Butler oscillator

The Butler oscillator is a crystal-controlled oscillator that uses the crystal near its series resonance point. They are used where a simple low-cost circuit is needed which can oscillate at high frquencies (>50MHz[1]) by using overtones of a crystal, and also giving low low phase noise.[2]

It was described by Butler in 1946 as the earthed grid oscillator, a derivative of the Hartley oscillator.[3] It is also known as the bridged-T oscillator or the grounded-base oscillator.[4]

Circuit operation

The classic Butler oscillator circuit is a two-stage circuit with two non-inverting stages, a grounded base stage and an emitter follower.[5] The crystal is inserted in series in the overall feedback path.[5]

The more common modern form of the circuit uses just the emitter follower stage.[1][6] The circuit may be analysed by considering it as a equivalent AC circuit with three parts. The emitter follower forms an amplifier with no phase shift. The crystal and its loading capacitor then produce a phase lag network, followed by the LC network of the resonant tank circuit. This then produces a phase lead, which overall meets the Barkhausen criteria for self-oscillation.[1]

The Butler circuit is a free-running or tuned oscillator. If the crystal is replaced temporarily with a low value resistor, the circuit will still oscillate at approximately the design frequency of the tank circuit. This allows the circuit to be set-up and adjusted initially without the crystal, and also encourages the selection of the correct crystal harmonic.[6] To avoid the circuit oscillating at the strong resonance of the crystal's fundamental, a small inductor may be placed in parallel with the crystal.[1][6]

Both the better-known Pierce and Colpitts oscillator circuits may be considered as derivatives of the Butler.[6][lower-roman 1]

References
  1. In the sense of circuit analysis, rather than historical origin.
  1. "Butler Crystal Oscillator Design" (PDF). Understanding Quartz Crystals and Oscillators. pp. 237–247.
  2. Cushing, Richard; Swift, Steven. "A Discrete, Low Phase Noise, 125 MHz Crystal Oscillator for the AD9850 Complete Direct Digital Synthesizer" (PDF). Analog Devices. AN-419.
  3. Butler, F. (June 1946). "Series-Resonant Crystal Oscillators". Wireless Engineer. 23: 157–160.
  4. Karlquist, Richard (January 1999), A New Type of Balanced-Bridge Controlled Oscillator (PDF), Hewlett-Packard, HPL-1999-6, archived from the original (PDF) on 2016-03-03 Butler, Meacham, Sultzer, ...
  5. Gottlieb, Irving M. (1997), Practical Oscillator Handbook, Newnes, pp. 164–165, ISBN 0-7506-3102-3
  6. "Crystal Theory" (PDF). Technical Notes. Somerset UK: EuroQuartz. Archived from the original (PDF) on 8 February 2015. Retrieved 8 February 2015.

Further reading
  • Carr, Joe (September 1999), "Crystals Made Clear I", Electronics World: 780–783
  • Carr, Joe (October 1999), "Crystals Made Clear II", Electronics World: 849–855
  • US 6741137, Sibrai, Andreas & Fritzwenwallner, Kurt, "High quality serial resonance oscillator", published 6 January 2003, issued 25 May 2004
  • US 3996530, Feistel, Claude Herbert & Gianos, Theodore, "Butler oscillator", published June 30, 1975, issued Dec 7, 1976, assigned to International Business Machines Corporation
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