Two types of direct-detection Doppler lidars are described: the multichannel spectral resolving approach and the edge technique approach. The two methods are compared in terms of fundamental system characteristics. The equations necessary to model the instruments and predict noise-limited performance are developed in forms that enable an easy comparison of the methods. Using those equations, simulation results are presented to compare the performance of the receiver systems. Results are shown for both aerosol Doppler and molecular Doppler systems. Instrument design considerations are discussed to clarify the choice of realistic values for the model parameters.