Abstract: Aiming at the difficulty to realize the conventional analog quadrature demodulation topology under the condition of the large time-bandwidth product signals, the quadrature demodulation method and the corresponding topologies based on the parallel sampling of the Double-IF signals are proposed. The method and the topologies which generate the digital in-phase and quadrature (I/Q) components by selecting the specific sampling instant of the Double-IF signals with the same echo information and the simple sign changer of the output of the ADCs are the extensions of the conventional analog quadrature demodulation topology and easy to be engineering implemented for real time. The poor performance and complex digital correction of the conventional analog quadrature demodulation topology are solved in the new method and the new topologies. The simulation experiment and analysis show that the new method and the new topologies have the good performance of generating the digital I/Q components of the large time-bandwidth product signals.