Fixed Frequency Oscillator
A fixed frequency oscillator is a device that generates a continuous waveform with a stable and unchanging frequency. It is commonly used in various electronic systems and applications where a precise and constant frequency is required. Among the given options, the crystal oscillator is a type of fixed frequency oscillator.

Crystal Oscillator
A crystal oscillator is an electronic circuit that uses the mechanical resonance of a vibrating crystal to generate a precise frequency. It is widely used in many electronic devices such as clocks, radios, microprocessors, and communication systems. The crystal used in the oscillator is typically made of quartz, which exhibits a phenomenon called piezoelectric effect, where it vibrates at a specific frequency when an electric field is applied.

Working Principle of Crystal Oscillator
The crystal oscillator circuit consists of an amplifier and a quartz crystal. The quartz crystal acts as a resonator, providing feedback to the amplifier circuit to sustain the oscillations at a specific frequency. The basic working principle can be explained as follows:

1. Quartz Crystal: The quartz crystal is carefully cut and shaped to vibrate at a specific frequency when an electric field is applied. The natural resonant frequency of the crystal is determined by its physical dimensions and the properties of the quartz material.

2. Amplifier Circuit: The amplifier circuit amplifies the electrical signal from the crystal and feeds it back to the crystal. This positive feedback creates a sustained oscillation at the resonant frequency of the crystal.

3. Frequency Control: The frequency of the crystal oscillator is primarily determined by the physical characteristics of the quartz crystal. It can be further controlled by external components such as capacitors, resistors, and inductors connected to the crystal oscillator circuit.

Advantages of Crystal Oscillator
- High Frequency Stability: Crystal oscillators offer excellent frequency stability over a wide range of operating conditions. The resonant frequency of the crystal is highly stable and does not drift significantly with temperature, aging, or other environmental factors.
- High-Quality Signal: Crystal oscillators produce a clean and pure sinusoidal waveform with low phase noise and harmonic distortion. This makes them suitable for applications that require precise timing and accurate frequency references.
- Wide Frequency Range: Crystal oscillators can be designed to operate at various frequencies ranging from a few kilohertz to several hundred megahertz, depending on the type and size of the crystal used.
- Compact and Reliable: Crystal oscillators are compact in size and can be easily integrated into electronic circuits. They are also highly reliable and have a long lifespan.

Conclusion
Among the given options, the crystal oscillator is a fixed frequency oscillator that utilizes the mechanical resonance of a quartz crystal to generate a stable and precise frequency. It offers high frequency stability, produces a high-quality signal, and is widely used in various electronic applications.