depth meter displaying "OUT"; expected reflected wave shape?

Discussion in 'Analog & Mixed-Signal Design' started by patpin, May 11, 2017.

  1. patpin

    Thread Starter Active Member

    Sep 15, 2012
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    On a boat I have the problem tha the Nasa Duet depth meter giving "OUT" on the display instead of the measured depth.
    Normally this means that the reflected wave is not detected.
    I have checked the cable signal (combined input/output coaxial cable to the tranducer) .
    I can see a 150µS 91V pulse and afterward a series of oscillations. I do not know the shape of reflected pulses. Anyone can help me on that one. I include an image of the pulse. (You have to take into account that the probe was 10:1 and multiply amplitude by 10).
    Whats the next step?
     
  2. AlbertHall

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    Jun 4, 2014
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    You will need to use a slower timebase to see the reflection (if there is one). The reflection will be after about 1.3mS for each metre of actual depth.
     
  3. patpin

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    Hello,
    The depth is actually 4meter and with 20mS/div I see... (image)
     
  4. AlbertHall

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    Oh, well, nice try!
    Without more information - a schematic preferably - it's impossible to get much farther. We don't even know whether the transmit pulse is correct. I would have expected a train of short pulses at whatever frequency the transducer sends.
     
  5. patpin

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    I guess nobody has a diagram of this depth meter or more info on the pulse?

    I'll allso try to clean the transducer surface..; maybe marine fouling...
    Do you have an idea what kind of wave shapes I should have seen?
     
    Last edited: May 11, 2017
  6. AlbertHall

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    No. I couldn't find any kind of technical spec.
     
  7. patpin

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    Thanks for yr help !!!
     
  8. AlbertHall

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    Keep your fingers crossed that someone who knows that system comes along :)
     
  9. patpin

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    Crossing helpt!! I now have a (rare) correct signal alternating a lack of reflected waves (1 on 10 respectively). See image for the correct one at 3.8m depth.
     
  10. patpin

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    Thing worked fine for some time but now I allways have OUT; It is not a through hull.
     
    Last edited: Nov 5, 2018
  11. ebeowulf17

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    Aug 12, 2014
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    According to Wikipedia, sound in water travels 1.48 meter/ms, so:

    4m depth x 2 = 8m round trip distance
    8m / 1.48m/ms ~ 5.4ms expected reflection time

    The timing on your scope doesn't match that perfectly, but close enough that you're clearly on the right track. Looks like 1 or 2 ms/division is the right timescale to see what's going on.

    Just to make sure I'm understanding this, when the system isn't working, do you see the same initial pulse, but without the reflection?
     
  12. patpin

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    Today I opened the depth meter and I saw a hot resistance of 47 ohm (attached picture) with some black coloring. It was not open yet. The meter functioned correctly for 4 hours (then I stopped it). I could not detect the reflected wave anymore...I looked on a10ms/ screen (1ms/div) and up to 200mV/div and nothing. The meter was depth stable and indicated 3.3 to 3.2meter. I expected a pulse at 3.9ms after the Trx pulse, which was also stable. No reflection at all.... I'll do more tests when it fails again and post results.
     
  13. ebeowulf17

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    Wait, you had the scope on it while it was working properly, reading ~3.2 meter, and the scope showed no reflection?

    Or are you saying it worked for hours, then you stopped it and hooked up a scope, and then it quit working?

    Two very different scenarios...
     
  14. patpin

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    First scenario! Don't understand it..
     
  15. ebeowulf17

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    Truly bizarre! I'm out of my depth here (pun intended!)

    Let me double check some more facts here. When you got a good scope image of the reflected signal, was the unit reading depth accurately right then? Or was it showing an error state and/or working very intermittently when you caught the reflection?

    I have a weird theory that might partially explain the behavior if you only see good reflections on the scope while the unit is malfunctioning and vice versa.
     
  16. patpin

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    While I scoped, the readings on the display where what they should be: correct.
     
  17. ebeowulf17

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    I'm not sure if you mean while you scoped and saw good reflections, while you scoped and couldn't find reflections, or both, so I'm no closer.

    Regardless, I'll share my "theory," which is really more of a wild guess: if your depth meter uses a transimpedance amplifier at its input stage the amplifier would try to actively sink or source current to keep the input line at zero volts at all times, while converting the current flow through the input into a voltage level at the output. This would explain how you can have no visible reflection trace on the scope when the meter is working. It's only when the transimpedance amplifier was non operational (for whatever reason) that you would see voltage on the input.

    I'll take my guess one step further and speculate that the transimpedance amp is switched off whenever a pulse is being transmitted.

    So, maybe the malfunction is in whatever switching mechanism activates and deactivates the transimpedance amplifier. If my theory is right:

    • When the input stage is working, oscilloscope shows flat line except for outbound pulses, and depth readings are accurate.
    • When input stage is malfunctioning, oscilloscope shows all reflections, noise, etc and depth readings are out of range.
     
  18. patpin

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    Sep 15, 2012
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    concerning post #16: I scoped and could not find the reflected wave and the readings where correct in that periode of >20 minutes.
    Thanks for yr explanation. I'll have to wait to check this when the readings are faulty again.... and pray it doesn't happen.
     
    Last edited: Nov 9, 2018
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