Oscillator is the basic element of ac signal sources and generates ac signals of known frequency and amplitude. The main applications of oscillators are as sinusoidal waveform sources in electronic measurement work. To generate ac voltage, the circuit is supplied energy from a dc source. That is why, although an oscillator can be considered as generating sinusoidal signal, it is to be noted that it merely acts as an energy converter, It converts a dc source of supply to alternating current of desired frequency.
Oscillators are generally an amplifier with positive feedback. An oscillator has a gain equal to or slightly greater than unity. In the feedback path of the oscillator, capacitor, inductor or both are used as reactive components. In addition to these reactive components, an operational amplifier or bipolar transistor is used as amplifying device. No external ac input is required to cause the oscillator to work as the dc supply energy is converted by the oscillator into ac energy.
(a) the design principle used
(b) the frequency range over which they are used and
(c) the nature of generated signals.
1. Classification According to Design Principle
(a) Positive feedback oscillators
(b) Negative feedback oscillators
2. Classification According to Frequency Band of the Signals
(a) Audio Frequency (AF) oscillators—frequency rage is 20 Hz to 20 kHz
(b) Radio Frequency (RF) oscillators—frequency range is 20 kHz to 30 MHz
(c) Video Frequency oscillators—frequency range is dc to 5 MHz
(d) High Frequency (HF) oscillators—frequency range is 1.5 MHz to 30 MHz
(e) Very High Frequency (VHF) oscillators—frequency range is 30 MHz to 300 MHz
3. Classification According to Types of Generated Signals
(a) Sinusoidal Oscillators: These are known as harmonic oscillators and are generally LC tuned-feedback or RC tuned-feedback type oscillator that generates a sinusoidal waveform which is of constant amplitude and frequency.
(b) Non-sinusoidal Oscillators: These are known as relaxation oscillators and generate complex non-sinusoidal waveforms that changes very quickly from one condition of stability to another such as square-wave, triangular-wave or sawtooth-wave-type waveforms.
The oscillators have a variety of applications. In some applications we need voltages of low frequencies, in others of very high frequencies. For example to test the performance of a stereo amplifier, we need a signal of variable frequency in the audio range (20 Hz-20 KHz). Next to amplifiers, oscillators are the most important analog circuit block. Oscillators can be found in almost every imaginable electronic system. For example all radio receiving systems must have a local oscillator. All transmitting systems require oscillators to define the carrier frequency. Similarly, most digital systems are clocked and require a master clock oscillator to operate. Signal sources, which are essential for testing electronic systems are also precise oscillators whose frequency and amplitude can be accurately set according to the requirement. A working knowledge of oscillators, their construction and operating characteristics is therefore essential to analog electronics. This module is intended to introduce the reader to oscillator types, their characteristics and considerations for their behaviour.
A few basic requirements for an oscillatory circuit are:
1. The circuit should contain a reactive or frequency dependent component—either an Inductor (L) or a Capacitor (C) and a dc supply voltage.
2. Overall gain of the amplifier circuit must be at least unity.
3. Self-regenerative or positive feedback results oscillations.
4. Oscillations of the circuit become damped due to circuit losses.
5. To overcome these circuit losses, voltage amplification is necessary.
6. Desired oscillations can be maintained by using some part of the output voltage as feedback to the tuned circuit that is of the correct amplitude and in-phase (0°).
7. To keep the output signal in phase with the input, the overall phase shift of the circuit must be zero.
In conclusion, oscillators are a crucial component in the field of electronics. They are used to generate AC signals of known frequency and amplitude and have a wide range of applications, from testing electronic systems to defining the carrier frequency in transmitting systems. A working knowledge of oscillators, their construction, and operating characteristics is essential to analog electronics.
