High Frequency Linear Array BeamForming

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SUMMARY

The discussion focuses on High Frequency Linear Array beamforming for imaging sonar, utilizing a linear Rx Array with 100 channels and a Tx probe operating at 420 KHz. The setup includes NI PXIe-6368 ADC cards for simultaneous data acquisition at 2 Msamples/s/channel. Participants highlight issues with high side lobes in the beam pattern, suggesting that inadequate anti-aliasing filters and synchronization problems may be contributing factors. Recommendations include implementing active low-pass filters and ensuring proper calibration of the system to mitigate reflections and phase discrepancies.

PREREQUISITES
  • Understanding of High Frequency Linear Array beamforming techniques
  • Familiarity with NI PXIe-6368 ADC cards and their synchronization
  • Knowledge of anti-aliasing filter design and implementation
  • Experience with MATLAB for data processing and beam pattern analysis
NEXT STEPS
  • Research advanced anti-aliasing filter designs suitable for 4x oversampling
  • Learn about calibration techniques for acoustic measurement systems
  • Explore the use of high-speed USB oscilloscopes like PicoScope for data acquisition
  • Investigate methods to reduce reflections in aquatic environments during testing
USEFUL FOR

Acoustic engineers, researchers in sonar technology, and professionals involved in signal processing and data acquisition systems will benefit from this discussion.

  • #31
Baluncore said:
1. The separation between the elements in the array is 14.3mm, which is much greater than the wavelength of 3.52 mm. That will lead to ambiguity of received phase. The main lobe will have a null at Atan( 3.52 / 14.3 ) = 13.82°;
The 100 element * 14.3 mm = 1430 mm long array will require a rigid mounting with alignment accurate to better than 0.3 mm at the ends of the array.

3. Can you describe the physical connections for the channels. Where are the filters and pre-amplifiers located, and are all the interconnections the same length. I would look for hum loops where ground currents flow in signal cables, or through the water.
As separation is greater than (wavelength/2), 'grating lobes' occurs but beyond +/-6 deg. That's why only +/-5 deg sector has been used here. The sensors are mounted on a rigid water tight hollow cylinder outer surface with preamplifiers/filters fitted inside the hollow cylinder. There is approx. 0.5m coaxial cable b/w sensor and corresponding preamplifier. The two water sealed 18m long cable sets each carrying 50 preamplifiers/filters output goes to surface through two glands in hollow cylinder.
 
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  • #32
Changing the RX array position very slightly changes the RX phase differences significantly. That suggests there is cross-coupling between the signals somewhere other than in the water.
I can't see how to progress investigating signal integrity without all detailed circuit and mounting information. Such as;
nauman said:
The sensors are mounted on a rigid water tight hollow cylinder outer surface with preamplifiers/filters fitted inside the hollow cylinder.
What is the tube made from? Is it grounded? How and where are the signals and power supply grounded.
Or;
nauman said:
The two water sealed 18m long cable sets each carrying 50 preamplifiers/filters output goes to surface through two glands in hollow cylinder.
Are the 18 m long cables each 50 twisted pairs, or 50 coaxial cables. How are the two ends of those transmission lines terminated to prevent reflection? and what about cross-talk?

When you compare the phase of an element against the two adjacent elements, is the phase difference of the first few cycles the same as the difference for the last few cycles, or does phase vary during the RX pulse? Are some, or all, channels unreliable?
 

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