to get the exact envelope of the 40khz signal

[24+]

hi, i want the exact envelope of the 40khz signal . i am using low pass filter but i am not getting the signal clearly. suggest me a solution. you can see the attachment.
thanks

 

[cdh7]

Hi,

You can get a pretty good result using a low pass filter followed by a Schmitt trigger to generate clean logic transitions. Select the cutoff frequency of the LPF so that the resulting 40 kHz ripple has a low enough amplitude to prevent the Schmitt trigger from switching.

I don't know what the amplitude of your signal is, so I just picked some values and did a small simulation as an example.

Regards,
Chris

 

[FvM]

The shown waveform can't be related to a low-pass filter response. There's obviously something wrong with your circuit.

 

[keith1200rs]

I would have thought you should be using a peak detect circuit not a low pass filter. You need rectification.

Keith