Method of detecting a disturbing signal for a digital data demodulator, and apparatus implementing such a method

Apparatus for implementing a method of detecting a disturbing signal for demodulating a modulated input signal (E) containing digital data, wherein, on the modulated input signal transposed into a baseband modulated signal (r.sub.k), the disturbing signal is detected by periodically inserting reference symbols by means of a fixed guard time delay applied to the received signal prior to demodulation, and wherein said method further comprises:

• a first step of estimating the square of the modulus of the cross-correlation of a received signal and of an expected baseband modulated signal (r.sub.k) at a determined instant when the reference symbols are assumed to be present;
• a second step of estimating the square of the correlation of the baseband modulated signal (r.sub.k) at the same instant, of multiplying by a coefficient which represents the square of the threshold for the estimated correlation coefficient, and of multiplying by the number of reference symbols used in a reference signal burst;
• and a third step of comparing with each other the two quantities obtained simultaneously during the preceding two steps;
• said apparatus comprising:
• a first circuit (10) for shaping the input signal (E) at intermediate frequency, for transposing this signal to baseband, and for sampling it;
• a second circuit (20) for synchronizing and shaping reference signals (S.sub.K.sup.*);
• a third circuit (30) for detecting the presence of disturbing signals by implementing said first, second and third steps;
• and
• a fourth circuit (40) for baseband demodulation of the baseband modulated signal (r.sub.k);
• wherein said first circuit is coupled to said second and fourth circuits, said second circuit is coupled to said third circuit, and said third circuit is coupled to said fourth circuit.