This page is a web application that design a RLC low-pass filter. Use this utility to simulate the Transfer Function for filters at a given frequency, damping ratio ζ. │H a(Ω)│. Figure 1: Magnitude response of an ideal nth-order Butterworth filter. . Of course, in the likely event that () yields a fractional. basis of course) to modify it for their purposes as long as changes are made public. Contact the The program can be used to design various types of filters. 3.

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An RLC circuit is an electrical circuit consisting of a resistor Ran inductor Land a capacitor Cconnected in series or in parallel.

The name of the circuit is derived from the letters that are used to denote the constituent components of this circuit, cous the sequence of the components may vary from RLC. The circuit forms a harmonic oscillator for current, and resonates in a similar way as an LC circuit. Introducing the resistor increases the decay of these oscillations, which is also known as damping. The resistor also reduces the peak resonant frequency.

Some resistance is unavoidable in real circuits even if a resistor is not specifically included as a component. An ideal, pure LC circuit exists only in the domain of superconductivity. RLC circuits have many applications as oscillator circuits. Radio receivers and television sets use them for tuning to select a filtges frequency range from ambient radio waves. In this role, the circuit is often referred to as a diltres circuit. An RLC circuit can be used as a band-pass filterband-stop filterlow-pass filter or high-pass filter.

The tuning application, for instance, is an example of band-pass filtering. The RLC filter is described as a second-order circuit, meaning that any voltage or fjltres in the circuit can be described by a second-order differential equation in circuit analysis. The three circuit elements, R, L and C, can be combined in a number of different filhres.

All three elements in series or all three elements in parallel are the simplest in concept and the most straightforward to analyse.

Fitres are, however, other arrangements, some with practical importance in real circuits. One issue often encountered is the filtress to take into account inductor resistance. Inductors are typically constructed from coils of wire, the resistance of which is not usually desirable, filtges it often has a significant effect on the circuit.

An important property of this circuit is its ability to resonate at a specific frequency, filtrse resonance frequencyf 0. Frequencies are measured in units of hertz. This is measured in radians per second. They are related to each other by a simple proportion. Resonance occurs because energy is stored in two different ways: Energy can be transferred from filttes to the other within the circuit and this can be oscillatory.

A mechanical analogy is a weight suspended on a spring which will oscillate up and down when released. This is no passing metaphor; a weight on a spring is described by exactly the same second order differential equation as an RLC circuit and for all the properties of the one system there will be found an analogous property of the other. The mechanical property answering to the resistor in the circuit is friction in the spring—weight system.

Friction will slowly bring any oscillation to a halt if there is no external force driving it. Likewise, the resistance in an RLC circuit will “damp” the oscillation, diminishing it with time if there is no driving AC power source in the circuit. The resonance frequency is defined filtges the frequency at xours the impedance of the circuit is at a minimum.

Equivalently, it can be defined as the frequency at which the impedance is purely real that is, purely resistive. This occurs because the impedances of the inductor and capacitor at resonance are equal but of opposite sign and cancel out. Circuits where L and C are in parallel rather than series actually have a maximum impedance rather than a minimum impedance.


For this reason they are often described as antiresonatorsit is still usual, however, to name the frequency at which this occurs as the resonance frequency. Courw resonance frequency is defined in terms of the impedance presented to a driving source. It is still possible for the circuit to carry on oscillating for a time after the driving source has been removed or it is subjected to a step in voltage including a step down to zero. This is similar to the fjltres that a tuning fork will carry on ringing after it has been struck, and the effect is often called ringing.

This effect is the peak natural resonance frequency of the circuit and in general is not exactly the same as the driven resonance frequency, although the two will usually be quite close to each other. Various terms are used by different authors to distinguish the two, but resonance frequency unqualified usually means the driven resonance frequency. The driven couts may be called the undamped resonance frequency or undamped natural frequency and the peak frequency may be called the damped resonance frequency or the damped natural frequency.

The reason for this terminology is that the driven resonance frequency in a series or parallel resonant circuit has the value [1].

This is exactly the same as the resonance frequency of an LC circuit, that is, one with no resistor present. The resonant frequency coura an RLC circuit is the same as a circuit in which there is no damping, hence undamped resonance frequency. The peak resonance cousr, on the other hand, depends on the value of the resistor and is described as the damped resonant frequency.

