Background normalization algorithms attempt to suppress the ambient and self-noise during the measurements of sonar, which enhance the detection performance and the display effect of weak signals. Conventional background normalization methods are usually sensitive to the accuracy of prior set filtering interval and threshold, while significant noise is still detected in low frequency. In this paper, an improved background normalization algorithm is proposed by thresholding the processing interval between several local peak values and local valley values. Compared to the existing scenarios, the proposed approach automatically calculates the filtering interval and threshold, with substantial resilience to the noise level in low frequency. Experimental results illustrate the effectiveness of our algorithm.

The pulse-output Direct Digital Synthesis (DDS), in which the overflow signal of the phase accumulator is used for the pulse output, can be applied to sonar and radar systems as a way of signal generation and modulation. Despite its simple hardware implementation, it suffers the drawback of phase jitter. In this paper, the pulse-output DDS for continuous wave (CW) signal generation is introduced and the phase jitter problem is analysed. By examining the instantaneous frequency of the overflow signal, this paper reveals that the generated CW pulse is far from being single frequency, instead two different frequencies exist. How these two frequency components are distributed with time is also considered. An inequality that helps select the value of system clock to achieve certain frequency precision is given. This paper also analyses the pulse duration error and initial time delay error. Experiment results are in good agreement with calculations.