A software-radio front-end for microwave applications

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 in over-sampling technique. The well-known Nyquist criterion states that wide-band digitisation of an RF-signal with a maximum frequency f requires a minimum sampling rate of 2/spl middot/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/spl middot/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/spl middot/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 are presented.