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.

Operation

Schematic diagram

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 $L_1$ and $L_2$ is given (e.g. in a simulator), the total effective inductance that determines the frequency of the oscillation is (coupling factor $k$):

$L_0 = L_1 + L_2 + k*\sqrt{L_1*L_2}$

(see )

History

Original Patent Drawing.

The Hartley oscillator was invented by Ralph V.L. 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 above basic schematic is essentially the same as in the patent drawing, except that the tube is replaced by a J-FET, 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.)

Applications

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.

See also

LC oscillators:

• Armstrong oscillator
• Colpitts oscillator
• Clapp oscillator
• Vačkář oscillator

Other (non-LC):

• Wien bridge oscillator
• Opto-electronic oscillator

References

• US 1356763, Hartley, Ralph Vinton Lyon, "Oscillation Generator", published June 1, 1915, issued October 26, 1920 
• Langford-Smith, F. (1952), Radiotron Designer's Handbook (4th ed.), Sydney, Australia: Amalgated Wireless Valve Company Pty., Ltd. 
• Record, F. A.; Stiles, J. L. (June 1943), "An Analytical Demonstration of Hartley Oscillator Action", Proceedings of the IRE 31 (6), ISSN 0096-8390 
• Rohde, Ulrich L.; Poddar, Ajay K.; Böck, Georg (May, 2005), The Design of Modern Microwave Oscillators for Wireless Applications: Theory and Optimization, New York, NY: John Wiley & Sons, ISBN 0-471-72342-8 
• Vendelin, George; Pavio, Anthony M.; Rohde, Ulrich L. (May, 2005), Microwave Circuit Design Using Linear and Nonlinear Techniques, New York, NY: John Wiley & Sons, ISBN 0-471-41479-4 

External links

• Hartley oscillator, Integrated Publishing