Abstract: An apparatus estimates the frequency difference existing between the carrier frequency of a received digital signal and the frequency of a signal from a local oscillator contained in a receiver of the received signal. The receiver performs quadrature demodulation on the received signal. The apparatus includes a detector that detects the noise level received in the band of the receive filters, a processor for computing discrete Fourier transforms on the received signal transposed into baseband, a summer for summing the energy levels detected on either side of the center frequency of the receive filters, a subtractor for subtracting the noise level from the energy bands, and a comparator for comparing the resultant energy levels and delivering an estimate &egr; of the frequency difference.

Abstract: The invention relates to a system for detecting the presence of a carrier wave in a digital signal that is available at a given frequency. The system delivers at least two baseband samples of the digital signal at each symbol time. The system comprises a timing estimator responsive to the baseband samples to deliver an error signal (&egr;) corresponding to the phase error between the clock frequency that is to be recovered and a local clock frequency, and a detector that generates a detection signal indicating that a carrier wave has been detected whenever a level representative of the variance of the error signal (&egr;) reaches a threshold value. The invention is particularly applicable to receivers of signals transmitted in DTMA mode.