Differentiator based spread spectrum modulator

Abstract

A spread spectrum clock generator which includes a pulse train generator circuit and a modulating circuit configured to produce a modulating signal relating to a time derivative of an output of the pulse train generator circuit. In one embodiment the modulating circuit includes a active differentiator circuit and in another embodiment the modulating circuit includes a passive differentiator circuit. A modulator is included which is configured to produce a spread spectrum clock output which is frequency modulated by the modulating signal.

What is claimed is: 
     
         1 . A spread spectrum clock generator comprising:
 a pulse train generator circuit;   a modulating circuit configured to produce a modulating signal relating to a time derivative of an output of the pulse train generator circuit; and   a modulator configured to produce a spread spectrum clock output which is frequency modulated by the modulating signal.   
     
     
         2 . The spread spectrum clock generator of  claim 1  wherein the modulating circuit includes an active differentiator circuit which includes an amplifier, a feedback resistance connected between an output of the amplifier and an inverting input of the amplifier and an input capacitance disposed intermediate the inverting input of the amplifier and the pulse generator circuit. 
     
     
         3 . The spread spectrum clock generator of  claim 2  further including feedback resistance control circuitry configured to control a magnitude of the feedback resistance in response to the pulse train generator circuit. 
     
     
         4 . The spread spectrum clock generator of  claim 3  wherein the feedback resistance includes at least two resistors and wherein the feedback resistance control circuitry includes a transistor switch connected across one of the at least two resistors. 
     
     
         5 . The spread spectrum clock generator of  claim 4  wherein the transistor is switchable between an ON and an OFF state in response to a level of the output of the pulse train generator circuit. 
     
     
         6 . The spread spectrum clock generator of  claim 1  wherein the modulator includes a voltage controlled oscillator. 
     
     
         7 . The spread spectrum clock generator of  claim 1  wherein the pulse train generator circuit includes control circuitry configured to control at least one of the rise and fall time durations of the output of the pulse train generator circuit. 
     
     
         8 . The spread spectrum clock generator of  claim 7  wherein the control circuitry is further configured to control both the rise and fall time durations of the output of the pulse train generator circuit. 
     
     
         9 . The spread spectrum clock generator circuit of  claim 8  wherein the control circuitry includes a first current source and a capacitor connected to be charged by the first current source and a second current source connected to discharge the capacitor. 
     
     
         10 . The spread spectrum clock generator of  claim 9  wherein the first current source operates to begin charging the capacitor in response to one of a rising and falling edge of a clock and wherein the second current source operates to begin discharging the capacitor in response to another one of the rising and falling edge of the clock. 
     
     
         11 . The spread spectrum clock generator of  claim 10  wherein the control circuitry includes delay circuitry for delaying a time at which the first current source operates to begin charging the capacitor and for delaying a time at which the second current source operates to begin discharging the capacitor. 
     
     
         12 . A spread spectrum clock generator comprising:
 a modulating circuit including an active differentiator circuit comprising an amplifier, a resistance connected between an inverting input of the amplifier and the amplifier output and an input capacitance connected between the inverting input of the amplifier and a source of sequential pulses; and   a voltage controlled oscillator having a control input responsive to an output of the amplifier, with the voltage controlled oscillator providing a spread spectrum clock output.   
     
     
         13 . The spread spectrum clock generator of  claim 12  further including a pulse train generator circuit configured to produce the sequential pulses, with the pulse train generator circuit including a limiter circuit that limits the magnitude of the sequential pulses. 
     
     
         14 . The spread spectrum clock generator of  claim 12  further including a pulse train generator circuit configured to produce the sequential pulses, with the pulse train generator circuit including rise and fall time control circuitry configure to control the rise and fall time durations of the sequential pulses. 
     
     
         15 . The spread spectrum clock generator of  claim 12  wherein the voltage control oscillator control input is further responsive to a primary frequency input which sets a primary frequency of the spread spectrum clock output, with the clock output being frequency modulated in response to the output of the amplifier. 
     
     
         16 . The spread spectrum clock generator of  claim 15  wherein a magnitude of the resistance is controllable with response to changes in a level of the sequential pulses. 
     
     
         17 . A method of generating a spread spectrum clock comprising:
 receiving a pulse train;   producing a modulating signal relating to a time differential of the pulse train; and   modulating a clock signal with the modulating signal to produce the spread spectrum clock.   
     
     
         18 . The method of  claim 17  wherein the producing the modulating signal includes providing an active differentiator circuit which includes an amplifier, a feedback resistance and an input capacitance. 
     
     
         19 . The method of  claim 18  further including providing the pulse train, with the pulses of the pulse train having controllable rise times and controllable fall times. 
     
     
         20 . The method of  claim 18  wherein the feedback resistance magnitude is controllable in response to levels of the pulse train.