Wideband 1-Bit Bandpass Delta Sigma Modulator Using Elliptic Filter in Noise Transfer Function

Abstract

A 1-bit band-pass delta-sigma modulator (BP-DSM), which utilizes oversampling technology, allows a modulated signal to be directly output by using a high-speed 1-bit digital pulse train, thus realizing miniaturization of transmitters. For 1-bit BP-DSM, the noise transfer function (NTF) is used to suppress quantization noise power in the transmission band and a guideline of out-of-band gain of $\vert \text {NTF}\vert < 1.5$ is used to prevent oscillation. However, in previous studies, such as Butterworth and inverse Chebyshev filters, the out-of-band gain was designed indirectly by tuning the zeros and poles in the transmission band and thus, was not stabilized sufficiently. Furthermore, even though the zeros of the NTF are identical to the poles of the loop filter, there are still widely used designs in which the zeros are set on the unit circle, making stabilization quite difficult. Therefore, in this paper, we propose a feasible implementation of the NTF for 1-bit BP-DSM with an elliptic filter that can be used to set not only in-band but also out-of-band gain, in which both the zeros and poles are set inside the unit circle. As a design result, a modulation bandwidth of 400 MHz as a relative bandwidth of 11%, a noise suppression bandwidth of 800 MHz, and an adjacent channel leakage power ratio of 50 dB were achieved at a center frequency of 3.6 GHz, enabling a wider bandwidth and higher SNR than before by improving the stability.

Keywords

• Delta-sigma modulator
• quantization
• 5G mobile communication
• software defined radio
• wideband