Animals such as dolphin and bat feature powerful biological sonar structure. The research on the mechanism and principle of signal processing performed by neural system will provide a novel viewpoint for signal processing. In this paper, a biological neural loop is established to handle Doppler signal based on neural coding theory of ordered space. From simulation, the system loop achieves fast operation and high resolution. Furthermore, Doppler signals of continuously-changing frequency can be properly coped with as well. The network structure for handling Doppler signals may take many forms. However, the structure proposed in the paper is possibly the simplest one, for it comprises only one or two neuron(s), ensuring rapid processing and high resolution.