Acoustic Crosstalk Elimination

Discussion in 'The Projects Forum' started by NewStudent, Jun 15, 2008.

  1. NewStudent

    Thread Starter Member

    Feb 16, 2008
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    I'm working on an ultrasonic ranger (40kHz) and struggling with acoustic crosstalk between the transmitter and the receiver. With extensive use of the oscilloscope I discovered that during the first 100µs and with a value of 40k for the Rb resistor (see circuit diagram) the target detection is considerably larger then the acoustic crosstalk. The comparator can easily detect this difference.

    Unfortunately, a 40k resistor limits target detection to short distances. However, longer distance detection can be archived if I increase the value of the Rb resistor by stages to 1M .

    Appropriate values for Rb:
    40k for the first 100µs
    85k for the second 100µs
    200k for the third 100µs
    450k for the fourth 100µs
    1M after 400µs

    Does anyone have a suggestion of how to implement the resistor switching?

    Thanks
     
  2. hgmjr

    Moderator

    Jan 28, 2005
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    214
    Can you post the receiver circuit so that we can see the other half of the ultrasonic measurement system?

    hgmjr
     
  3. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    Which resistor is Rb?

    The comparator needs to have a resistor to ground at its pin 2 input because its input floats high without it.
    The comparator needs to have a pullup resistor at its pin 1 output because its output is the collector of an NPN transistor and the resistor pulls it high when it should be high.
     
  4. NewStudent

    Thread Starter Member

    Feb 16, 2008
    25
    0

    The Rb 40K resistor is located between OpAmp pin 6 and pin 7 (see the provided Circuit Diagram).

    Can you suggest appropriate values for the missing resistors?

    Thanks
     
  5. NewStudent

    Thread Starter Member

    Feb 16, 2008
    25
    0
    The receiver circuit is posted as "Circuit Diagram". Here is the transmitter circuit.

    Thanks
     
  6. hgmjr

    Moderator

    Jan 28, 2005
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    Have you considered an analog switch IC such as the CD4066. This device contains 4 electronics switches. I seen them used in gain switching circuits in the past. I checked and digikey has them in stock.

    I imagine your plan is to gain switch your receiver by starting with the lowest gain setting and then on 100us intervals you would bump the gain up to the next value and repeat this process until you get a sufficiently large return signal. At that point you would then do your math to determine the range.

    hgmjr
     
  7. NewStudent

    Thread Starter Member

    Feb 16, 2008
    25
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    Yes, that is the idea.

    I tried two CMOS switches the CD4016 and the ADG451. I couldn't make the idea work with either switch and thought that the problem was the turn on/off propagation delay. With the CD4016 the “Turn-On Propagation Delay” = 40ns. With the ADG451 the tON = 70ns typ. and tOFF = 60ns typ. Reading the data sheet of the CD4066 I find “Propagation Delay Time Control Input to Signal” = 125µs. Since I am trying to switch resistors every 100µs I don't think that it will be fast enough either. Do you have suggestion for a faster switch or another approach?

    I am including the circuit diagram of my attempt to use the ADG451. Do you see any problems with my implementation?

    Thanks
     
  8. hgmjr

    Moderator

    Jan 28, 2005
    9,030
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    I was actually thinking that you would follow this procedure:

    1. Select the least sensitive gain setting.
    2. Fire the Transmitter and see if you get a return signal. If yes, then calculate the distance. If not, then select the next gain setting up and fire the transmitter again.
    3. Repeat step 2 until you get a distance measurement or reach the most sensitive gain in your arsenal.

    If this approach is suitable, then you would not need to be that concerned with the speed of the switch since the delay would only extend the overall time to make the measurement.

    hgmjr
     
  9. Audioguru

    New Member

    Dec 20, 2007
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    896
    The CD4066 switches very fast, a lot faster than 125us.
    The datasheet from Texas Instruments has a Turn-on and Turn-off Propagation Delay of typically only 20ns when it has a 10V supply and 35ns when it has a 5V supply.
     
  10. NewStudent

    Thread Starter Member

    Feb 16, 2008
    25
    0
    Thanks for identifying the deficiency in my circuit design. Can you suggest appropriate values for the two additional resistors (red), see the New Circuit Diagram.
    Thanks for your help
    NewStudent
     
  11. NewStudent

    Thread Starter Member

    Feb 16, 2008
    25
    0
    Thanks I was misreading the datasheet. A delay of only 20ns should not be a problem.
     
  12. NewStudent

    Thread Starter Member

    Feb 16, 2008
    25
    0
    Thanks hgmjr
    What I'm trying to do is:
    1. Select the least sensitive gain setting.
    2. Fire the Transmitter.
    3. Start measuring the time required for the returning echo.
    4. Wait ~400µs and then increase the gain setting.
    5. Repeat step 4 until I detect a returning echo or reach maximum gain.
    The returning signal strength decreases drastically with distance. My plan is to increase the gain over time to compensate.
     
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