PLL circuit, communication device, and loopback test method of communication device

1. A PLL circuit that generates a spread spectrum clock (SSC), comprising:

a phase comparing unit that receives a reference clock signal and a feedback clock signal and generates a control voltage in accordance with a phase difference between the reference clock signal and the feedback clock signal;

a voltage controlled oscillator that oscillates at an oscillation frequency in accordance with the control voltage and generates an output clock signal as the SSC;

a phase interpolator that generates a phase-shifted signal obtained by shifting phase of the output clock signal by a phase shift amount selected from plurality of phase shift amounts mutually different by integral multiple of a basic delay amount;

a frequency-dividing circuit that generates the feedback clock signal by dividing frequency of the phase-shifted signal to supply the feedback clock signal to the phase comparing unit; and

controlling unit that controls the phase interpolator to gradually change a phase shift amount applied to the phase-shifted signal by a unit of the basic delay amount at timings predetermined in accordance with a modulation profile of the SSC,

wherein the controlling unit controls a total phase shift amount applied to the phase-shifted signal in one period of the feedback clock signal in a way that a difference between the total phase shift amount and a total phase shift amount in a previous one period of the feedback clock signal is always equal to or less than one basic delay amount, and

wherein the controlling unit regularly increases or decreases the total phase shift amount applied to the phase-shifted signal in one period of the feedback clock between a first phase shift amount and a second phase shift amount in switching a modulation degree of the output clock signal from a first modulation degree to a second modulation degree, the first modulation degree being defined by the first phase shift amount corresponding to k basic delay amounts, and the second modulation degree being defined by the second phase shift amount corresponding to k+ 1 basic delay amounts.

2. The PLL circuit according to claim 1 , wherein the frequency-dividing circuit comprises a first frequency divider that generates a control clock signal by dividing frequency of the phase-shifted signal and a second frequency divider that generates the feedback clock signal by further dividing frequency of the control clock signal, and

the controlling unit determines a timing of changing the phase shift amount by counting the number of pulses of the control clock signal.

3. The PLL circuit according to claim 1 , wherein the modulation profile of the SSC includes a modulation frequency.

4. The PLL circuit according to claim 1 , wherein the modulation profile of the SSC includes a maximum modulation degree.

5. The PLL circuit according to claim 1 , wherein the phase comparing unit includes a phase comparator, a charge pump, and a loop filter.

6. The PLL circuit according to claim 5 , wherein the phase comparator may be a phase and frequency comparator.

7. A method of generating a spread spectrum clock signal (SSC):

comparing by a comparing unit a phase of a reference clock signal and a feedback clock signal and generating a control voltage in accordance with a phase difference between the reference clock signal and the feedback clock signal;

receiving a control voltage by a voltage controlled oscillator that oscillates at an oscillation frequency in accordance with the control voltage and generating a spectrum-modulated output clock signal;

receiving the spectrum-modulated output clock signal by a phase interpolator and generating a phase-shifted signal obtained by shifting phase of the output clock signal by a phase shift amount selected from plurality of phase shift amounts mutually different by integral multiple of a basic delay amount;

supplying along a feedback path the phase-shifted signal or a signal obtained by dividing frequency of the phase-shifted signal to the comparing unit as the feedback clock signal; and

controlling by a controlling unit periodic changes to a modulation degree of the output clock signal by controlling the phase interpolator to change a phase shift amount at timings predetermined in accordance with a modulation profile of the SSC,

wherein the controlling unit controls a total phase shift amount applied to the phase-shifted signal in one period of the feedback clock signal in a way that a difference between the total phase shift amount and a total phase shift amount in a previous one period of the feedback clock signal is always equal to or less than one basic delay amount, and

wherein the controlling unit regularly increases or decreases the total phase shift amount applied to the phase-shifted signal in one period of the feedback clock between a first phase shift amount and a second phase shift amount in switching a modulation degree of the output clock signal from a first modulation degree to a second modulation degree, the first modulation degree being defined by the first phase shift amount corresponding to k basic delay amounts, and the second modulation degree being defined by the second phase shift amount corresponding to k+1 basic delay amounts.

8. A method of generating a spread spectrum clock signal according to claim 7 , wherein the modulation profile of the SSC includes a modulation frequency.

9. A method of generating a spread spectrum clock signal according to claim 7 , wherein the modulation profile of the SSC includes a maximum modulation degree.

10. A method of generating a spread spectrum clock signal according to claim 7 , wherein the phase comparing unit includes a phase comparator, a charge pump, and a loop filter.

11. A method of generating a spread spectrum clock signal according to claim 7 , wherein the phase comparator may be a phase and frequency comparator.