Butler oscillator

Single transistor emitter-follower circuit

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 frequencies (>50MHz[1]) by using overtones of a crystal, and also giving 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]

AC equivalent circuit

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 an 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][i]

References

  1. ^ In the sense of circuit analysis, rather than historical origin.
  1. ^ a b c d "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. ^ a b Gottlieb, Irving M. (1997), Practical Oscillator Handbook, Newnes, pp. 164–165, ISBN 0-7506-3102-3
  6. ^ a b c d "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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