Hi Oschi
Its an analog (chirp) signal. That goes between the unit and the transducer. I have tested this with my oscilloscope a while ago.
Just briefly because the amplitude of the signal was that high that it still maxed out with my x100 probe.
I wanted to measure this again, but with the oscilloscope of the university nearby. They can handle high voltages and have better probes. But then COVID happend and only students where allowed. So I never got to it.