Introduction to oscillators

      Oscillator is an electronic circuit that makes a repetitive electronic signal, frequently a sine wave or a square wave. It is essential in many different types of electronic equipment. Like a quartz watch, which uses a quartz oscillator to keep record of what time it is. An AM radio transmitter uses an oscillator to generate the carrier wave for the station, and an AM radio receiver uses a special form of oscillator called a resonator to tune in a station. There are also oscillators in computers, metal detectors and even stun guns.

       The two main types of electronic oscillator are linear or harmonic oscillator and the nonlinear or relaxation oscillator.

       The harmonic or linear oscillator creates a sinusoidal output. The simplest physical understanding of a harmonic oscillator consists of a mass m on which a force acts that is linear in a location from equilibrium. By applying Hooke’s law, a spring creates a force that is linear for small displacements and it shows a simple realization of a harmonic oscillator.

                                                         Figure 1 

Types of Linear or Harmonic Oscillators

1. RC Oscillator

In a Resistance-Capacitance Oscillator or simply an RC Oscillator, the phase shift occurs between the input to a RC network and the output from the same network by using RC elements in the feedback branch. The input is shifted through the amplifier stage and 180^o again through a second inverting stage giving a of phase shift which is the same as thus providing the required positive feedback. 

                                                        Figure 2 

2. LC Oscillator

It consists of a capacitor and a coil connected in parallel. The circuit has an inductive coil, L and a capacitor, C. The capacitor saves energy in the form of an electrostatic field and which produces a potential across its plates, while the inductive coil stores its energy in the form of an electromagnetic field. The capacitor is charged up to the DC supply voltage, V by putting the switch in a certain position. When the capacitor is fully charged the switch changes to position the charged capacitor is now connected in parallel across the inductive coil so the capacitor begins to discharge itself through the coil. 

                                                         Figure 3 

3. Crystal Oscillator

In crystal oscillator the primary frequency determining element is a quartz crystal. Since quartz crystal inherent characteristics of the crystal oscillator which held to extreme accuracy of frequency stability. Temperature compensation may be related to crystal oscillators to improve thermal stability of the crystal oscillator. 

                                                       Figure 4 

Crystal oscillators are generally fixed frequency oscillators where stability and accuracy are the fundamental considerations. For example, to design a stable and accurate LC oscillator for the upper HF and higher frequencies, crystal oscillator is used to prevent resorting to some sort of crystal control. 

4. Colpitts Crystal Oscillator

The input signal to the base of the transistor is inverted at the transistors output. The output signal at the collector is then taken through a shift of 180^o phase shifting network in a series resonant mode. The output is also returned to the input which is in-phase with the input providing the positive feedback.

                                                         Figure 5