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I was reading about old crystal radios and read that in a crystal receiver the radio signal in the air is first selected by a tuning coil and then converted to audio signal by a diode.

I am confused since I used to know that diode's role is only to allow current in one direction. How can a diode act as a demodulator (remove carrier from an RF signal)?

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  • \$\begingroup\$ "Only" is such a deceptive word... \$\endgroup\$ – Ignacio Vazquez-Abrams Jan 17 '16 at 1:09
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    \$\begingroup\$ You need to do some research. There are plenty of good references on the web - search for 'diode detector' or 'envelope detector'. \$\endgroup\$ – Chu Jan 17 '16 at 1:10
  • \$\begingroup\$ do you mean diodes also function as detectors? they dont pass high frequencies?? \$\endgroup\$ – user16307 Jan 17 '16 at 1:11
  • \$\begingroup\$ Not by themselves, but in conjunction with a resistor/capacitor arrangement. \$\endgroup\$ – Chu Jan 17 '16 at 1:14
  • \$\begingroup\$ please see this the guy shows a circuit without a capacitor and says diode itself is converting RF to audio: youtu.be/X3Z5iM8NCXc?t=481 What do you say? \$\endgroup\$ – user16307 Jan 17 '16 at 1:35
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The diode does not "remove the carrier", but what it does do is "add the baseband" signal to the mix. The output of an ideal diode includes the baseband signal (proportional to the "envelope" of the modulated siganl) along with the original signal and its sidebands, and a whole bunch of harmonic distortion at mulitples of the carrier frequency.

It doesn't really matter all that much if you filter out all of that higher-frequency junk or not if you're just driving an earphone with it — you can't hear that stuff anyway because of the limited frequency response of your own ears (and of the earphone, too). All you hear is the baseband signal that the diode created.

The reason that more complex radios use a filter is that those other components can cause problems in any subsequent audio amplifier stages. A relatively simple low-pass filter (usually just a resistor and a capacitor) takes care of that issue up front, leaving you with a clean audio-only signal.

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  • \$\begingroup\$ i found a very interesting explanation about these(the guys says the earphone cannot react the carrier waves so it only vibrates to the audio component): youtu.be/0-PParSmwtE?t=517 \$\endgroup\$ – user16307 Jan 17 '16 at 3:41
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    \$\begingroup\$ @user16307 Which is exactly what the middle paragraph of Dave's answer says... \$\endgroup\$ – Ecnerwal Jan 17 '16 at 3:52

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