The invention relates to a communication system comprising at least a signal transmitter and a signal receiver, said signal receiver including at least a phase-locked loop which comprises an error detector and control means for controlling a local oscillator which may cover a certain frequency band part by part to minimize an error determined by said error detector.
The invention also relates to:
a signal receiver intended to be used in such a communication system,
an integrated circuit which comprises at least such a phase-locked loop,
a control method of controlling a local oscillator used in such a phase-locked loop, and
a computer program comprising means for implementing such a control method.
The invention has highly interesting applications in the field of telecommunications, for example, for receiving digital signals broadcast by satellite or by cable.
Such a phase-locked loop is notably described in paragraphs 1xe2x80x941 and 4xe2x80x944 of the book xe2x80x9cPhaselock Techniquesxe2x80x9d by Floyd M. Gardner published in the USA by John Wiley and Sons, Inc. in April 1967. In this book the scanning of the frequency band which the oscillator covers is obtained by applying to the input of the local oscillator a voltage that rises linearly. This voltage is delivered, for example, by an independent voltage generator.
It is an object of the invention to improve this scanning method for accelerating the locking of the loop.
Therefore, a system according to the invention and as described in the opening paragraph is characterized in that said control means comprise:
means for choosing an initial part of the band to be covered, which is close to the center of said frequency band,
means for choosing as the next part of the band to be covered the part that has not yet been covered and is closest to the initial part in the direction of the rising or falling frequencies, as a function of a predefined criterion.
In lieu of linearly covering the frequency band in one direction or the other, as this is the case in cited prior art, the invention permits to determine an order in which the frequency band is covered part by part as a function of a predefined criterion, starting from a substantially central position relative to the frequency band to be covered. The invention also permits on average to limit the number of parts of the band to be covered to obtain the locking of the loop.
Various criterions may be used. In a first embodiment the next part to be covered is chosen alternatively in one direction and then in the other. In a second embodiment the part is chosen in a totally random fashion. And in a third particularly advantageous embodiment the part is chosen as a function of the sign of the error obtained for the previous part of the band.