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I tried to simulate a modulation circuit which uses a comparator, following this video.

The output I get at the 1 kΩ resistor is weird; it should be the same shape as the send signal above.

May I know what values of capacitor and inductor are used, and whether the resistor value at the output also affects the shape of the send signal?

Is my LTspice output correct (oscillating wave output seems triangular) and should the carrier signal, audio signal and send signal be AC or DC? The video diagram shows all of them are DC.

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  • \$\begingroup\$ "The output I get at the (1k resistor) is weird" Can you please post screenshot of that simulation as well? What comparator (part number) does the simulation use? \$\endgroup\$ Commented Aug 31, 2022 at 7:24
  • \$\begingroup\$ @akwky, hi, it is updated :) \$\endgroup\$ Commented Aug 31, 2022 at 7:42
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    \$\begingroup\$ @jessicasmith - Hi, (a) The first comment asked you to add the comparator's part number used in your simulation. Even after your update, I can't see where you have given that info. Please clearly add it to your question. (b) You said that you are following the linked video. However I skipped through the video and did not find your circuit shown in the video. Therefore please also add to your question the exact timestamp where that circuit is used, so site members can see more context. (c) Why did you choose the passive component values in your schematic? Again, I didn't see those in the video. \$\endgroup\$ Commented Aug 31, 2022 at 8:31
  • \$\begingroup\$ @jessicasmith - Hi, I have rolled-back (reversed) your latest edit. You should not significantly change a question after it has been answered (clarifications not withstanding). There is also no context to the new circuit - the original text question related to the original circuit, so just adding a new circuit makes things unclear what you are trying to "say". You also didn't address the points in the comments above. Please try to follow the site etiquette here: Respond to comments and don't change questions are two parts of that. \$\endgroup\$ Commented Aug 31, 2022 at 11:01

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You have your band pass filter and 1 kΩ resistor swapped with each other. It should look like this: -

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However, the use of a comparator is very hit and miss for obtaining a harmonically generated carrier at 159 MHz (where the parallel tuned filter is set) so I wouldn't recommend this circuit. There is an easier circuit using diodes that can perform AM such as the one here: -

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This is a 1 MHz carrier AM modulator and can be received on a standard AM radio receiver in the MW band: -

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You can swap the modulation waveform with an audio waveform providing you limit the amplitude to no more than 2.5 volts p-p.

Be also aware that the use of such a modulator is probably illegal in most countries.

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