Calibration for spread spectrum clock generator and method therefor
In one form, a spread spectrum clock generator includes a clock generator and a modulator. The clock generator modulates a frequency of a reference clock signal using a modulation signal to provide a spread spectrum clock signal. The clock generator has a characteristic transfer function that varies with values of a parameter. The modulator generates the modulation signal according to a desired profile conditioned by an inverse of the characteristic transfer function of the clock generator at a current value of the parameter.
1. A spread spectrum clock signal generator comprising:
a clock generator for modulating a frequency of a reference clock signal using a modulation signal to provide a spread spectrum clock signal, said clock generator having a characteristic transfer function that varies with values of a frequency of said reference clock signal and a nominal frequency of said reference clock signal can vary over a range of frequencies; and
a modulator for generating said modulation signal according to a desired profile conditioned by an inverse of said characteristic transfer function of said clock generator at a current nominal value of said frequency of said reference clock signal, wherein said modulator comprises an inverse transfer function filter and said modulator changes coefficients of said inverse transfer function filter according to said current nominal value of said frequency of said reference clock signal.
2. The spread spectrum clock signal generator of claim 1 wherein said modulator comprises:
a calibration circuit for determining said coefficients of said inverse transfer function filter at said current nominal value of said frequency of said reference clock signal during a calibration mode.
3. The spread spectrum clock signal generator of claim 2 wherein said modulator comprises:
a profile memory having an output for providing a sequence of signals representative of said desired profile; and
wherein said inverse transfer function filter has an input coupled to said output of said profile memory in a normal operation mode, and an output for providing said modulation signal.
4. The spread spectrum clock signal generator of claim 3 wherein said calibration circuit comprises:
a training signal generator for generating a training signal and providing said training signal to said input of said inverse transfer function filter during said calibration mode; and
an adaptive algorithm coefficient generator having a first input coupled to a second output of said clock generator, a second input coupled to said output of said training signal generator, and an output coupled to said inverse transfer function filter.
5. The spread spectrum clock signal generator of claim 4 wherein said adaptive algorithm coefficient generator iteratively provides said coefficients to said inverse transfer function filter to reduce an error between said training signal and a signal at said second output of said clock generator.
6. The spread spectrum clock signal generator of claim 4 further comprising a switch for switching said input of said inverse transfer function filter between said output of said profile memory in said normal operation mode and said output of said training signal generator in said calibration mode.
7. The spread spectrum clock signal generator of claim 4 wherein:
said training signal generator provides said training signal at a frequency corresponding to said frequency of said reference clock signal.
8. A spread spectrum clock signal generator comprising:
a clock generator comprising:
a reference divider having an input for receiving a reference clock signal, and an output;
a phase/frequency detector and charge pump having a first input coupled to said output of said reference divider, a second input for receiving a feedback signal, and an output;
a lowpass filter having an input coupled to said output of said phase/frequency detector and charge pump, and an output;
a voltage controlled oscillator having an input coupled to said output of said lowpass filter, and an output for providing said spread spectrum clock signal;
a loop divider having a first input coupled to said output of said voltage controlled oscillator, a second input, and an output coupled to said second input of said phase/frequency detector and charge pump; and
a delta-sigma modulator having an input for receiving a modulation signal, and an output coupled to said second input of said loop divider,
a modulator for generating said modulation signal according to a desired profile conditioned by an inverse of a characteristic transfer function of said clock generator at a nominal frequency of said reference clock signal and providing said modulation signal to said input of said delta-sigma modulator, wherein said nominal frequency of said reference clock signal can vary over a range of frequencies, and wherein said modulator comprises an inverse transfer function filter and said modulator changes coefficients of said inverse transfer function filter according to a current nominal frequency of said reference clock signal.
9. The spread spectrum clock signal generator of claim 8 wherein said modulator comprises:
a calibration circuit for determining said coefficients of said inverse transfer function filter at said current nominal frequency of said reference clock signal.
10. The spread spectrum clock signal generator of claim 9 wherein said modulator comprises:
a profile memory having an output for providing a sequence of signals representative of said desired profile; and
wherein said inverse transfer function filter has an input coupled to said output of said profile memory in a normal operation mode, and an output for providing said modulation signal.
11. The spread spectrum clock signal generator of claim 10 wherein said calibration circuit comprises:
a training signal generator for generating a training signal and providing said training signal to said input of said inverse transfer function filter during a calibration mode; and
an adaptive algorithm coefficient generator having a first input coupled to a second output of said clock generator, a second input coupled to said output of said training signal generator, and an output coupled to said inverse transfer function filter.
12. The spread spectrum clock signal generator of claim 11 wherein said adaptive algorithm coefficient generator iteratively provides said coefficients to said inverse transfer function filter to reduce an error between said training signal and a signal at said second output of said clock generator.
13. The spread spectrum clock signal generator of claim 12 further comprising a switch for switching said input of said inverse transfer function filter between said output of said profile memory in said normal operation mode and said output of said training signal generator in said calibration mode.
14. The spread spectrum clock signal generator of claim 8 wherein said clock generator further comprises
a reference oscillator having an output for providing said reference clock signal wherein said reference clock signal is characterized as being a square wave signal.
15. A method comprising:
modulating a reference clock signal using a modulation clock signal to provide a spread spectrum clock signal, said modulating having a characteristic transfer function that varies with values of a nominal value of said reference clock signal over a desired range;
generating said modulation signal according to a desired profile conditioned by an inverse of said characteristic transfer function of said modulating at a current nominal value of said reference clock signal; and
changing said generating using coefficients that vary according to said current nominal value of said reference clock signal.
16. The method of claim 15 wherein said generating comprises:
measuring said inverse of said characteristic transfer function of said modulating at said current nominal value of said reference clock signal during a calibration mode.
17. The method of claim 15 wherein said modulating comprises modulating using a phase locked loop.
18. The method of claim 17 wherein said modulating further comprises varying a value of a loop divide ratio of said phase locked loop.
19. The method of claim 15 wherein said generating said modulation signal according to said desired profile comprises generating said modulation signal according to a triangular wave profile.
20. The method of claim 15 wherein said generating said modulation signal according to said desired profile comprises generating said modulation signal according to an asymmetric triangular wave profile.