A quadrature sub-sampling direct conversion mixer capable of sampling two or more bands concurrently using a single sampling frequency is presented. The implementation of the mixer to sample a band in quadrature and downconvert it to baseband is discussed and it is shown how this idea could be extended to sample in quadrature two or more bands concurrently. The proposed circuit is analyzed in detail and the results are validated using Spectre RF simulations for a 0.18 mum CMOS process.