Peak Detector Design

Hi, I am building a peak detector and voltage comparator for my returning signal of the ultrasonic transducer.

Attached are the circuit diagram of peak detector and voltage comparator and the screenshot of oscilloscope after peak detector and voltage comparator:As you can see, it kind of works, it detect the peak at the very beginning, however, what I am interested is the returning signal after the first pulse (the signal that appears in the middle of the screen), and my peak detector is not able to detect it.

Any idea about how to do it?

Thank you.

You have to disable(blank) the input until you want the peak detector to operate.
You may need to put the peak detector inside the feedback loop of an op amp as well, since the diode only conducts when you get above it's turn on voltage. I assume that the first pulse is the transmit signal getting directly into the receiver, so you blank for a fixed time during and after the transmit pulse,
cheers,
Richard