| 摘要 |
A digital sinewave generator can be used in digital equipment, e.g. instead of a local oscillator for modulation/demodulation of signals. If there is no simple relationship between sampling rate and the frequency of the sinewave, successive sample values must be calculated since storage of these values would be prohibitive. For a sinewave Rejk phi sampled at successive instants k=0, 1, 2, . . . etc. separated by a phase angle phi , quadrature components Ak+jBk=Rejk phi can be calculated using the algorithm: Ak+1=cos phi Ak-sin phi Bk Bk+1=sin phi Ak+cos phi Bk. Unfortunately it is impossible for cos2 phi +sin2 phi to be exactly equal to unity if calculations are performed with finite precision using an even number base, which as the effect of causing the modulus of a sinewave generated by this algorithm to tend exponentially to 0 to INFINITY . The present invention overcomes this by using the above algorithm to obtain approximations Ak+1' and Bk+1' to the desired result at terminals (9 and 10) and then multiplying (22,23) the approximations Ak+1' and Bk+1' by a correction factor ( 1-1/2 epsilon ) to obtain better approximations Ak+1 and Bk+1 which are stable about the desired amplitude R. <IMAGE> Such a generator can be used as a local oscillator in a modem for example. |
