Spread spectrum capability is contained within the Fractional-N output dividers associated with each output clock. When applied, triangle wave modulation of any spread spectrum amount, SS%AMT, from ±0.25% to ±2.5% center spread and -0.5% to -5% down spread between 30 and 63kHz may be generated, independent of the output clock frequency. Five variables define Spread Spectrum in FODx (see Table below).
To calculate the spread spectrum registers, first determine the value in decimal of the FOD output divider, N, for the nominal output frequency without spread spectrum. The VCO frequency is divided by two to account for a fixed divide by 2 between the VCO output and the input to the FOD. Convert the integer portion into hex to define ODx_intdiv.