What is the basic principle of sonar signal simulator based on DDS technology?
This article aims to develop a kind of high-resolution imaging sound signal simulator. The imaging sonar receiving acoustic array uses a 48-element equally-spaced linear array, an operating frequency of 800 kHz, an action distance of 0.5 to 25 meters, and an angular resolution of 0.35 °. Imaging sonar beamforms the received array signals to achieve the acquisition of acoustic images.
The incident sound waves are incident parallel to the normal direction of the array at an angle of θ, the elements are numbered from left to right in order 1, 2, ... i, t + 1, ... N, and the element spacing is d. If primitive No. 1 is selected as the time reference point and the received signal is Acos2πft, then there is a difference in sound path Δ = dsinθ between two adjacent primitives. Where c is the speed of sound. Since the imaging sonar is a narrow-band active sonar, the phase difference between the received signal of the element I and element 1 is φi = 2π (i-1) d / λsinθ, where λ is the wavelength. Therefore, in order to orient the linear array in the direction of θ0, it is only necessary to delay the signal of the i-th element by τi (θ0) = 2π (i-1) d / λsinθ0.
The above is the basic principle of linear array beamforming, but this is only an approximation in the case of the far field. For near-field conditions, the error produced by such an approximation can be significant. For the high-frequency imaging sonar in this paper, since the entire working range belongs to near-field conditions, the focusing method must be used when beamforming. The basic principle is the same as above, but the delay (or phase shift) of each element signal is not linear, and this article will not elaborate on this.
The signal analog quantity used for imaging sonar is generally the same as the number of primitives. The output of each channel simulates the signal of one primitive in the sonar array. Because the working distance of imaging sonar is relatively short, and the noise level of the high frequency band in the underwater acoustic environment is very low, the received signal-to-noise ratio is usually high. For this reason, no additional noise is added to the output of the signal simulator. Imaging sonar works in a strong reverberation environment. Because the simulation of reverberation is difficult and the impact on imaging is not serious, the simulation of reverberation is not considered in the design, only focusing on the simulation of near-field targets Echo.
According to the azimuth and distance of the point target to be simulated input by the user, the signal simulator calculates the phase difference of the corresponding target echo reaching each element of the receiving array, and then generates a corresponding multi-channel sinusoidal signal according to these phase differences. Add these signals to the input end of the imaging sonar, instead of the actual array output, so that the imaging sonar can be easily debugged and measured under the conditions of the land laboratory.
Traditional sonar signal simulators usually use a fixed oscillator to generate sinusoidal signals at the same frequency as the sonar system. The local oscillator signal is passed through a set of multi-tap analog delay lines, and then the signals are drawn from the different taps of the delay line as the output of the simulator. There are several defects and deficiencies in this signal simulator structure.
First, since the analog device is used to form the tapped delay line structure, the minimum variable delay length is limited. Especially considering the scale and cost of the system hardware, the number of taps on the delay line is generally not large, which causes a large error between the delay time and the theoretical value, thereby reducing the accuracy of the simulator.
Secondly, in order to simulate the echo signals of different azimuth targets, the outputs of different tap delay lines must be switched or combined, and then output as a primitive signal to the sonar device. Therefore, the scale of the whole simulator is huge, and it can only simulate a few discrete azimuth and distance targets. It cannot simulate the point target echo at any azimuth distance, otherwise it will be difficult to achieve without increasing complexity.
In addition, the tapped delay line formed by analog devices is difficult to ensure channel consistency and difficult to debug. And the frequency range of the delay line is narrow. If the frequency parameter changes, it will not be used normally, so the application range is narrow, and the cost performance is very low.
In order to overcome these defects of the traditional sonar signal simulator, this paper uses DDS technology to design and implement a new type of signal simulator. This DDS-based simulator structure can achieve accurate simulation of point target echo signals at any azimuth distance, is suitable for different frequency parameters and has a certain expansion capability, and thus has a high cost performance.
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