Costa's loop
A Costas Loop (German: Costas loop ) is in digital communications technology is a possibility for demodulation of binary phase modulated (BPSK) signals. An extended form of this loop can also be used for demodulating quadrature modulated (QPSK) signals. The Costas Loop was developed in 1956 by John P. Costas as a special variant of the phase-locked loop .
When transmitting phase-modulated digital signals, such as those used in digital mobile radio , the receiver must be able to reconstruct the correct phase position in order to obtain the transmitted bit data stream . The core components of the Costas Loop used in receivers are a voltage-controlled oscillator ( VCO , VCXO ), a loop filter, several multipliers and two low-pass filters. If the phase position of the VCO is correctly set, the control signal via the loop filter is 0. If there is a clock deviation, this is readjusted by the control loop. If the phase position is correctly set, the transmitted bit data stream can be tapped directly at the output of the Costas Loop.
The advantage of the Costas Loop for digital demodulation of QPSK signals, in contrast to demodulation using a phase locked loop (PLL), is that the phase noise is lower and it locks into place more quickly if the phase position is unfavorable. In addition, the engagement can thus (Engl. Lock ) to wrong carrier frequencies are more suppressed.
literature
- Friedrich Jondral: Message Systems . J. Schlembach Fachverlag, 2006, ISBN 3-935340-50-8 , p. 150 to 159 .
Individual evidence
- ^ John P. Costas, Synchronous Communications , in: Proceedings of the IRE, December 1956, pp. 1713-1716
Web links
- Various Costas loops for demodulating BPSK and QPSK, in English (PDF file; 95 kB)