If difference of frequencies is passed through high Q value, narrow band bandpass filter, then, output of filter will respond to difference frequency generated which falls inside its passband. As oscillator frequency is swept across the range, filter responds to those frequencies in input signal that difference with oscillator's frequency fall within passband.

Proper calibration with the oscilloscope will display the curve in the frequency domain when swept frequency is produced with voltage controlled oscillator.

In commercial spectrum sweeping devices, bandwidth of band-pass filter finds out spectral resolution bandwidth and this is associated to minimum bandwidth which can be detected by instrument.

Spectrum analyzers are universally applied in measurement of frequency response, noise and distortion characteristics of numerous kinds of Radio Frequency equipment circuitry. This is done simply and effectively by comparing input and output spectra of circuitry. Telecommunications spectrum analysis involves use of spectrum analyzers to find out occupied bandwidths and to track interference sources; this is very significant in GSM cell planning. They are applied in finding wireless transmitters' compliance with stated standards for purity of emissions.

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