High Frequency Linear Array BeamForming

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Discussion Overview

The discussion revolves around the challenges and observations related to High Frequency Linear Array beam forming for imaging sonar applications. Participants explore issues related to the setup, data acquisition, and processing of signals from a linear receiver array consisting of 100 channels, focusing on the performance of the beam pattern and the effects of aliasing and filtering.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Experimental/applied

Main Points Raised

  • Nauman describes the setup involving a transmitting probe and a receiver array, noting discrepancies between simulated and actual beam pattern results.
  • Some participants suggest that aliasing may be occurring due to insufficient anti-alias filtering, given the 4x oversampling rate.
  • Concerns are raised about the need for steep anti-alias low-pass filters for each channel, with suggestions for active filtering methods.
  • Nauman mentions the use of bandpass filters centered at 420 KHz with specific attenuation characteristics, prompting questions about their effectiveness against aliasing.
  • Participants inquire about the calibration of the setup and the potential impact of reflections from the water tank's surfaces on the results.
  • There is discussion about the windowing function used during data acquisition and its effect on the quality of the received signals.
  • Nauman reports that the phases of the digitized waveforms do not match the simulated ones, raising questions about synchronization and potential issues in the setup.

Areas of Agreement / Disagreement

Participants express differing views on the adequacy of the filtering and calibration methods employed, with no consensus reached on the root causes of the observed issues in beam pattern performance.

Contextual Notes

Participants note limitations related to the oversampling rate and the characteristics of the filters used, as well as the potential for reflections in the experimental setup, but do not resolve these issues.

  • #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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