frequency response at its output because a linear system can not create frequencies not present at its input. Phase noise and jitter The superior close-in phase noise performance of a DDS stems from the fact that it is a feed-forward system. In a traditional phase locked loop (PLL), the frequency divider in the feedback path acts to multiply the phase noise of the reference oscillator and, within the PLL loop bandwidth, impresses this excess noise onto the VCO output. A DDS, on the other hand, reduces the reference clock phase noise by the ratio because the fractional division of the clock derives its output. Reference clock jitter translates directly to the output, but this jitter is a smaller percentage of the output period (by the ratio above). Since the maximum output frequency is limited to , the output phase noise at close-in offsets is always at least 6dB below the reference clock phase noise. At offsets far removed from the carrier, the phase-noise floor of a DDS is determined by the power sum of the DAC quantization noise floor and the reference clock phase noise floor. See also Numerically controlled oscillator Digital-to-analog converter Reconstruction filter Crystal oscillator Table-lookup synthesis Multiple wavetable synthesis Digital synthesizer References External links and further reading Tutorial on Digital Signal Synthesis (From Analog Devices) L. Cordesses, "Direct Digital Synthesis: A Tool for Periodic Wave Generation (Part 1)" IEEE Signal Processing Magazine, DSP Tips & Tricks column, pp. 50–54, Vol. 21, No. 4 July 2004. L. Cordesses, Direct Digital Synthesis: A Tool for Periodic Wave Generation (Part 2) IEEE Signal Processing Magazine, DSP Tips & Tricks column, pp. 110–117, Vol. 21, No. 5, Sep. 2004. Digital signal processing Electronic oscillators