A 3-30 MHz Tunable Continuous-Time Bandpass Sigma-Delta A/D Converter for Direct Conversion of Radio Signals

modulators are still limited to IFs of several tens of megahertzs [6], [9]. The disadvantages of the CT approach are less accuracy and lower dynamic range. Abstract— In this paper we present the design and simulation results of a CMOS fourth-order bandpass deltasigma modulator, intended for the A/D conversion of RF signals directly coming from a shortwave receiver antenna. The modulator tuning range covers the 3-30 MHz frequency band. The continuous-time loop filter is based on two Gm-C resonators, with low-power transconductors and MOS capacitors. The modulator is implemented in a 0.35um triplemetal standard CMOS technology. Circuit-level simulations demonstrate that the modulator achieves SNDR higher than 60 dB for all the nominal frequency range, for a signal bandwidth of 10 kHz, and a power consumption of about 6 mW. All these proposals allow the IF signal-processing stage of the receivers to be moved to the digital domain. However, in these receivers, RF signals are still first downconverted to IF by means of an analog mixer, so mixer circuit non-idealities (such as non-linearity or mismatch of components) can still degrade sensibly the expected (theoretical) performance. In our work, following this trend of moving to the digital domain as many blocks as possible, we aim to digitalize the RF signal coming from the antenna before any mixing. This way, by moving the mixing to the digital domain, a perfect matching between mixers is achieved and, therefore, no high performance channel-selection filters are required.