What Is a Costas Loop?

A costas loop is an analog circuit that can regenerate a carrier and its phase from an input signal without any or with minimal carrier content. It is a phase-locked loop that relies on frequency content of the input signal sidebands. The phase-locked loop maintains the phase relationship of a generated sinusoidal signal to a reference signal and makes use of a voltage-controlled oscillator, which generates a sinusoidal signal with a frequency and phase that can be controlled by a steering voltage and a phase comparator circuit. When the steering voltage or the correction voltage increases, the instantaneous phase lead of the voltage-controlled oscillator output may increase relative to a reference signal.

For communication circuits, the phase-locked loop may be used to set the carrier frequency as well as to generate a voltage. The typical input into the costas loop is the so-called double sideband suppressed carrier signal. Using multipliers and low-pass filters, the output of the voltage-controlled oscillator and the incoming double sideband suppressed carrier are mixed so that the double sideband suppressed carrier signal is shifted in frequency spectrum into the baseband, usually audio, spectrum. The latter is commonly known as demodulation of the message from the modulated carrier. In the digital costas loop, the multipliers and the synchronous detection function described are done by digital signal processors.

Moreover, the double sideband suppressed carrier is a special type of signal because it has no carrier. The simplest form of modulation is the double sideband amplitude modulation, wherein a carrier is modified so that its envelope is made proportional to the message. Simple circuits can demodulate this double sideband amplitude modulation, a carrier with two sidebands. A simple diode detector rectifies the carrier and passes the result into a low-pass filter, and the result is the message.

If the carrier is suppressed, the costas loop will reconstruct the carrier using a feature called carrier phase recovery. At startup, the free-running voltage-controlled oscillator is not yet locked to the carrier frequency of the incoming double sideband suppressed carrier. It should be noted that the costas loop actually calculates the carrier frequency based on the sideband positions in the frequency spectrum. Using the low-pass filter in-phase and quadrature, or 90 degrees lagging, versions of the voltage-controlled oscillator output, a low-pass filtered product of the demodulated sidebands produces the correction signals to synchronize the voltage-controlled oscillator to the center of the sidebands, and this is the carrier frequency. Meanwhile, the message is being extracted so long as the voltage-controlled oscillator is locked onto the suppressed carrier.