I have the following setup:
2 piezo transducers are placed 0.7 meter apart from eachother in water facing eachother. 1 piezo (PA) acts as the sender, the other piezo (PB) acts as the receiver.
PA sends periodically a pulse that the receiver picks up. The pulse gets generated by a microcontroller switching a MOSFET according to the following circuit.
With the send pulse:
PB receives the pulse and is connected to the following amplifier circuit.
With the amplified receiving trace:
The two circuits are only connected to eachother through earth (microcontroller + send-circuit are supplied by a 230V AC to 5V DC converter, the receive-circuit is supplied by 2 benchsupplies).
Extra logic is added so I can detect if the pulse arrives or not. This works, however I seem to get coupling on my scope image from PA onto PB. For now I filter this away in my code but I rather fight the cause instead of the symptoms. As you can see, the send pulse also is visible on the receive trace (the left most pulse). Both traces are captured with the same scope, though at different times and with only 1 probe connected.
Is there a way to surpress this signal? Later on, the setup will consists of more piezo's as well as both sending/receiving capabilities and I rather keep crosstalk (?) to a minimum.
Following the questions in the comments:
1) Are you sure that the coupling is electrical and not, say acoustic ?
=> Yeah I'm pretty sure that it's electrical. If I connect both probes the left most receiving pulse is at the same time as the send pulse. If you take the time difference between both pulses (around
476us) and the speed of sound in water (
1480 m/s), you can calculate the distance between the 2 piezos
x = 1480 * 476e-6 = 70cm
2) Why is the send a pulse and not a sinusoid ? => I'm still tweaking the send pulse circuit. I send a single pulse as the setup is to detect if something (fish) goes through the circle (only 2 piezos at the moment at the diameter). Hence receiving a pulse is nothing in between, not receiving a pulse is something in between. The setup is NOT for transmitting data.
3) Is the output of the micro controller on the send side a single pulse or a a sequence of pulses tuned to the resonant frequency of the piezo transducer ? => It's a pulse of 40us at the moment.The pulse gets created on the down slope.
4) Are they immersed in water ? => Yeah they are immersed in water. Not deep at the moment but later on this will be in the meters.
5) Are the leads to the transducers well insulated from the water ? => No! The leads are not shielded. So maybe some electromagnetical interference? I also think they are not twisted, it has a housing around it. I will pry one open to see further. Also the water is rainy water, with apparently some algaea too.