The coherent lidar system for speed measurement and the characteristics of the echo signal are discussed. And the
principle and the advantages of the digital quadrature transform are analyzed. A digital quadrature detection scheme
based on polyphase filter for coherent lidar system for speed measurement is presented by using the idea of software
radio. Firstly the amplified analog signal is sampled to digital signal, and then is re-sampled with parity, time-aligned by
delay filtering, finally the signal is divided into in-phase component and quadrature component. The scheme can be used
to decrease the hardware complexity of system to a large extent, to avoid aliasing between the positive and the negative
spectrums, to improve the SNR, and to obtain the signal in the quadrature form so as to fit the follow-up processing.
Simulations and experiments indicate that the scheme is capable of resisting the noise compared with the traditional
methods and is prone to real-time implementation, which, in turn, makes a contribution to greatly improve the
performance of coherent Doppler lidar system for speed measurement and to orient the direction of a moving target.