Crystal Oscillator Design
Key Takeaways
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A crystal oscillator comprises a crystal that vibrates mechanically to generate frequency.
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Crystal oscillators require an amplifier and positive feedback to generate oscillations.
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Crystal oscillator design can be made to operate between series resonant frequency fs and parallel resonant frequency fp.
Quartz crystal is an excellent choice for crystal oscillators, as it eliminates the net phase shift
Many electronic systems use crystal oscillators, including phones, laptops, tablets, TV, radio, and microcontrollers. Crystal oscillator design is incorporated into these electronic circuits to supply a stable frequency. Compared to LC and RC tank circuits, the frequency stability of crystal oscillator designs is much better.
All About Oscillators
Oscillator circuits are used in electronics for generating periodic, time-varying signals. Usually, oscillators are incorporated in electronic boards for clock frequency generation or some logic implementation in communication circuits. The oscillator circuits can be implemented in different ways, a few of which are shown below.
Oscillator Types
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Oscillator |
Description |
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RC Oscillator |
RC oscillators are designed using the components-resistor, capacitor, and inverting amplifier. The variations in component values change the frequency of oscillation in this circuit. This is a low-cost oscillator with a shorter start-up time. |
|
Ceramic Oscillator |
Even though ceramic oscillators or resonators are easy to manufacture, they lack precision in oscillation frequency. The ceramic oscillators have a faster start-up time. |
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Crystal Oscillator |
The crystal oscillator comprises a crystal that vibrates mechanically to generate frequency. The frequency of the crystal oscillator is dependent on the molecular composition of the crystal, the angle at which the crystal is cut, etc. The crystal oscillator offers precision and temperature stability. |
Crystal Oscillator Classifications
Crystal oscillators can be classified into two types:
- Fundamental oscillator - This crystal oscillator operates at the fundamental crystal oscillation frequency.
- Overtone oscillator - This crystal oscillator operates at even multiples of the fundamental frequency.
Crystal Oscillator Hardware Requirements
Crystal oscillators require an amplifier and positive feedback for operation. The amplifier can be a transistor, op-amp, FET-based, or digital amplifier. When selecting an amplifier, you must consider the frequency to be generated.
For example, low-frequency oscillators can incorporate op-amp amplifiers. Low to high frequency can be implemented using a transistor amplifier, and high frequency using a FET amplifier. The crystal-based amplifier and positive feedback complete the crystal oscillator circuit.
Crystal Oscillator Operating Conditions
In an oscillator, the amplifier output is fed to the positive feedback network, which sends the amplifier input.
In a crystal oscillator, two conditions must be met for the oscillator to operate effectively.
- The loop gain must be equal to unity.
- The loop phase shift must be 1, 2𝛑, 4𝛑, etc. radians.
The above two conditions are generally called Barkhausen conditions.
The power fed back to the amplifier input should be enough to supply the amplifier input, oscillator requirements, and circuit losses. The loop phase shift determines the crystal oscillator precision. As the phase shift changes, the frequency of oscillation also varies. Quartz crystal is an excellent choice for crystal oscillators, as it eliminates the net phase shift. It exhibits frequency stability, no drift, and temperature stability.
Crystal Oscillator Design Considerations for Frequency Range
The figure below represents a simplified crystal oscillator circuit. With the Barkhausen condition satisfied, any slight disturbance will cause oscillating. The oscillator frequency that meets the Barkhausen condition is amplified.
In a quartz crystal oscillator, the crystal is encapsulated between two metalized electrodes that produce the series resonance condition in the oscillator circuit. The quartz crystal producing the mechanical vibrations can be represented as equivalent to a series RLC circuit. While operating a crystal oscillator, it tends to move towards series resonance.
When the equivalent parameters resistance, inductance, and capacitance of crystal are represented as Rs, Ls, Cs , respectively, then the frequency of oscillation can be designed in accordance with series resonance conduction.
The series resonant frequency can be given by the equation:
Similarly, the electrical connections to the crystal oscillator establish a capacitance Cp in the circuit, which leads to parallel resonance conditions. When the reactance (Ls, Cs related reactances) of the crystal equivalent circuit equals the reactance of Cp, then there arises a parallel resonance condition. The parallel resonant frequency is given by the equation:
Crystal oscillator design can be made to operate between fs and fp. Cadence can help you with the design, analysis, and hardware implementation of crystal oscillators. Leading electronics providers rely on Cadence products to optimize power, space, and energy needs for a wide variety of market applications. If you’re looking to learn more about our innovative solutions, talk to our team of experts or subscribe to our YouTube channel.