Architecture and algorithms for generating general-purpose modulation signals

In this paper, we introduce the architecture for implementing a wideband general purpose digital modulator which can generate commonly-used modulation format signals. The symbol rate of modulation signals can be from 500kS/s up to 45MS/S. In order to use the same analog low-pass filters (LPF) to cancel all image signals from D/A converter (DAC) under different output signal bandwidth, we employ Cascaded Integrator Comb (CIC) filter to interpolate so as for DAC to work within a specific range. In the meantime, we propose an algorithm to counteract the passband droop of CIC filter. Considering about the effect of sinc(x) by DAC on wideband signals, a similar algorithm is presented. For reducing the implementation complexity, all integrated adverse effects were compensated for by modifying the coefficients of Pulse Shaping Filter (PSF). The algorithm is verified by experiments and the modulator can produce digital modulation signals with good Error Vector Magnitude (EVM) and adjacent channel leakage power ratio (ACLR).