Direct RF sampling mixer with recursive filtering in charge domain

Abstract

We present a direct RF sampling technique in which an input RF signal is converted to a current waveform, gated and integrated on a sampling capacitor. A rotating capacitor shares this charge with the main sampling capacitor and transfers it to a subsequent discrete-time switched-capacitor filter stage. This action creates a first order IIR filter which serves as an anti-alias filter for subsequent stages. The transfer function of this stage can be changed by adjusting the clock signal controlling the rotating capacitor. This approach has been validated and incorporated in a commercial Bluetooth receiver IC realized in a digital 130 nm CMOS that meets or exceeds performance of other conventional Bluetooth radio architectures.