spread spectrum clocking_xilinx pre-production_Free running(10)

Practical limits on the modulation rates may also be considered。 For example, if a modulation rate is too slow, such as lower than 20 kHz, it may cause audible interference due to demodulation by non-linearity of the circuit components。 If a modulation rate is too fast, it can cause signal timing issues or excessive jitter and/or loss of DLL lock。

FIG。 6 shows an illustrative adaptive SSC circuit 320 for generating a clock signal having an adaptive SSC profile 404 similar to that described above in regard to FIG。 4, according to some embodiments。 Specifically, the adaptive SSC circuit 320 comprises a phase-locked loop (“PLL”) circuit, comprising an oscillator 602, a phase frequency detector 604, a charge pump 606, a low-pass filter 608, a voltage-controlled oscillator 610 and a divider 612。 The output clock signal 614 is fed into divider 612 for frequency down conversion。

The phase frequency detector 604 compares phase between a reference clock produced by the oscillator 602 and the output signal of the divider 612 and produces an error signal between them。 The charge pump 606 and the low-pass filter 608 make the voltage input to the voltage-controlled oscillator 610 proportional to the error signal, thus changing the output frequency。 This PLL feedback circuit allows the generation of an output clock signal 614 that follows the reference clock from the oscillator 602 in phase and that may be multiple times the reference clock frequency。

The PLL circuit may be modified to produce the adaptive SSC profiles 404 described herein by the addition of one or more components in the feedback loop。 According to some embodiments, the voltage input to the voltage-controlled oscillator 610 may be modulated directly by a voltage modulator 616 to produce the SSC clock signal with the adaptive SSC profiled 404。 Output frequency of the voltage-controlled oscillator 610 is controlled by the value of the input voltage。 For example, higher voltage input to voltage-controlled oscillator 610 may cause lower output frequency。

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