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I try to understand the AM transmitter on the figure below, but I cannot figure out two essential things. The carrier frequency is created by the crystal oscillator chip. enter image description here

  1. Why does it suffice to put the audio input in parallel to the battery, in order to add this signal (baseband signal) to the carrier frequency?

  2. Why do we need the capacitor?

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    \$\begingroup\$ Do you have any information about the grey box in the diagram that you could pass on to us? \$\endgroup\$ – Andrew Morton Dec 30 '17 at 17:10
  • \$\begingroup\$ I don't have any information about the crystal oscillator. Sorry \$\endgroup\$ – null Dec 30 '17 at 18:38
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This is a very crappy "AM transmitter" circuit which isn't worth its name.

It "works" by modulating the supply voltage with an Audio signal.

The output of a crystal oscillator switches between negative and positive supply. By changing that supply with a signal, the amplitude of the oscillator's output signal will change with that signal.

The "modulation depth" of this circuit will be bad as the crystal will only work for a certain voltage range limiting the amount of AM modulation possible.

It would make more sense to have the 470 ohm resistor in the Bat +5V line instead of the Bat - line as then ground of the audio circuit and this circuit would be the same.

The capacitor is needed to prevent +5 V getting into the audio circuit, without the capacitor the audio circuit might be damaged if it does not have a DC-blocking capacitor of its own.

As I said, this is a really crappy circuit and although it will do "something" it is more a "hack" than a well designed circuit. So don't expect too much from it.

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  • \$\begingroup\$ Thanks. That means this is no feedback loop, because the current flows only in one direction? At first I thought the audio signal was fed into the carrier signal within a feedback loop. \$\endgroup\$ – null Dec 30 '17 at 18:41
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    \$\begingroup\$ No, the only feedback in this circuit will be in the crystal oscillator itself. All oscillators need feedback to oscillate. But that feedback has nothing to do with the AM modulation. I advise you to look at "proper" circuits with feedback to learn about that, not at this monstrosity of a circuit. These "hack" circuits are really bad also for misleading beginners. You should first learn and understand circuits that are actually used in the real world, only then can you understand why this circuit is such a bad example. \$\endgroup\$ – Bimpelrekkie Dec 30 '17 at 20:33
  • \$\begingroup\$ Why would they bother sharing a pin where there are 4 for free? \$\endgroup\$ – copper.hat Dec 30 '17 at 21:08

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