Design and implementation of a tunable 40 MHz-70 MHz Gm-C bandpass ΔΣ modulator

The discrete-time ΔΣ modulator is transformed to the continuous-time ΔΣ 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 ΔΣ modulator is mapped to a stable continuous-time ΔΣ 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 ΔΣ loop filter