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 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]
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] [i]
An electronic oscillator is an electronic circuit that produces a periodic, oscillating or alternating current (AC) signal, usually a sine wave, square wave or a triangle wave, powered by a direct current (DC) source. Oscillators are found in many electronic devices, such as radio receivers, television sets, radio and television broadcast transmitters, computers, computer peripherals, cellphones, radar, and many other devices.
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Resonance is a phenomenon that occurs when an object or system is subjected to an external force or vibration that matches its natural frequency. When this happens, the object or system absorbs energy from the external force and starts vibrating with a larger amplitude. Resonance can occur in various systems, such as mechanical, electrical, or acoustic systems, and it is often desirable in certain applications, such as musical instruments or radio receivers. However, resonance can also be detrimental, leading to excessive vibrations or even structural failure in some cases.
In physics and engineering, the quality factor or Q factor is a dimensionless parameter that describes how underdamped an oscillator or resonator is. It is defined as the ratio of the initial energy stored in the resonator to the energy lost in one radian of the cycle of oscillation. Q factor is alternatively defined as the ratio of a resonator's centre frequency to its bandwidth when subject to an oscillating driving force. These two definitions give numerically similar, but not identical, results. Higher Q indicates a lower rate of energy loss and the oscillations die out more slowly. A pendulum suspended from a high-quality bearing, oscillating in air, has a high Q, while a pendulum immersed in oil has a low one. Resonators with high quality factors have low damping, so that they ring or vibrate longer.
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