Apparatus for reducing spurious frequency components in the output signal of a direct digital synthesizer

Apparatus for reducing spurious frequency components from the output signal of a direct digital synthesizer with minimum degradation of the fundamental waveform of the output signal and wherein said synthesizer has a phase accumulator for receiving digital multibit modulation data words from a source and providing output data words, a wave function converter for converting said output data words into multibit digital waveform data words and a digital-to-analog converter for converting the digital waveform data words into said output signal and which exhibits a frequency and waveform dictated by the digital modulation data words and wherein the output signal contains spurious frequency components resulting from the operation of the direct digital synthesizer, said reducing apparatus comprising:

• means for generating a dither waveform representing a band limited waveform;
• means for summing said dither waveform with a said modulation data word prior to application of the summation thereof to said accumulator so that said output signal becomes modulated by said dither waveform providing a dither modulated analog output signal;
• means for providing a sample dither signal corresponding with said dither waveform;
• and
• means for combining said sample dither signal and said dither modulated analog output signal in such a manner that said dither waveform is cancelled from the fundamental waveform of the analog output signal.

Apparatus for reducing spurious frequency components from the output signal of a direct digital synthesizer with minimum degradation of the fundamental waveform of the output signal and wherein said synthesizer has a phase accumulator for receiving digital multibit data words and providing output data words, a wave function converter for converting data words into multibit digital waveform data words and a digital-to-analog converter for converting the digital waveform data words into said output signal and which exhibits a frequency and waveform dictated by the digital modulation data words and wherein the output signal contains spurious frequency components resulting from the operation of the direct digital synthesizer, said reducing apparatus comprising:

• means for generating a dither waveform representing a band limited waveform;
• means for summing said dither waveform with a data word and providing a summation data word prior to application thereof to said periodic wave function converter so that said output signal becomes modulated by said dither waveform providing a dither modulated analog output signal;
• means for providing a sample dither signal corresponding with said dither waveform;
• and
• means for combining said sample dither signal and said dither modulated analog output signal in such a manner that said dither waveform is cancelled from the fundamental waveform of the analog output signal.