A charge pump-based direct frequency modulator

The architecture of a charge pump-based direct frequency modulator (DFM) is proposed. Based on a hybrid time/digital fractional-N phase lock loop (PLL), this architecture has several advantages. First, the data rate is independent of the PLL loop bandwidth. Since the modulation signal does not need boosting, the digital pre-emphasis filter is removed and thus, information about the PLL´s inverse transfer function is not needed. This also eliminates complicated calibration circuits for reducing the phase errors induced by the mismatch between the pre-emphasis filter and the PLL transfer function. The circuit complexity is further reduced because only an additional programmable charge pump is needed. Simulation results on a GMSK modulator demonstrate that the proposed architecture can achieve data rates 14 times greater than the loop bandwidth.