The Hartley oscillator is an electronic oscillator circuit in which the oscillation frequency is determined by a tuned circuit consisting of capacitors and inductors, that is, an LC oscillator. The circuit was invented in 1915 by American engineer Ralph Hartley. The distinguishing feature of the Hartley oscillator is that the tuned circuit consists of a single capacitor in parallel with two inductors in series (or a single tapped inductor), and the feedback signal needed for oscillation is taken from the center connection of the two inductors.
History
The Hartley oscillator was invented by Hartley while he was working for the Research Laboratory of the Western Electric Company. Hartley invented and patented the design in 1915 while overseeing Bell System's transatlantic radiotelephone tests; it was awarded patent number 1,356,763 on October 26, 1920. Note that the basic schematic shown below labeled "Common-drain Hartley circuit" is essentially the same as in the patent drawing, except that the tube is replaced by a JFET, and that the battery for a negative grid bias is not needed.
In 1946 Hartley was awarded the IRE medal of honor "For his early work on oscillating circuits employing triode tubes and likewise for his early recognition and clear exposition of the fundamental relationship between the total amount of information which may be transmitted over a transmission system of limited band-width and the time required."(The second half of the citation refers to Hartley's work in information theory which largely paralleled Harry Nyquist.)
Operation
The Hartley oscillator is distinguished by a tank circuit consisting of two series-connected coils (or, often, a tapped coil) in parallel with a capacitor, with an amplifier between the relatively high impedance across the entire LC tank and the relatively low voltage/high current point between the coils. The original 1915 version used a triode as the amplifying device in common plate (cathode follower) configuration, with three batteries, and separate adjustable coils. The simplified circuit shown to the right uses a JFET (in common-drain configuration), an LC tank circuit (here the single winding is tapped) and a single battery. The circuit illustrates the Hartley oscillator operation:
the output from the JFET's source (emitter, if a BJT had been used; cathode for a triode) has the same phase as the signal at its gate (or base) and roughly the same voltage as its input (which is the voltage across the entire tank circuit), but the current is amplified, i.e. it is acting as a current buffer or voltage-controlled voltage-source. this low impedance output is then fed into the coil tapping, effectively into an autotransformer that will step up the voltage, requiring a relatively high current (compared with that available at the top of the coil). with the capacitor-coil resonance, all frequencies other than the tuned frequency will tend to be absorbed (the tank will appear as nearly 0Ω near DC due to the inductor's low reactance at low frequencies, and low again at very high frequencies due to the capacitor); they will also shift the phase of the feedback from the 0° needed for oscillation at all but the tuned frequency.
Variations on the simple circuit often include ways to automatically reduce the amplifier gain to maintain a constant output voltage at a level below overload; the simple circuit above will limit the output voltage due to the gate conducting on positive peaks, effectively damping oscillations but not before significant distortion (spurious harmonics) may result. Changing the tapped coil to two separate coils, as in the original patent schematic, still results in a working oscillator but now that the two coils are not magnetically coupled the inductance, and so frequency, calculation has to be modified (see below), and the explanation of the voltage increase mechanism is more complicated than the autotransformer scenario.
A quite different implementation using a tapped coil in an LC tank feedback arrangement is to employ a common-grid (or common-gate or common-base) amplifier stage, which is still non-inverting but provides voltage gain instead of current gain; the coil tapping is still connected to the cathode (or source or emitter), but this is now the (low impedance) input to the amplifier; the split tank circuit is now dropping the impedance from the relatively high output impedance of the plate (or drain or collector).