Editing Costas loop (section)

== Classical implementation ==
[[File:Costas loop model.svg|500px|center|thumb|Costas loop working in the locked state.]]
In the classical implementation of a Costas loop,<ref>{{cite journal
 |url          = http://rfdesign.com/images/archive/0102Feigin20.pdf
 |title        = Practical Costas loop design
 |first        = Jeff
 |last         = Feigin
 |date         = January 1, 2002
 |journal      = [[RF Design]]
 |pages        = 20–36
 |access-date  = February 17, 2010
 |archive-url  = https://web.archive.org/web/20120211194804/http://rfdesign.com/images/archive/0102Feigin20.pdf
 |archive-date = February 11, 2012
 |url-status     = dead
}}</ref> a local [[voltage-controlled oscillator]] (VCO) provides [[quadrature phase|quadrature]] outputs, one to each of two [[phase detector]]s, ''e.g.'', [[product detector]]s. The same phase of the input [[signal (information theory)|signal]] is also applied to both phase detectors, and the output of each [[phase detector]] is passed through a [[low-pass filter]]. The outputs of these low-pass filters are inputs to another phase detector, the output of which passes through a noise-reduction filter before being used to control the voltage-controlled oscillator. The overall loop response is controlled by the two individual low-pass filters that precede the third phase detector, while the third low-pass filter serves a trivial role in terms of gain and phase margin.

The above figure of a Costas loop is drawn under the "locked" state, where the VCO frequency and the incoming carrier frequency have become the same due to the Costas loop process. The figure does not represent the "unlocked" state.