Performance limitation by finite gbw in continuous-time quadrature bandpass sigma-delta modulators

Continuous-time quadrature bandpass sigma-delta modulators seriously suffer from finite gain bandwidth of the amplifiers they contain. In this paper, the performance limitation of the continuous-time quadrature bandpass sigma-delta modulator caused by finite gain bandwidth is presented. Finite gain bandwidth results in an unstable behavior of the complex integrators and a deviation from the center frequency. In order to improve stability of the complex integrator by finite GBW, it is desirable to employ a lossy integrator. The deviation from the center frequency affects the in-band noise power of the modulator. A compensation solution for reducing the deviation from the center frequency is proposed by means of transconductors at the amplifier inputs.