A bandpass subsampled delta-sigma modulator for narrowband cellular mobile communications

Abstract

Radio receivers for digital cellular systems must respect tight constraints in terms of selectivity and dynamic range.Delta-sigma analog-to-digital converters offer a response to the last problem thanks to their high resolution. On the other hand, selectivity is often achieved by filtering the received signal at an intermediate frequency (IF). Limitations of A/D converters impose usually an extra IF conversion of the signal to a lower frequency before being able to properly digitize it.We propose a new delta-sigma converter operating directly on the IF signal allowing the design of simple receivers offering both a high resolution and a good selectivity. Undersampling the IF signal allows moreover to reduce the load of the digital section of the receiver.

Bandpass ΣΔ modulator employing undersampling of RF signals for wireless communication

“…The only replica that is useful for the feedback is the one at the IF frequency. This replica will be severely attenuated by the DAC in the feedback loop which behaves as a low-pass filter when implemented as a zeroth-order-hold and which has a frequency response [10] (1)…”

“…The architecture studied here employs undersampling of the IF signal, while at the same time, oversampling the information bandwidth to provide signal-to-noise ratio (SNR) improvement [10]. Preliminary experimental results from a prototype of the proposed architecture were published by the authors in [11].…”

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.…”

Undersampled LC Bandpass ΣΔ Modulators with Feedback FIRDACs

“…We notice that when the excess loop delay is greater than 1 sampling period, the compensation transfer function, equation (6), does not cancel all the undesired terms of the equation (5). Therefore, we have to increase the order of the useful FIRDAC to cancel these terms by using its coefficients u i :…”

“…Undersampling can be used to reduce the sampling frequency of Continuous-Time (CT) bandpass Σ∆ modulators, [6]- [8]. The design of CT Σ∆ modulators is usually based on Discrete-Time to Continuous-Time equivalence [9], [10].…”