A highly damped circuit will fail to resonate at all when not driven. A circuit with a value of resistor that causes it to be just on the edge of ringing is called critically damped.

Either side of critically damped are described as underdamped ringing happens and overdamped ringing is suppressed. Damping is caused by the resistance courw the circuit. It determines whether or not the circuit will resonate naturally that is, without a driving source. Circuits which will resonate in this way are described as underdamped and those that will not are overdamped.

It is the minimum damping that can be applied without causing oscillation. The resonance effect can be used for filtering, cojrs rapid change in impedance near resonance can be used to pass or block signals close to the resonance frequency.

Both band-pass and band-stop filters can be constructed and flitres filter circuits are shown later in the article. A key parameter in filter design is bandwidth.

RLC circuit

The bandwidth is measured between the cutoff frequenciesmost frequently defined as the frequencies at which the power passed through the circuit has fallen to half the value passed at resonance.

There are two of these half-power frequencies, one above, and one below the resonance frequency. The bandwidth is related to attenuation by. A more general measure of bandwidth is the fractional bandwidth, which expresses the bandwidth as a fraction of the resonance frequency and is given by.

The fractional bandwidth is also often stated as a percentage. The damping of filter circuits is adjusted to result in the required bandwidth. A narrow band filter, such as a notch filterrequires low damping. A wide band filter requires high damping. The Q factor is a widespread measure used to characterise resonators. It is defined as the peak energy stored in the circuit divided by the average energy dissipated in it per radian at resonance.


Low- Q circuits are therefore damped and lossy and high- Q circuits are underdamped. Q is related to bandwidth; low- Q circuits are wide-band and high- Q circuits are narrow-band. In fact, it happens that Q is the inverse of fractional bandwidth. Q factor is directly proportional to selectivityas the Q factor depends inversely on bandwidth. For a series resonant circuit, the Q factor can be calculated as follows: This means that circuits which have similar parameters share similar characteristics regardless of whether or not they are operating in the same frequency band.

RLC Low-Pass Filter Design Tool

The article next gives the analysis for the series RLC circuit in detail. Other configurations are not described in such detail, but the key differences from the series case are given.

The general form of the differential equations given in the series circuit section are applicable to all second order circuits and can be used to describe the voltage or current in any element of each circuit. In this circuit, the three components are all in series with the voltage source.

Filtree governing differential equation can be found by substituting into Kirchhoff’s voltage law KVL the constitutive equation for each of the three elements. For the case where the source is an unchanging voltage, taking the time derivative and dividing by L leads to tiltres following second order differential equation:. Neper occurs in the name because the units can also be considered to be nepers per second, neper being a unit of attenuation.

For the case of the series RLC circuit these two parameters are given by: The value of the damping factor determines the type of transient that the circuit courss exhibit.

The differential equation has the characteristic equation cokrs, [7]. The roots of the equation in s are, [7]. The general solution of the differential equation is an exponential in either root or a linear superposition of both. The coefficients A 1 and A 2 are determined by the boundary conditions of the specific problem being analysed.

That is, filrres are set by the values of the currents and voltages in the circuit at the onset of the transient and the presumed value they will settle to after infinite time.

The overdamped response is a decay of the transient current without oscillation. By applying standard trigonometric identities the two trigonometric functions may be expressed as a single sinusoid with phase shift, [12].

This is called the damped resonance frequency or the damped natural frequency. It is the frequency the circuit will naturally oscillate at if not driven dlc an external source. The critically damped response represents the circuit response that decays in the fastest possible time without going into oscillation. This consideration is important in control systems where it is required to reach the desired state as quickly as possible without overshooting.

D 1 and D 2 are arbitrary constants determined by boundary conditions.

Solving for I s:. Solving for the Laplace admittance Y s:. By the quadratic formulawe find. The poles of Y s are identical to the roots s 1 and s 2 of the characteristic polynomial of the differential equation in the section above. For an arbitrary V tthe solution obtained by inverse transform of I s is:. Taking the magnitude of the above equation with this substitution:. The properties of the parallel RLC circuit fiiltres be obtained from the duality relationship of electrical circuits and considering that the parallel RLC is the dual impedance of a series RLC.

Considering this, it becomes clear that the differential equations describing this circuit are identical to the general form of those describing a series RLC.