A software-radio front-end for microwave applications

In modern communication, sensor and signal processing systems digitisation methods are gaining importance. They allow for building software configurable systems and provide better stability and reproducibility. Moreover digital front-ends cover a wider range of applications and have better performance compared with analog ones. The quest for new architectures in radio frequency front-ends is a clear consequence of the ever increasing number of different standards and the resulting task to provide a platform which covers as many standards as possible. At microwave frequencies, in particular at frequencies beyond 10 GHz, no direct sampling receivers are available yet. A look at the roadmap of the development of commercial analog-to-digital-converters (ADC) shows clearly, that they can neither be expected in near future. We present a novel architecture, which is capable of direct sampling of band-limited signals at frequencies beyond 10 GHz by means of an over-sampling technique. The wellknown Nyquist criterion states that wide-band digitisation of an RF-signal with a maximum frequency f. requires a minimum sampling rate of 2 · f. But for a band-limited signal of bandwidth B the demands for the minimum sampling rate of the ADC relax to the value 2 · B. Employing a noise-forming sigma-delta ADC architecture even with a 1-bit-ADC a signal-to-noise ratio sufficient for many applications can be achieved. The key component of this architecture is the sample-and-hold switch. The required bandwidth of this switch must be well above 2 · f. We designed, fabricated and characterized a preliminary demonstrator for the ISM-band at 2.4 GHz employing silicon Schottky diodes as a switch and SiGe-based MMICs as impedance transformers and comparators. Simulated and measured results will be presented.