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I am trying to do amplitude modulation using a diode, but my circuit will not work. This is the circuit I am using:

schematic

simulate this circuit – Schematic created using CircuitLab

I don't know why this won't work as I have found other circuits online with a similar design.

My questions are:

  • Why wont this circuit work?
  • Do you know a better way to do amplitude modulation with a crystal oscillator?

The circuit below is one that I have built but I am trying to use a crystal oscilator.

enter image description here

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    \$\begingroup\$ circuits online with a similar design ... please show some \$\endgroup\$
    – jsotola
    Apr 14 at 22:06
  • \$\begingroup\$ I just edited to show the online design. I built that circuit but took out Rsum and Rf for 1MHz and it worked \$\endgroup\$ Apr 14 at 22:40

3 Answers 3

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A classic amplitude modulator is the diode bridge one as in the figure: enter image description here

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  • \$\begingroup\$ Would this circuit work for a higher frequency carrier wave (25MHz)? \$\endgroup\$ Apr 15 at 11:00
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    \$\begingroup\$ Components suitable for working at this frequency are necessary, the band pass filter must be centered on the carrier and have a band double that of the modulating signal. \$\endgroup\$
    – Franc
    Apr 15 at 11:16
  • \$\begingroup\$ Is there a max frequency for diodes? \$\endgroup\$ Apr 15 at 11:23
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    \$\begingroup\$ You can see this from the device datasheets. \$\endgroup\$
    – Franc
    Apr 15 at 12:43
  • \$\begingroup\$ If you did not make these images yourself, you should credit the source where you got them by posting links to those sources in your answer. \$\endgroup\$ Apr 18 at 9:35
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  1. That's not a crystal oscillator.

  2. I can't think of a reason why that circuit SHOULD work.

  3. Where on earth did you find it?

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  • \$\begingroup\$ I just edited the question to show where I got the idea for. \$\endgroup\$ Apr 14 at 22:42
  • \$\begingroup\$ @MarcusMarchese, I don't see any link to where you got the circuit from. \$\endgroup\$
    – Transistor
    Apr 14 at 22:45
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    \$\begingroup\$ 2. I can. It is a stripped-down version of a ring modulator. Varying the current through a diode makes it appear as a varying resistance. If a low amplitude carrier is riding on a higher amplitude modulating signal, the diode acts as the series leg of a 2-element attenuator, with Rf as the shunt leg. This was done in broadcast audio processing stuff in the 60's by CBS Labs, called a vario-losser. Ward used it as an AGC circuit in their station intercom. en.wikipedia.org/wiki/Ring_modulation \$\endgroup\$
    – AnalogKid
    Apr 14 at 23:10
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A more sophisticated AM modulator which can be found in various texts on the subject is the following which however needs to be sized according to the applicant's requirements and this is a fairly difficult task for those who do not have much experience:

enter image description here

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