Experimenting with some ultrasonic transducers (noise on receiver)

Discussion in 'Wireless & RF Design' started by irmanao, Jul 10, 2018.

  1. irmanao

    Thread Starter Member

    Apr 29, 2017
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    I am experimenting with some ultrasonic transducers and have encountered a small issue with the receiver. At this stage the receiver is just the second transducer without any additional circuitry. The received signal sits on top of a 50Hz noise signal (presumably from mains). In yellow is the transmitter and in purple the receiver. You can see the slight curve at the end there and if I adjust the time scale the 50Hz noise is visible. Is there a way to filter that out? I assume the noise isn't present when the probe isn't connected. Will this problem go away after the amplifier stage is connected? Any ideas?
    upload_2018-7-10_16-39-49.png
    thanks
     
  2. ericgibbs

    Moderator

    Jan 29, 2010
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    hi irmanao,
    I assume the ultrasound is around 33Khz to 45Khz frequency. please confirm
    It is usual to have some form of tune circuit in the receiver.
    Please post more details of the project.
    E
     
  3. irmanao

    Thread Starter Member

    Apr 29, 2017
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    This is the simple circuit at this point (the application is for measurements inside liquids): upload_2018-7-10_19-57-2.png
    I changed the inductor to 21uH for the correct resonance of 1MHz at the transmitter (the transducer is 1200pF). The transducer is at 1MHz and i receive that 1MHz at the receiver albeit with extra noise from reflection and it being on top of the 50Hz sine wave. The resistor parallel to the receiver is missing from my circuit.
    thanks
     
    Last edited: Jul 10, 2018
  4. ericgibbs

    Moderator

    Jan 29, 2010
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    hi,
    If you need to increase the range, I would add a 1MHz tuned pre-amp between the SP2 transducer RX and the PIC.
    Also adding a comparator after the preamp, you could raise the RXP to a digital level and use a PIC interrupt pin.
    E
     
  5. irmanao

    Thread Starter Member

    Apr 29, 2017
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    When you say 1MHz tuned pre-amp, you mean an opamp with enough gain bandwidth for the desired amplification and frequency, correct?
    My main concern was the 50Hz sine wave that my signal is sitting on but i will add the amplification stage and maybe some filtering and will be back in a few days with the results.
    thanks a lot
     
  6. ericgibbs

    Moderator

    Jan 29, 2010
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    hi,
    The tuned pre-amp could be AC coupled and have a pass band centred at 1MHz.
    What range and accuracy are you requiring.?
    E
     
  7. RichardO

    Late Member

    May 4, 2013
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    Do you really have a 1 MegaHertz transducer?

    That is a very high frequency for transmission through air. A few centimeters at most using a *lot* of gain in the receiver.

    In water, the attenuation is very roughly 1dB/cm/MHz.

    You will need a very fast op-amp to get much gain at 1 MHz. A current feedback op-amp will likely work for low gains.

    Once you have your receiver tuned to the transducer frequency, the 50 Hz will go away.
     
  8. irmanao

    Thread Starter Member

    Apr 29, 2017
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    I'm trying to build a water flow meter for a 14cm (5.5inch) pipe. Yes the transducer is 1MHz. What do you mean exactly when you say tuned pre -amp?
    thanks
     
  9. RichardO

    Late Member

    May 4, 2013
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    Okay. The 1 MHz makes sense to me now.

    It seems to me that you need something moving with the water to create a reflection. What are you getting a reflection from?

    Are you planning on measuring the Doppler frequency? What change in frequency are you expecting?


    Pre-amp means the first amplifier in a chain of amplifiers. This is a low noise amplifier for the small signal received from the transducer. The tuning is used so only the frequency of the transducer is amplified. For instance, the 50 Hz will not be amplified.
    I am sure that @ericgibbs can explain this better. :D
     
  10. ericgibbs

    Moderator

    Jan 29, 2010
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    hi,
    These two LTSpice plots show the AC response of a very basic tuned preamp, depending upon you application it will need refining.
    The V2,4,5 voltage sources are for simulation only.
    The plots show a 50Hz half cycle with a short 1MHz burst superimposed, which represents an [vrx] Echo signal.
    Note the 50Hz component does not get amplified.

    We also need more details of your project.
    E
     
    irmanao likes this.
  11. irmanao

    Thread Starter Member

    Apr 29, 2017
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    Thanks for the replies. I just came back from a trip so i'll post again in a while because the fast opamp that is needed is not available where i live and i'll have to order it. I'll be using the Time Of Flight methodology as shown in the picture:
    : upload_2018-7-12_21-4-1.png
     
  12. irmanao

    Thread Starter Member

    Apr 29, 2017
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    I managed to remove the 50Hz component with a bandpass filter and amplify the signal with enough resolution. The end result is workable and accurate. I used an LM6172 chip which has 100MHz unity gain bandwidth.
    (picture: purple=output, yellow=input)


    thanks for the help guys
     
  13. ericgibbs

    Moderator

    Jan 29, 2010
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    hi,
    Thats good to hear.
    What are the flow rate limits you can detect and the resolution level.?
    E
     
  14. irmanao

    Thread Starter Member

    Apr 29, 2017
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    I was speaking about the signal, i haven't measured any flow yet. I am however planning on having <100ps resolution and with the pipe that i'll be using which is fairly large (15cm) that's plenty for accurate results.
    thanks
     
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