Oscillators

Any circuit which is used to generate a periodic voltage without an a.c. input signal is called an oscillator . To generate the periodic voltage, the circuit is supplied with energy from a d.c. source. Oscillators are classified according to the waveshapes they produce and the requirements needed for them to produce oscillations.

Classification of Oscillators

Oscillators can be classified into two broad categories according to their output waveshapes, Sinusoidal and Nosinusoidal.

Sinusoidal Oscillators

A sinusoidal oscillator produces a sine-wave output signal. Ideally, the output signal is of constant amplitude with no variation in frequency. The degree to which the ideal is approached depends upon such factors as class of amplifier operation, amplifier characteristics , frequency stability, and amplitude stability .

Nonsinusoidal Oscillators

Nonsinusoidal oscillators generate complex waveforms, such as square, rectangular, trigger, sawtooth, or trapezoidal. Because their outputs are generally characterized by a sudden change, or relaxation, they are often referred to as Relaxation Oscillator . The signal frequency of these oscillators is usually governed by the charge or discharge time of a capacitor in series with a resistor. Some types, however, contain inductors that affect the output frequency. Thus, like sinusoidal oscillators, both RC and LC networks are used for determining the frequency of oscillation .

Basic Oscillator

An oscillator must provide amplification . Amplification of signal power occurs from input to output. In an oscillator, a portion of the output is fed back to sustain the input, as shown in figure. Enough power must be fed back to the input circuit for the oscillator to drive itself as does a signal generator.

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This oscillator consists of a capacitor and a coil connected in parallel. Basic LC circuit generates a sine wave that loses voltage in every cycle. To overcome this, additional voltage is applied to keep the oscillator from losing voltage. However, to keep this oscillator going well, a switching method is used. A vacuum tube (or a solid-state equivalent such as a FET) is used to keep this LC circuit oscillating. The advantage of using a vacuum tube is that they can oscillate at specified frequencies such as a thousand cycles per second.

Hartley Oscillator

The cathode is tapped to the coil so when current flows through the coil, there is a voltage kick in the grid coil . The amount of feedback is controlled by changing the cathode tap. The Hartley oscillator has some frequency instabilities .

The Colpitts oscillator is very similar to the Hartley oscillator, but instead of a tapped grid coil, it has tapped capacitance . The tap between the two capacitors is grounded and the feedback is obtained from the coupling capacitor, C1 . The amount of feedback depends on the ratio of C2 to C3. The capacitor part of the LC circuit consists of both C2 and C3, which determines the oscillating frequency . This oscillator has more frequency stablities than the Hartley oscillator.

Crystal Oscillator

This is a type of oscillator that is controlled by a crystal . The big advantage of a crystal oscillator is high frequency stablility . Common crystals used are tourmaline, Rochelle salts, and quartz. The crystal makes a voltage difference when voltage is applied to the two plates on the crystal. When AC is applied, the crystal compresses and stretches, in other words it vibrates . The natural frequency of a crystal's vibrations is found to be more constant than the oscillations in a LC circuit. The thinner the crystal is, the faster it vibrates. The LC circuit is the electricial equilavent of a crystal.

The RC Oscillator which is sometimes called a Phase Shift Oscillator , produces a sine wave output signal using regenerative feedback from the Resistor/Capacitor combination. This regenerative feedback from the RC network is due to the ability of the capacitor to store an electric charge , (similar to the LC tank circuit). This Resistor/Capacitor feedback network can be connected as shown to produce a leading phase shift (Phase Advance Network) or interchanged to produce a lagging phase shift (Phase Retard Network) the outcome is still the same as the sine wave oscillations only occur at the frequency at which the overall phase-shift is 360^o. By varying one or more of the resistors or capacitors in the phase-shift network, the frequency can be varied and generally this is done using a 3-ganged variable capacitor .

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