Hartley oscillator

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Schematic diagram

The Hartley oscillator is an L C electronic oscillator that derives its feedback from a tapped coil in parallel with a capacitor (the tank circuit). Although there is no requirement for there to be mutual coupling between the two coil segments, the circuit is usually implemented as such. A Hartley oscillator is essentially any configuration that uses a pair of series-connected coils and a single capacitor (see Colpitts oscillator for the equivalent oscillator using two capacitors and one coil). It was invented by Ralph Hartley, who filed for a patent on June 1, 1915 and was awarded patent number [1] 1,356,763 on October 26, 1920.

Hartley oscillators may be series or shunt fed. A Hartley oscillator is made up of the following:

Two inductors coupled by mutual inductance (may be a two-winding transformer)
One tuning capacitor

Advantages of the Hartley oscillator include:

The frequency is varied using a variable capacitor
The output amplitude remains constant over the frequency range
The feedback ratio of the tapped inductor remains constant

Disadvantages include:

Harmonic-rich content of the output
It is not suitable for a pure sine wave

[edit] Uses

Part of Scott 310E circuit diagram

The Hartley oscillator was extensively used on all broadcast bands including the FM 88-108MHz band. An example is given of the Scott 310E RF oscillator for its FM section.