DDFS with New Sinusoid Approximation based on Harmonics Removal.
Hiroomi Hikawa · International Symposium on Circuits and Systems · 2009
This paper proposes a new sine wave approximation method and the proposed method is used as the phase amplitude converter of direct digital frequency synthesizer (DDFS). As the circuit size of the proposed method grows exponentially in proportion to the purity of the generated signal, an error compensation ROM is combined to improve the signal purity while keeping the circuit size small. VHDL simulations are conducted to test the DDFS and the results show that the circuit size of the proposed DDFS is about 40% smaller than that of the conventional DDFS. I. INTRODUCTION Frequency synthesizers play important roles in modern digital communication systems. Direct digital frequency synthesizers (DDFS) [1] are preferred in some modern communication systems because of their significant advantages over PLLbased synthesizers. In the most common DDFS architecture, sine computation is carried out by a phase-amplitude converter (PAC) using ROM-based lookup table. In this technique, increasing the number of memory locations and the length of memory words can improve the frequency resolution and spurious performance. However, the larger ROM consumes higher power or lowers the operation speed [2]. Therefore, various memory-compression techniques have been reported to decrease ROM size while keeping the frequency resolution and spectral purity unchanged. One of the such ROM size reduction scheme is linear interpolation [3], in which the sine wave is segmented and each segment is approximated by a linear function. The number of segment has to be increased to have better approximation of sine wave. However, it is disadvantageous that the algorithm requires multipliers that requires additional hardware. Another data compression technique for the ROM based PAC is the use of two smaller coarse and fine ROMs [4]. The coarse ROM stores a coarse approximation of the sine wave and the fine ROM is used to store the error between the true value and the approximated value from the coarse ROM. The sine wave is obtained by adding two ROM outputs. This paper proposes a new method to approximate sine wave by removing the harmonic frequencies contained in a triangular waveform. The function obtained by the approximation is a piecewise linear function that is similar to the one given by the liner interpolated approximation, but the method uses no multiplier. Adder