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Hartley oscillator is actually oscillator with L-C resonant circuit.
JFET Hartley oscillator realized with N-channel JFET is given in figure bellow.
Simulate JFET Hartley oscillator.


Connected together, capacitor and inductor forms so called L-C resonant circuit. L-C resonant circuit makes natural electric oscillations that are dumped. In order to have stable oscillations, dumped oscillations, i.e. power losses has to be compensated. In that sense, active amplifier has to be applied to keep oscillator running. JFET is good choice for amplifier since it is discrete component that can amplify signal on high frequencies and has high input impedance. In the scheme given above two coupling capacitors CS1 and CS2 and one coupling inductor LP are used to prepare quiescent operating point. Both coupling capacitors and inductor has to have as higher value as possible.
Oscillation frequency of JFET Hartley oscillator is:


Similar with JFET Hartley oscillator is JFET Colpitts oscillator.
Simulate JFET Hartley oscillator.

External Links:

JFET Hartley Oscillator on EDN
JFET Hartley Oscillator on Electronics-Tutorials
JFET Hartley Oscillator on Wikipedia

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