I thought I would create a space for anyone who would like to talk about electrical circuits that use time variant parameters to modulate a circuit’s electrical reactance, or alternatively—as the case may be—resistance.
This effect can be seen in the form of inductance and capacitance variation with respect to two signals; first, a primary signal passing through a device or circuit element; and second, an axillary signal applied to control the device or circuit element’s parameter—which can be considered as the “bias signal” or “control signal”. Ironically this happens all the time, and without anyone realizing it, that is, unless you’re an audiophile or radio engineer. For example a simple electrolytic capacitor can, among other types, vary its absolute magnitude of capacitance from the DC level at which it is operated at . For audio this adds second order harmonics to the signal (read distortion), for radio, it adds unwanted frequency domain content. Returning to resistive devices, a solid-state circuit like a lambda diode falls under the category of a time variant resistance and most importantly, produces a negative slope for its V-I curve, while nearly all other resistive devices show a positive slope .
There are a lot of interesting uses for the above examples in real world applications. LF and RF oscillators can be (and are) made using the negative slope charictaristiscs of the lambda diode or gunn diode. Mainly, time-variant circuit elements are used in radio for frequency sweeping circuits. One of the most used for that purpose is the variactor diode, which changes effective capacitance based on the applied DC bias level . In its usual application (frequency sweeping), a variactor diode's capacitance is changed at a rate that is very, very slow, with respect to the primary signal. When you start to increase the rate of change in the bias signal, and thus effectively changing the amplitude of effective capacitance in the circuit with respect to the applied input signal, strange things start to happen—or at least can theoretically happen given the right conditions.
Double frequency amplitude variation of circuit elemtent's reactance is what I personally will be covering in this thread. For anyone else who is interested, feel free to join in as well as talk about other applications and allied ideas related to the thread's theme. For the visual people out there, I've got pictures and diagrams coming soon. Arguments are welcome, but let's keep things about why something can or cannot work, as opposed to attacking people for what they think.
 CliftonLaboratories.com - Capacitance Change with Applied Voltage. (This is an awesome website for anyone who is interested in seeing fun experiments and judicious use of test equipment.)
 LambdaDiode.7zip (A collection of documents on negative resistance--negative slope V-I devices)