What are the advantages of adding an RC low pass filter after an envelope detector? I am asking this because I often notice (in multiple studies) that this type of circuit logic (in the figure below) is used for information demodulation by first using a high-pass filter, then an envelope detector, and further a low pass filter, but I am wondering why.

The envelope detector already works as a low pass filter by filtering the high-frequency inputs and giving an envelope of the input signal as output. I can imagine using the low pass filter to further diminishing high-frequency noise, but then again, why not use a bandpass filter before enveloping the signal?

enter image description here


1 Answer 1


The filters before and after the demodulator (the envelope detector) do different things.

The filter before the detector is there to make sure that you are detecting the correct carrier frequency. It typically has a band pass filter rather than just a high pass filter as your example shows.

Here is a modulated carrier signal:

enter image description here

That is a 16kHz carrier modulated with a 10Hz signal.

You would use a band pass filter centered around 16kHz to pick the carrier out of noise.

After passing it through a detector, you have a signal that looks like this:

enter image description here

You can see that there's still some of the carrier there (the tops of the waves are fuzzy) and that the signal is slightly distorted (the bottoms are squared off.)

A band pass filter after the demodulator cleans up the remains of the carrier and reduces the distortion:

enter image description here

That's not optimal, but it is better than without the filter. Those photos are from some experiments I made a few months ago for a gadget I ended up not building.

  • The bandpass filter before the detector selects the carrier.
  • The bandpass filter after the detector selects the "information" signal - whatever was modulated on the carrier.

If you know the content of the noise and interference, you may be able to simplify the bandpass before the detector to just a high pass or just a low pass filter.

If you know the content of the signal, you may be able to simplify the filter after the detector to just a low pass filter - or even omit it entirely.

  • \$\begingroup\$ Thanks! It is much more clear to me now, especially because of the provided photos. I am only wondering if in your case, both of the bandpass filters are exactly the same? \$\endgroup\$
    – Gabriel
    Commented Jul 21, 2021 at 6:42
  • \$\begingroup\$ Nope. A band pass that will pass 16kHz is nothing like a band pass for 10Hz. \$\endgroup\$
    – JRE
    Commented Jul 21, 2021 at 6:58
  • \$\begingroup\$ Ok, got it! Thanks. \$\endgroup\$
    – Gabriel
    Commented Jul 21, 2021 at 13:48

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