Shortening the coaxial leads of ultrasonic sensors

Discussion in 'General Electronics Chat' started by ardali, May 10, 2015.

  1. ardali

    Thread Starter New Member

    Dec 4, 2012
    9
    0
    Hi all,
    Ultrasonic sensors I use (car parking sensor type) had 6 meter 75 om coax leads. I shortened them to about 30 cm. This did not effect transmission. However reception fails. Adding cut pieces in parallel reception is possible but erratic and noissy. Oddly, connecting a 15 cm coax piece to try some capasitors same erratic reception is possible (without any capasitor).
    Any suggestions ?
    Thanks in advance.
    O.Ardali
     
  2. Papabravo

    Expert

    Feb 24, 2006
    10,148
    1,791
    Clearly your basic premise is false. Shortening the coax did affect the transmission.
    Without more details on the transmitter, and the receiver, and the nature of the signal, I can't help you, and your attempts to fix the problem are not based on any kind of reasonable plan. I don't suppose you have access to a VNA do you?
     
  3. ardali

    Thread Starter New Member

    Dec 4, 2012
    9
    0
    No I do not have Access to a VNA.
    TX transducer (with shortened leads) transmits a burst of 40 kHz signal and this I can receive on a sensor with the original 6 meter lead (verified on the osciloscope) but not on sensors with shortened lead. By shortening the leads I assumed I had removed some capasitance and I was trying to see the effect adding some parallel capasitance.
     
  4. Papabravo

    Expert

    Feb 24, 2006
    10,148
    1,791
    Except the coaxial cable is actually modeled as distributed inductance and distributed capacitance. Never mind all that, why are they using coaxial cable for a VLF signal. The wavelength is many times the 6 meter length of the original cable and I doubt that the attenuation difference is significant. I'm guessing it is reflections that are biting you.

    What happens if you use twisted pair?

    Have you tried terminating the receiver end with a 75Ω resistor?
    What is the output impedance of the transmitter?
    Schematics?

    Sorry my crystal ball is in for repairs.
     
    cmartinez likes this.
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