Introduction: From Phase-Locked Loop to Costas Loop

Abstract

The Costas loop can be considered an extension of the phase-locked loop (PLL). The PLL, invented 1932 by French engineer Henri de Belleszice, was first used for the synchronous reception of amplitude modulated radio signals with carrier (c). If the modulation index is less than 100%, the modulated signal is always in phase with the carrier signal (c), irrespective of the polarity of the modulating signal. The PLL then reconstructs a local carrier (loc) that is in phase the carrier (c) of the AM signal. The modulating signal is recovered by multiplying the AM signal with the local carrier (c) with subsequent lowpass filtering. This method of synchronous reception fails, however, when an AM signal with suppressed carrier is transmitted. Here the modulated signal is in phase with the (suppressed) carrier when the modulating signal is positive, but it is in antiphase with the carrier when the modulating signal is negative. When this AM signal is fed to the input of a PLL, it would create a local carrier (loc) that is in phase with the carrier when the modulating signal is positive, but in antiphase with the carrier when the modulating signal is negative. Recovery of the modulating signal is therefore not possible. The situation would be the same when the transmitted signal is a Binary Phase Shift Keying (BPSK) signal, a Quadrature Phase Shift Keying (QPSK) signal, or a Quadrature Amplitude Modulation (QAM) signal. This problem has been solved by the invention of the Costas loop by American engineer James P. Costas in 1956. The Costas loop is able to lock onto a modulated signal also in cases where the carrier is suppressed. When we compare the topology of a Costas loop with that of a PLL, we recognize that the PLL is built up from a cascade of three circuits: (1) a phase detector, (2) a loop filter, and (3) a voltage-controlled oscillator (VCO). There is a feedback path from the output of the VCO to the input of the phase detector. In case of the PLL, the input signal is propagated through this cascade of three blocks. There is only one signal path in the PLL. In contrast, there are two signal paths in the Costas loop: (1) a so-called “I branch” (in phase), (2) a so-called “Q branch” (quadrature phase). It is shown that with that configuration the loop is able to create a local carrier that is correctly locked to the carrier of the received signal even in cases when the carrier is suppressed. The invention of the Costas loop had a dramatic impact on any kind of communication receivers; one or more Costas loops are found in TV, radio, satellite communications, mobile phones, modems, etc.