Design and implementation of a tunable 40 MHz-70 MHz Gm-C Bandpass deltasigma modulator

The discrete-time [Delta][Sigma] modulator is transformed to the continuous-time [Delta][Sigma] modulator. For either nonreturn to-zero or return to-zero feedback, the loop transfer functions of the continuous-time and discrete-time modulators can be made exactly equivalent. Hence, a stable discrete-time [Delta][Sigma] modulator is mapped to a stable continuous-time [Delta][Sigma] modulator having the same SNR. The anti-alias (image rejection) properties of the continuous-time modulator are described. Experimental results for a tunable 40 MHz-70 MHz continuous-time Gm-C modulator are shown to verify the theory. A master-slave tuning scheme is developed to tune the Gm-C [Delta][Sigma] loop filter. Index Terms - Analog-to-digital converter, bandpass [Delta][Sigma] ADC's, continous-time filtering, converter, [Delta][Sigma] modulator.