Hello Forum,

I am designing an Ultrasonic Reciever and Transmitter circuit, in which the Reciever and Transmitter Transducers are connected with the same cables. That means that the Excitation signal and the Received signal will be transported by the same medium. The Excitation signal is a unipolar 400kHz square wave 0-20v and I'm protecting the Reciever transducer with a T/R switch (so the excitation signal does not excite the Reciever Transducer ), This works fine. But, when Receiving the signal I'm getting both the Receiving Transducer and the Transmitter Transducer Signals and they are basically coupled. I want to get only the Received signal from the receiver transducer. I was thinking of using a simple diode but it is not working. Do you have any suggestions? Is there any component that only blocks low amplitude voltages around 100mV. Is there are low Voltage transmitter switch. (Normally closed with high voltages open with low voltages)?

Thank you very much.
-Enrique

 

Two diodes in parallel back to back would have that characteristic. They would block signals below about 0.5V but pass higher voltages.

 

You could use a mixer with an op amp, invert the transmit signal and mix it with the recieived signal, this will cancel out the transmitted signal. Germanium or Shotkley diodes will conduct at 200mV.

 

The circuit shown will not work. You discovered that already. What you need is a driver that presents an open circuit when it is not driving, and a saturating transistor shunt on the receive end, along with a series FET switch to connect the receive amplifier. If you can find a circuit of the sonar depth finders of a few years back they used that scheme.