Envelope detector design

I am working on a project in which we are trying to design and build an ultrasonic thickness gauge that will calculate the thickness of a given part based on the time difference between sending a signal and receiving a reflection back (using the known speed of sound through the medium).

As we are in the initial stages of prototyping, we are currently using an Arduino nano as the MCU for our prototype. We have successfully amplified the pulse generated by the MCU to a higher voltage, which is then used to excite the ultrasonic transducer (which is in contact with the part that is being measured). The reflection is then detected by the second element of the transducer and converted into an electric signal that is centred around the natural frequency of the transducer (5MHz). This signal is then processed (amplified and filtered) into a level that would be detectable by the MCU (circuit and scope screenshot as shown below). However due to MCU's limited sampling rate, the signal will not be reliably detected. Therefore I was wondering if we would be able to devise an envelope detector that would extract the amplitude modulation of the reflection signal, effectively converting a very high frequency into a lower frequency signal, which I am hoping would be detected by the MCU.



According to what I have read so far, this would be achieved by first rectifying the signal using a “super-diode” (precision rectifier configuration as shown below) and then integrate it through an op-amp integrator. However, it seems that because I am relying on single supply and the frequency of the signal of interest is very high, the rectifier will not work as I expected, in fact I did not get anything out of it. Is there perhaps a way of designing an envelope detector entirely from transistors instead of op-amps?



Any pointers or suggestions will be highly appreciated!!

Thanks in advance!

Joe

I suspect that what you want is a peak detector, rather than an envelope detector. Either way, I think if you use a fast enough diode and opamp, you should be able to create your ideal diode. You didn't show any parameters on your schematic so it's impossible to tell you why it didn't work.

The op amp that we are using is LM7171, which is pretty fast, but the thing is we only have single supply here. for the rectifier, we using LM7171, R1=R2=1K, diodes are 1N4007

Hi,

1N4148 is way more suitable. Faster, low capacitance.

Klaus

Thank you all. I have changed the diodes, and problem solved. Now is just matter of program the MCU