Hartley oscillator

The Hartley oscillator is an electronic oscillator circuit that uses an inductor and a capacitor in parallel to determine the frequency. Invented in 1915 by American engineer Ralph Hartley, the distinguishing feature of the Hartley circuit is that the feedback needed for oscillation is taken from a tap on the coil, or the junction of two coils in series.
A Hartley oscillator is essentially any configuration that uses two series-connected coils and a single capacitor (see Colpitts oscillator for the equivalent oscillator using two capacitors and one coil). Although there is no requirement for there to be mutual coupling between the two coil segments, the circuit is usually implemented this way.
It is made up of the following:

• Two inductors in series, which need not be mutual
• One tuning capacitor

Advantages of the Hartley oscillator include:

• The frequency may be adjusted using a single variable capacitor
• The output amplitude remains constant over the frequency range
• Either a tapped coil or two fixed inductors are needed

Disadvantages include:

• Harmonic-rich content if taken from the amplifier and not directly from the LC circuit.

Note that, if the inductance of the two partial coils L₁ and L₂ is given (e.g. in a simulator), the total effective inductance that determines the frequency of the oscillation is (coupling factor k):

(see )