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. The modulator generates the modulation signal according to a desired profile conditioned by an inverse of the characteristic transfer function of the clock generator.
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 at a predetermined nominal frequency; and
a modulator for generating said modulation signal according to a desired profile at said predetermined nominal frequency conditioned by an inverse of said characteristic transfer function of said clock generator, wherein said modulator comprises a profile memory that stores a waveform table of values of said modulation signal that provides a uniform frequency change over a predetermined period of time, and having an output for providing said modulation signal according to said desired profile, wherein said modulation signal is pre-distorted by said inverse of said characteristic transfer function of said clock generator and stored in said profile memory in pre-distorted form.
2 . The spread spectrum clock signal generator of claim 1 wherein said desired profile corresponds to a uniform frequency change over a predetermined period of time.
3 . The spread spectrum clock signal generator of claim 2 wherein said desired profile comprises a triangular wave profile over said predetermined period of time.
4 . The spread spectrum clock signal generator of claim 1 further comprising:
a reference oscillator for providing said reference clock signal as a square wave signal.
5 . (canceled)
6 . (canceled)
7 . The spread spectrum clock signal generator of claim 1 wherein said clock generator forms a phase locked loop, said clock generator comprising:
a loop divider having an input for receiving a divide value; anda delta-sigma modulator having an input for receiving said modulation signal, and an output for providing said divide value.
8 . A spread spectrum clock signal generator comprising:
a reference oscillator having an output for providing a reference clock signal wherein said reference clock signal is characterized as being a square wave having a substantially constant frequency;
a clock generator having a first input coupled to said output of said reference oscillator for receiving said reference clock signal, a second input for receiving a modulation signal, and an output for providing a spread spectrum clock signal, said clock generator having a characteristic transfer function; and
a modulator having an output coupled to said second input of said clock generator for providing said modulation signal according to a desired profile conditioned by an inverse of said characteristic transfer function of said clock generator, wherein said modulator comprises a profile memory that stores a waveform table of values of said modulation signal that provides a uniform frequency change over a predetermined period of time, and having an output for providing a sequence of signals representative of an amplitude of said modulation signal, wherein each of said sequence of signals is pre-distorted by said inverse of said characteristic transfer function of said clock generator and stored in said memory in pre-distorted form, and said modulator uses said reference clock signal to access locations of said profile memory.
9 . (canceled)
10 . (canceled)
11 . The spread spectrum clock signal generator of claim 8 wherein said clock generator forms a phase locked loop, said clock generator comprising:
a loop divider having an input for receiving a divide value; and
a delta-sigma modulator having an input for receiving said modulation signal, and an output for providing said divide value.
12 . The spread spectrum clock signal generator of claim 11 wherein said clock generator further comprises:
a reference divider having an input for receiving said 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; and
an output divider having an input for receiving said output of said voltage controlled oscillator, and an output for providing said spread spectrum clock signal,
said loop divider having a first input coupled to said output of said voltage controlled oscillator, a second input coupled to said output of said delta-sigma modulator, and an output coupled to said second input of said phase/frequency detector and charge pump.
13 . (canceled)
14 . The spread spectrum clock signal generator of claim 8 wherein said profile memory provides said sequence of signals at a second frequency that is a fraction of a first frequency of said reference clock signal.
15 . A method comprising:
generating a spread spectrum clock signal by modulating a reference clock signal with a modulation signal, said generating having a characteristic transfer function; and
generating said modulation signal according to a desired profile conditioned by an inverse of said characteristic transfer function, wherein said generating said modulation signal comprises:
pre-distorting said desired profile according to said inverse of said characteristic transfer function to form samples of said modulation signal according to a pre-distorted profile;
storing said samples of said modulation signal in a memory in pre-distorted form, wherein said memory stores a waveform table of values of said modulation signal that provides a uniform frequency change over a predetermined period of time; and
reading out values from said memory to form said modulation signal.
16 . The method of claim 15 further comprising:
generating said reference clock signal as a square wave having a substantially constant frequency.
17 . The method of claim 15 wherein said generating said spread spectrum clock signal comprises generating said spread spectrum clock signal using a phase locked loop.
18 . The method of claim 17 wherein said generating said spread spectrum clock signal 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.