Are you looking at LC or RC oscillators? For LC, I think varactors are the
simplest approach. I remember in vacuum tube days we had reactance tube
circuits (or something like that) where circuitry caused the current
through the tube to be 90 degrees out of phase with the applied voltage
(reactive). You then varied the grid voltage to vary what that current was
(thus varying the reactance).

For RC oscillators, you can vary the charge/discharge current. This
technique is used in the XR2206. You can also vary the threshold voltages
for where the charge changes to a discharge. This is used to FM NE555
astables.

Harold


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