Spread spectrum modulation of a clock signal for reduction of electromagnetic interference
First Claim
1. A spread spectrum frequency modulated oscillator circuit, comprising:
- a reference component;
a controlled oscillator;
a first circuit coupled to the reference component and controlled oscillator and configured to supply a first control signal to the oscillator to cause the oscillator to oscillate at a frequency corresponding to a value of the reference component;
a second circuit configured to supply a random signal to the oscillator to cause the frequency of the oscillator to dither; and
a third circuit configured to control (1) a rate of change of the oscillator frequency such that the rate of change is a fixed percentage of the oscillator frequency, and (2) an amount of frequency change in the oscillator frequency such that the amount of frequency change is a fixed percentage of the oscillator frequency.
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Abstract
A spread spectrum frequency modulated oscillator circuit usable as a clock comprises a reference component such as a resistor, a voltage controlled oscillator and a first circuit coupled to the reference component and voltage controlled oscillator and configured to supply a first control signal to the oscillator to cause the oscillator to oscillate at a frequency corresponding to a value of the reference component. A second circuit configured to supply a random signal to the oscillator causes the frequency of the oscillator to dither. To cause the oscillator to exhibit random frequency modulation that is fast enough to reduce EMI but not too fast for controlled devices such as switching regulators to track, the oscillator includes a third circuit configured to control (1) a rate of change of the oscillator frequency such that the rate of change is a fixed percentage of the oscillator frequency, and (2) an amount of frequency change in the oscillator frequency such that the amount of frequency change is a fixed percentage of the oscillator frequency.
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Citations
20 Claims
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1. A spread spectrum frequency modulated oscillator circuit, comprising:
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a reference component;
a controlled oscillator;
a first circuit coupled to the reference component and controlled oscillator and configured to supply a first control signal to the oscillator to cause the oscillator to oscillate at a frequency corresponding to a value of the reference component;
a second circuit configured to supply a random signal to the oscillator to cause the frequency of the oscillator to dither; and
a third circuit configured to control (1) a rate of change of the oscillator frequency such that the rate of change is a fixed percentage of the oscillator frequency, and (2) an amount of frequency change in the oscillator frequency such that the amount of frequency change is a fixed percentage of the oscillator frequency.
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2. A spread spectrum frequency modulated oscillator circuit, comprising:
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a controlled oscillator having a frequency control input port;
a first closed loop circuit coupled to the input port of the controlled oscillator, the first closed loop circuit comprising a variable bandwidth filter, a peak detector and a variable gain amplifier, wherein the peak detector and the variable gain amplifier are configured to produce a constant amplitude signal from the filter as a function of filter bandwidth;
a random signal generator coupled to the first control loop of the oscillator circuit to modulate output signal frequency; and
a second closed loop circuit comprising the filter and the voltage controlled oscillator, the voltage controlled oscillator producing an output signal of frequency determined by the first closed loop circuit, dithered randomly by the random signal generator and in which bandwidth of the filter is controlled by the output signal produced by the voltage controlled oscillator. - View Dependent Claims (3, 4, 5, 6, 7, 8)
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9. A spread spectrum frequency modulated oscillator circuit. comprising:
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an input adapted for coupling to a frequency setting resistor through which a reference current is caused to flow;
a frequency controlled resistor;
a controlled oscillator;
an integrator circuit coupled to the frequency setting resistor and configured to drive the oscillator at a controlled frequency determined by a value of the frequency controlled resistor, and a random signal generator coupled to the integrator circuit for producing random fluctuations in the controlled frequency. - View Dependent Claims (10, 11, 12, 13, 14, 15, 16)
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17. A method of reducing electromagnetic interference generated by a frequency modulated oscillator circuit implementing spread spectrum modulation, comprising the steps of:
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controlling a rate of frequency change of the output frequency of the oscillator circuit such that the rate of frequency change is a fixed percentage of the output frequency; and
controlling an amount of frequency change of the output frequency of the controlled oscillator circuit such that the amount of frequency change is a fixed percentage of the output frequency. - View Dependent Claims (18)
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19. A frequency modulated oscillator circuit, comprising:
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a controlled oscillator having first and second input ports;
an integrator responsive to a reference signal and an output of the oscillator for providing a first control signal to the first input port of the oscillator to cause the oscillator to attain a mean output frequency prescribed by the reference signal; and
a random signal generator connected in a first circuit for supplying a random signal to the integrator, and in a second circuit for supplying the random signal to the second input port of the oscillator to cause the output frequency of the oscillator to dither. - View Dependent Claims (20)
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Specification