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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) by using
The more common modern form of the circuit uses just the emitter follower stage. The circuit may be analysed by considering it as an equivalent AC circuit with three parts. The emitter follower forms an amplifier with no
Two-transistor Butler
* {{Electronic oscillators Electronic oscillators
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) by using overtone
An overtone is any resonant frequency above the fundamental frequency of a sound. (An overtone may or may not be a harmonic) In other words, overtones are all pitches higher than the lowest pitch within an individual sound; the fundamental i ...
s of a crystal, and also giving low phase noise
In signal processing, phase noise is the frequency-domain representation of random fluctuations in the phase of a waveform, corresponding to time-domain deviations from perfect periodicity (jitter). Generally speaking, radio-frequency enginee ...
.
It was described by Butler in 1946 as the earthed grid oscillator, a derivative of the Hartley oscillator
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 engine ...
. It is also known as the bridged-T oscillator or the grounded-base oscillator.
Circuit operation
The classic Butler oscillator circuit is a two-stage circuit with two non-inverting stages, a grounded base stage and anemitter follower
In electronics, a common collector amplifier (also known as an emitter follower) is one of three basic single-stage bipolar junction transistor (BJT) amplifier topologies, typically used as a voltage buffer.
In this circuit, the base termi ...
. The crystal is inserted in series in the overall feedback path.
The more common modern form of the circuit uses just the emitter follower stage. 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
In physics and mathematics, the phase (symbol φ or ϕ) of a wave or other periodic function F of some real variable t (such as time) is an angle-like quantity representing the fraction of the cycle covered up to t. It is expressed in such a s ...
. The crystal and its loading capacitor then produce a phase lag network, followed by the LC network of the resonant tank circuit
An LC circuit, also called a resonant circuit, tank circuit, or tuned circuit, is an electric circuit consisting of an inductor, represented by the letter L, and a capacitor, represented by the letter C, connected together. The circuit can act ...
. This then produces a phase lead, which overall meets the Barkhausen criteria for self-oscillation.
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. 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.
Both the better-known Pierce and Colpitts oscillator circuits may be considered as derivatives of the Butler.
References
Further reading
* * * *External links
Two-transistor Butler
* {{Electronic oscillators Electronic oscillators