A sigma-delta demodulator for digital radio

A new architecture for bandpass sigma-delta A/D conversion and demodulation with particular applications in radio reception is described. The proposed architecture is efficient in terms of silicon area usage and has improved performance over previous topologies. A chopping scheme shifts signals from the passband to DC and a lowpass modulator is then used to perform the conversion. In the new implementation, the passband location is limited only by the maximum sampling rate achievable, which depends on the RC time constant of the sampling switch and capacitor. In previous converters the maximum passband frequency was limited by the speed of the modulator clock, which depends on the much slower integrator settling time. Hence, the new converter can digitize bands at much higher frequencies than the earlier converters, while the analogue anti-aliasing filter requirements are less strict than for undersampling converters. An IC has been fabricated to demonstrate the architecture and test results are presented for an AM-modulated input, along with simulated results for an FM-modulated input