Frederic Gourgue

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

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 reduction of the load of the digital section of the receiver.<<ETX>>

Poles and zeros of bandpass filters for high‐performance sigma‐delta modulators

High-performance bandpass sigma-delta modulators find applications in digital mobile radio for accurate analogue-to-digital conversion in receivers. An approach based on the positioning of the poles and zeros is proposed for the digital design of the high-order loop filters which are required. Inverse notch filter sections are cascaded in a first step and then poles and zeros are moved to meet the performance objectives and achieve system stability. A distinct feature of the analysis and design technique presented is that it takes stability into account explicitly. Examples illustrate the practicality of the method as well as the level of performance which can be achieved in theory. the integrated circuit realization still has to be investigated to confirm the potential of the proposed design approach.

Radio receivers for cellular networks: Possible architectures using a ΣΔ-converter

We first explain the constraints set on a receiver for digital cellular systems before showing how they are handled with the standard architectures (direct conversion and superheterodyne). We introduce then a new approach based on the direct sampling of the intermediate frequency in a superheterodyne receiver using a passband analog- to- digital ΣΔ -converter. We give design rules to minimize the complexity of the system and study within that scope the interest of under sampled ΣΔ -converters addressing more especially the stability problems. Undersampling allows to reduce the processing load in the digital part of the converter, compared to a standard ΣΔ conversion. The last chapter is dedicated to a presentation of cellular networks in order to justify the constraints set on receivers designed for this environment.RésuméAprès avoir exposé les contraintes imposées à un récepteur pour système cellulaire numérique, les auteurs montrent comment il est possible d’y répondre avec les architectures usuelles (conversion directe et superhétérodyne). Ils proposent alors une nouvelle approche s’appuyant sur l’échantillonnage direct du signal à fréquence intermédiaire d’un récepteur superhétérodyne grâce à un convertisseur analogique- numérique ΣΔ passe- bande. Des règles de conception pour réduire la complexité du système et étudier dans ce cadre l’intérêt des convertisseurs ΣΔ sous- échantillonnés en regardant particulièrement le problème de la stabilité sont présentées. Le sous- échantillonnage permet de réduire la charge de calcul de la partie numérique du convertisseur par rapport à l’approche ΣΔ classique. Les contraintes imposées aux récepteurs destinés à être utilisés dans un réseau cellulaire sont justifiées par la présentation des principes d’un tel réseau.