Performance of interpolation algorithms for PWM driven power amplifiers

The new radio broadcasting standards as Digital Radio Mondiale (DRM) and HD Radio impose new demands on AM transmitters. Nowadays, two modulating designs dominate over the AM transmitter market - Pulse Width Modulation (PWM) technology and digital technology. In both cases, it is profitable to implement the modulator in all-digital form, which enables to use powerful DSP algorithms. This paper presents performance study of digital PWM generator, which is the key component of the PWM AM transmitters. As only the natural PWM is baseband distortion-free modulation scheme, the digital PWM generator must calculate the intersection of the modulating signal and a piecewise linear reference signal, requiring a low-distortion interpolating algorithm. Moreover, the digital PWM generator output stage must offer exceptional time resolution and low jitter. Therefore, the accuracy of interpolating algorithm and time resolution of PWM generator are surely the critical points of the digital PWM generator design. Both phenomena are discussed in this paper. The different interpolating algorithms are tested for Signal-to-Sampling Noise Ratio (SSNR) and the digital PWM generator time resolution impact on spectral properties is simulated.