A
Costas loop is a
phase-locked loop used for
carrier phase
recovery from suppressed-carrier
modulation signals, such as from double-
sideband suppressed carrier signals. It was invented by
John P. Costas at
General Electric in the 1950s. Its invention was described as having had "a profound effect on modern digital communications".
The primary application of Costas loops is in wireless receivers. Its advantage over the PLL-based detectors is that at small deviations the Costas loop error voltage is sin(2(
θi−
θf)) vs sin(
θi−
θf). This translates to double the sensitivity and also makes the Costas loop uniquely suited for tracking doppler-shifted carriers esp. in
OFDM and
GPS receivers
Implementation
In the usual implementation of a Costas loop,{{cite journal
|url = http://rfdesign.com/images/archive/0102Feigin20.pdf
|format=PDF|title = Practical Costas loop design
|author = Jeff Feigin
|date = January 1, 2002 |journal = RF Design
|pages = pp. 20–36
}} a local
voltage-controlled oscillator provides
quadrature outputs, one to each of two
phase detectors,
e.g.,
product detectors. The same phase of the input
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 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.
References