Its possible to use a single transducer for transmit and receive - but this requires a transmit/receive changeover circuit to allow the transducer to both transmit high voltage signals and be connected to a sensitive receive amplifier. Vehicle reversing/parking sensors typically are like this.Using a separate transducer for receive and transmit means the transmitter can be optimized for high power output and the receiver optimized for sensitivity, as well as isolating the receiver from transmitter and making the circuitry simpler (no transmit/receive signal switching).
I call that diode switching! The cheap parking sensor units have transformers to boost output voltages to 40V or so and I suspect (been a while since I looked at it) they use a large value resistor and diodes to protect the input circuit, and have more circuitry to switch between 4 different transducers (all with own tuned transformer)... I think the transmit pulse was actually 4 cycles at 40kHz...
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