peak detector design

[Joe Wu]

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

And below are the screenshot of oscilloscope after peak detector and voltage comparator:
[URL unfurl="true"]https://res.cloudinary.com/engineering-com/image/upload/v1487646346/tips/pic_62_2_ja67vl.bmp[/url]
[URL unfurl="true"]https://res.cloudinary.com/engineering-com/image/upload/v1487646378/tips/pic_62_1_qltlhp.bmp[/url]
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.

 

[IRstuff]

You designed your circuit to do exactly that, by designing the time constant to be that long.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

https://www.youtube.com/watch?v=BKorP55Aqvg

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[Joe Wu]

I designed according to this:
the capacitor-resistor combination will establish the time constant 𝜏, which should be kept to a value above the period of the main component (carrier, 5MHz), but much lower than the modulating signal (in this case dictated by the frequency of repetition 7.8kHz).

T1 = 1/5000000 = 0.2µs

T2 = 1/7800 = 128µs

𝜏 = R*C = 100p*220k = 22µs

∴ T1 < 𝜏 < T2

 

[IRstuff]

OK, regardless, your return is coming back at about 8 us.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

https://www.youtube.com/watch?v=BKorP55Aqvg

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[kcotton]

your peak detector must be disabled during the main bang,
may be a N-MOS to short the net Output_to5th to GND, during your transmit pulse and 1us or so after that pulse