Diode envelope detector question?

He says RF is filtered out by a cap. Isn't the supposed to be a resistor in series with a load to form a low-pass RC filter?

Second question is once the RF is removed how does low frequency ENVELOPE become a low frequency SIGNAL. Envelope is made of carrier frequency, once carrier is filtered, seems nothing is supposed to be left?

https://www.youtube.com/watch?v=e_gTCU2fnD8

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Update 2:

"The capacitor smooths out the carrier ripple to make it negligible which leaves you with the modulation (low-freq) signal." ONCE AGAIN, "envelope" is MADE OF RF, if RF is removed nothing is left, how does low frequency signal appear. This is not explained.

Update 3:

We can say that capacitor removes the RF oscillations, but not the envelope, that is, it converts the envelope into actual LF signal.

Update 4:

Philomel Modulation just shapes the carrier, there is no low frequency signal being transmitted.

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2 Answers

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  • qrk
    Lv 7
    4 months ago
    Favourite answer

    The carrier is rectified.

    What is left is an envelope that is pulsing at the carrier frequency.

    The capacitor smooths out the carrier ripple to make it negligible which leaves you with the modulation (low-freq) signal. Your output in this example is DC shifted, but a series coupling capacitor at the wiper of the pot will fix this.

    The variable resistor on the output is the resistance that forms the low-pass filter in conjunction with the capacitor.

    I suggest you cobble this circuit in LTSpice so you can look at the waveforms with and without the capacitor. Leave out the transformer as that's not important for the learning part.

  • 4 months ago

    The RF signal is made up of two signals. The carrier and the modulation. If you remove the RF carrier you are left with the Half wave modulation which is the intelligence /data.

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