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For example, I have an AC signal V1 which I'm modulating with another AC signal V2. When V2=0 the output will be also 0 (since a simplified transfer function of the analog multiplier is V1*V2=W). This corresponds to a modulation depth of 100%. I would like to control the depth so that it could be lower when needed. Simply adjusting the level of V2 will not work off course because that would simply divide the gain I'm applying to the signal. I somehow need to merge a DC voltage with V2 but now sure how... Any suggestions will be appreciated!

schematic

simulate this circuit – Schematic created using CircuitLab

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If V2 is a pure sinewave, and V1*V2=W, you aren't amplitude modulating the carrier, but DSBSC modulating it. As the "SC" in DSBSC means "Suppressed Carrier", there is no carrier energy, so the concept of "modulation depth", the ratio of signal amplitude to carrier amplitude, doesn't apply there.

The difference is that in DSBSC, the modulated output is 0 when V2=0 and inverted (180 degree phase "shifted") when V2 < 0.

The secret to AM is to add a DC offset equal to V2 maximum amplitude, so that V2 never crosses 0. Instead, when V2 = -1 (the negative peak of the sinewave) it cancels out the DC offset and W = v1 * (V2+DC) = 0, for 100% modulation depth.

You can now reduce V2 gain keeping the DC offset unchanged, and modulation depth will reduce as desired.

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  • \$\begingroup\$ Thanks for the answer. Please see the updated schematic on my question. Is that the idea? \$\endgroup\$
    – user34920
    Commented Feb 2, 2015 at 14:25

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