Spread-spectrum clock signal generator

1. A spread-spectrum clock signal generator comprising:

a first circuit loop, receiving a reference signal at a reference frequency and generating an output signal at an output frequency dependent on the reference frequency; and

a second circuit loop comprising a modulator circuit generating a modulation signal (Im) at a modulation frequency, the modulation signal being injected into the first circuit loop so as to induce a modulation of the frequency of the output signal,

the first circuit loop being adapted to make said output frequency equal to a first output frequency value when the modulator signal is absent, while in presence of the modulation signal the output frequency varies with respect to the first output frequency value according to the modulation signal,

wherein the first circuit loop is a frequency-locked loop and has a bandwidth sufficiently higher than the modulation frequency, so that the output frequency tracks the modulation signal, and

wherein a third circuit loop is provided comprising a frequency-offset correction arrangement for evaluating a frequency offset between an average of the output frequency and said reference frequency, and for generating a frequency-offset correction signal which is injected into the first circuit loop for correcting the evaluated frequency offset.

2. The spread-spectrum clock signal generator of claim 1 , in which said circuit loop comprises:

a frequency-difference detector arrangement, receiving the reference signal and a feedback signal corresponding to the output signal, and generating a frequency-difference signal sensitive to a pure frequency difference between the reference signal and the output signal;

a controlled oscillator controlled by an oscillator control signal derived from the frequency-difference signal, and generating the output signal, the output frequency being determined by the oscillator control signal; and

a feedback circuit path for deriving the feedback signal from the output signal.

3. The spread-spectrum clock signal generator of claim 2 , in which the modulation signal and the frequency-offset correction signal are injected into the frequency-locked loop in such a way to condition the oscillator control signal.

4. The spread-spectrum clock signal generator of claim 3 , in which said third circuit loop comprises:

a frequency-offset correction signal generator receiving the reference signal and an additional feedback signal derived from the output signal and generating frequency-offset correction signal injected into the first circuit loop;

the controlled oscillator, and

an additional feedback path for deriving the additional feedback signal from the output signal.

5. The spread-spectrum clock signal generator of claim 4 , in which the frequency-offset correction signal generator comprises:

a digital counter adapted to count in a first or in a second direction in response to the reference signal or to the additional feedback signal, respectively;

an evaluation arrangement for evaluating a digital count value present in the digital counter at prescribed time intervals; and

a digital-to-analog converter for generating the frequency-offset correction signal starting from the evaluated count value.

6. The spread-spectrum clock signal generator of claim 5 , in which the digital count value present in the digital counter is evaluated synchronously with the modulation signal.

7. The spread-spectrum clock signal generator of claim 4 , in which the third circuit loop is a phase-locked loop, and the frequency-offset correction signal generator includes a phase detector for generating a phase error signal indicative of a phase difference between the reference signal and the additional feedback signal, the frequency-offset correction signal being derived from the phase error signal.

8. The spread-spectrum clock signal generator of claim 7 , in which the phase-locked loop has a bandwidth sufficiently lower than the modulation frequency, so that the phase-locked loop is insensitive to the modulation signal.

9. The spread-spectrum clock signal generator of claim 8 , in which the additional feedback path coincides with the feedback path.

10. A method of generating a clock signal, comprising:

providing a reference signal at a reference frequency to a first, frequency-locked circuit loop, for generating the clock signal in such a way that a frequency of the clock signal depends on and is locked to the reference frequency;

generating a modulation signal in a second circuit loop, and injecting the modulation signal into the first frequency-locked circuit loop for inducing a modulation of the frequency of the clock signal; the first circuit adapted to make the frequency of the clock signal equal to a first output frequency value when the modulation signal is absent, while in the presence of the modulation signal the frequency of the clock signal varies with respect to the first output frequency value according to the modulation signal;

in a third circuit loop, evaluating a frequency offset between an average frequency of the clock signal and the reference frequency, generating a corresponding correction signal depending on the evaluated offset and injecting the correction signal into the first circuit loop for compensating the evaluated frequency offset.