Bandpass ΣΔ modulator employing undersampling of RF signals for wireless communication

Abstract

The rapid development of digital wireless systems has led to a need for high-resolution and high-speed bandpass analog-to-digital converters. Continuous-time bandpass 61 modulators are very suitable for such high frequency applications. In this paper, analysis and simulation of a continuous-time bandpass 61 modulator for use in modern cellular/PCS receivers is given. The design employs undersampling relative to a radio receiver's radio frequency (RF) or intermediate frequency (IF) center frequency, while oversampling the signal bandwidth. This technique enables clocking at a frequency much lower than the RF/IF frequency, allowing use of standard CMOS technology and reducing the complexity and power consumption of subsequent digital signal processing stages. The analysis shows that it is possible to achieve a loop transfer function that matches a standard discrete-time bandpass 61 modulator while operating at sample rates significantly lower than conventional bandpass architectures.

Design of an undersampled BP ΣΔ modulator using LC and time-interleaved resonators

“…The delays between the coefficients are chosen to be a half sampling period in order to give more flexibility for the design method. The choice of the DAC transfer function is critical for undersampled modulators in order to preserve the feedback signal [10]. It has been shown that return-to-zero (RZ) [5] and raised-cosine [2] signals could be used in undersampled architectures.…”

Novel Digital Compensation Approaches for Envelope Tracking Amplifiers

“…Crest factor reduction is applied to the generated signal. We assume that undersampling is used in the feedback branch, limiting the useful bandwidth of the forward branch to 38.4 MHz, as in [13]. In our system model, the target gain g for the predistorted amplifier is 13 dB.…”

The MEMS pulsed digital oscillator (PDO) below the Nyquist limit

“…In order to overcome this limitation, undersampling techniques have been used, e.g. in RF digital receivers including ADC architectures based on bandpass sigma delta modulators working in under-Nyquist conditions [5][6][7]. These structures allow working in the IF band, thus reducing the number of stages, obtaining a wide spurious-free dynamic range, allowing a high level of integration, reducing power consumption, etc.…”