TABLE OF CONTENTS Direct digital frequency synthesis (DDFS) is an oscillator scheme wherein a digital representation of the desired signal is generated and then applied to a D/A converter to convert the digital representation to an analog waveform. Advances in high-speed microelectronics, particularly the microprocessor, make DDFS practical at frequencies in the very-high-frequency band and below. Systems can be compact, use low power, and provide fine frequency resolution with virtually instantaneous switching of frequencies. DDFS is finding increasing application, particularly in conjunction with PLL synthesizers.
DDFS uses a single-frequency source (clock) as a time reference. One method of digitally generating the values of a sine wave is to solve the digital recursion relation as follows
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This is solved by 7 = cos (2 ?fnt). There are at least two problems with this method, however. The noise can increase until a limit cycle (nonlinear oscillation) occurs. Also, the finite word length used to represent 2 cos (2 ?ft) places a limitation on the frequency resolution. Another method of DDFS, direct table lookup, consists of storing the sinusoidal amplitude coefficients for successive phase increments in memory. The continuing miniaturization in size and cost of ROM make this the most frequently used technique.
One method of direct table lookup outputs the same N points for each cycle of the sine wave, and changes the output frequency by adjusting the rate at which the points are computed. It is relatively difficult to obtain fine frequency resolution...
