Asymmetrical Waveform Compensation for Concurrent Dual-Band 1-bit Band-Pass Delta-Sigma Modulator with a Quasi-Elliptic Filter

Abstract

The 1-bit band-pass delta-sigma modulator (BP-DSM) achieves high resolution if it uses an oversampling technique. This method can generate concurrent dual-band RF signals from a digitally modulated signal using a 1-bit digital pulse train. It was previously reported that the adjacent channel leakage ratio (ACLR) deteriorates owing to the asymmetrical waveform created by the pulse transition mismatch error of the rising and falling waveforms in the time domain and that the ACLR can be improved by distortion compensation. However, the reported distortion compensation method can only be performed for single-band transmission, and it fails to support multi-band transmission because the asymmetrical waveform compensated signal extends over a wide frequency range and is itself a harmful distortion outside the target band. Unfortunately, the increase of out-of-band power causes the BP-DSM unstable. We therefore propose a distortion compensator for a concurrent dual-band 1-bit BP-DSM that consists of a noise transfer function with a quasi-elliptic filter that can control the out-of-band gain frequency response against out-of-band oscillation. We demonstrate that dual-band LTE signals, each with 40MHz (2×20MHz) bandwidth, at 1.5 and 3.0GHz, can be compensated concurrently for spurious distortion under various combinations of rising and falling times and ACLR of up to 48dB, each with 120MHz bandwidth, including the double sided adjacent channels and next adjacent channels, is achieved.