Pseudorandom dither for frequency synthesis noise

An apparatus for reducing spurious output response in a direct digital frequency synthesizer, employing a phase angle accumulator connected to a sine function converter to convert phase data to digital amplitude numbers and being further connected to a Digital-to-Analog converter which generates an analog signal according to the amplitude numbers, comprising:

• a pseudorandom number generator;
• scaler means for receiving generated pseudorandom numbers and for scaling a magnitude of said pseudorandom numbers by a predetermined amount;
• and
• summation means for adding together two numbers to generate a sum, having a first input connected to an output of said sine function converter, a second input connected to said sine function converter, a second input connected to an output of said scaler, and an output connected to the input of said Digital-to-Analog converter.

A method of reducing spurious response from a digital frequency synthesizer where the synthesizer employs a phase angle accumulator connected to a sine function converter to convert phase angle data to digital sine amplitude numbers, and with an output of the sine converter connected to a Digital-to-Analog converter which generates an analog signal in response to the sine amplitude numbers, comprising the steps of:

• providing a sequence of randomly varying numbers;
• adding one of said randomly varying numbers to each of said sine amplitude numbers;
• and
• truncating a resulting sum before input to said Digital-to-Analog converter.

An apparatus for reducing spurious output response in a direct digital frequency synthesizer, employing a phase angle accumulator connected to a sine function converter to convert phase data to digital amplitude numbers and being further connected to a Digital-to-Analog converter which generates an analog signal according to the amplitude numbers, comprising:

• summation means for adding together two or more numbers to generate a sum, having a first input connected to an output of said sine function converter and an output connected to the input to said Digital-to-Analog converter;
• a random number generator connected to a second input of said summation circuit;
• and
• scaler means connected between said summation means second input and said random number generator for receiving generated random numbers and for scaling a magnitude of said random numbers by a predetermined amount.