page_title: Low-pass filter
section_title: RLC filter
hierarchical_section_title: Low-pass filter / Electronic low-pass filters / Second order / RLC filter
context_page_description: A low-pass filter is a filter that passes signals with a frequency lower than a selected cutoff frequency and attenuates signals with frequencies higher than the cutoff frequency. The exact frequency response of the filter depends on the filter design. The filter is sometimes called a high-cut filter, or treble-cut filter in audio applications. A low-pass filter is the complement of a high-pass filter.
  In optics, high-pass and low-pass may have the different meanings, depending on whether referring to frequency or wavelength of light, since these variables are inversely related. High-pass frequency filters would act as low-pass wavelength filters, and vice versa. For this reason it is a good practice to refer to wavelength filters as "Short-pass" and "Long-pass" to avoid confusion, which would correspond to "high-pass" and "low-pass" frequencies.
  Low-pass filters exist in many different forms, including electronic circuits such as a hiss filter used in audio, anti-aliasing filters for conditioning signals prior to analog-to-digital conversion, digital filters for smoothing sets of data, acoustic barriers, blurring of images, and so on.
context_section_description: An RLC circuit  (the letters R, L and C can be in a different sequence) is an electrical circuit consisting of a resistor, an inductor, and a capacitor, connected in series or in parallel.  The RLC part of the name is due to those letters being the usual electrical symbols for resistance, inductance and capacitance respectively.  The circuit forms a harmonic oscillator for current and will resonate in a similar way as an LC circuit will.  The main difference that the presence of the resistor makes is that any oscillation induced in the circuit will die away over time if it is not kept going by a source.  This effect of the resistor is called damping. The presence of the resistance also reduces the peak resonant frequency somewhat.  Some resistance is unavoidable in real circuits, even if a resistor is not specifically included as a component.  An ideal, pure LC circuit is an abstraction for the purpose of theory.
  There are many applications for this circuit.  They are used in many different types of oscillator circuits.  Another important application is for tuning, such as in radio receivers or television sets, where they are used to select a narrow range of frequencies from the ambient radio waves.  In this role the circuit is often referred to as a tuned circuit.  An RLC circuit can be used as a band-pass filter, band-stop filter, low-pass filter or high-pass filter. The RLC filter is described as a second-order circuit, meaning that any voltage or current in the circuit can be described by a second-order differential equation in circuit analysis.
caption: RLC circuit as a low-pass filter