What’s a Costas cycle?

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A costas loop is an analog circuit capable of regenerating a carrier and its phase from an input signal with no or minimal carrier content. It is a phase locked loop that relies on the frequency content of the sidebands of the input signal. The phase locked loop maintains the phase relationship of a sine wave generated signal with a reference signal and uses a voltage controlled oscillator, which generates a sine wave signal with a frequency and phase that can be controlled by a drive voltage and a phase comparator circuit. As the driving voltage or correction voltage increases, the instantaneous phase lead of the voltage controlled oscillator output can increase relative to a reference signal.

For communication circuits, the phase locked loop can 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 voltage controlled oscillator output and the incoming double sideband suppressed carrier are mixed so that the double sideband suppressed carrier signal is shifted across the frequency spectrum in the inband spectrum basis, usually audio. The latter is commonly known as demodulation of the message from the modulated carrier. In the digital costas ring, the described multipliers and synchronous detection function are performed by digital signal processors.

Also, double sideband suppressed carrier is a special type of signal because it has no carrier. The simplest form of modulation is double sideband amplitude modulation, in which a carrier is changed 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 cycle will rebuild the carrier using a function 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 in-phase and quadrature filter, or 90-degree delayed, versions of the voltage-controlled oscillator’s 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 extracted as long as the voltage controlled oscillator is locked to the suppressed carrier.

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