A Multi-Bit Fir Filtering Technique For Two-Point Modulators With Dedicated Digital High-Pass Modulation Path

This paper describes an effective way of relaxing the nonlinearity problem of the digitally-controlled oscillator (DCO) in the two-point modulator design. A separate coarse varactor array dedicated for the high-pass modulation significantly simplifies the nonlinearity calibration of the DCO with a few-bit control. In addition, a finite-impulse response (FIR) filter is designed for the multi-bit high-pass modulation path to reduce the quantization noise, while offering a time-interleaving operation to minimize the DCO sensitivity to the coupling during switching time. A two-point modulator based on a semi-digital fractional-N phase-locked loop (PLL) is implemented in 0.18 mu m CMOS. Simulation results show that the proposed modulator can achieve 10Mb/s GFSK/GMSK modulations with the EVM of -37dB.