= Two-tone output of a linear resonant circuit
A linear time-invariant circuit sends each sinusoidal input to an output at the same <frequency>, multiplied by its complex transfer function. Two inputs therefore give the sum of two phase-shifted <sinusoids>. If their output <amplitudes> are equal, the sum factors into a carrier at the mean <frequency> and a signed cosine envelope at half the <frequency> difference. The envelope magnitude repeats at the full difference <frequency>. Unequal <amplitudes> give a nonzero minimum; a nonlinear detector is needed to generate a separate difference-frequency signal.
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