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I am attempting to design an envelope detector, however, I'm having problems with regards to diagonal clipping and negative peak clipping of a certain input.

envelope detector picture

While trying to follow this video's simulation file, without modifying its input, I noticed that there isn't a lot of hassle when picking for the RC values since the modulation index was okay for the input wave.

Input: input1 (modulation index: 0.5)

Output: output1

However, I am unsure of what RC values to choose when my input is: $$f_m = 10kHz\quad f_c=600kHz \quad \text{amplitude}=1V \quad \text{modulation index}=0.5$$

I first tried getting the geometric mean of my messenger frequency and my carrier frequency but the plot shows that these values large

Input: input5

Output: output2

I then tried lowering my RC value in an attempt to fix the diagonal clipping, but as seen on the plot, the output still isn't a clean sine wave (compared to the very first output. Decreasing the RC values further would then result to negative clipping and ripples on the upper portion of the output.

Some questions I want to ask are:

  1. How do I get the RC values that would still maintain the shape of the envelope given the input? Is it possible to do so without adding components to the envelope detector?
  2. If it is not possible to get the optimal RC values, is it still possible to retrieve the message in later stages? (i.e. inputting the output to a high pass filter/low pass filter or amplifier)
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  • \$\begingroup\$ You appear to talk about two plots where you're trying stuff, but only show one -- I assume it's the "negative clipping" one. \$\endgroup\$
    – TimWescott
    Jan 7, 2021 at 19:58
  • \$\begingroup\$ Decrease R1 and/or C1, allowing C1 to discharge faster. (A separate LPF after a buffer amplifier can be used to remove the sawtooth if you need to). There are also tricks, like returning R1 to an -ve supply.to eliminate the -ve peak clipping you see. \$\endgroup\$
    – user16324
    Jan 7, 2021 at 20:00
  • \$\begingroup\$ I'm not sure you mean "clipping" \$\endgroup\$ Jan 7, 2021 at 23:16
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    \$\begingroup\$ Why did you change from 5 volts to 1 volt amplitude? \$\endgroup\$
    – Andy aka
    Jan 8, 2021 at 10:40

2 Answers 2

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If your diode is of silicon, voltage drops around 0.7 V and peak of output is only 1 - 0.7 = 0.3 V at maximum. That's why blue plot looks apart from green plot however you change RC constant.

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  • \$\begingroup\$ This is the correct answer. You need more carrier amplitude \$\endgroup\$
    – RussellH
    Aug 16, 2022 at 3:53
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Sure you have got distortion on the sim. You will get it in real life. Try making the input volts 10 volts to reduce diode effects. If this helps then place a DC voltage source in series with your AC generator. Experiment with a start value of say half a volt. Most AM detectors are pre-biased to remove some diode nonlinearity. Then reduce the RC time constant accepting more ripple. Now you can post-filter to get the ripple down to an acceptable level. Often an RC section is adequate. I have used full-wave bridge to get low 2F ripple that is easier to post-filter. I have benched a germanium pre-biased full-wave bridge that performs well. If you have spice models for these then you could try. Now things can get worse when modulation % is increased like 90%. When you do this on the sim you will have to do more optimization.

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