Electronic Oscillators

The oscillator is an electronic device capable of producing electronic oscillations the form of signal waves, popularly sine waves, and square waves. Oscillations refer to the forward and backward movement. It also converts one form of a power supply to another. For example,

DC (Direct Current) to AC (Alternating Current)

The AC signal is generally in the form of a sinusoidal wave. It is because the variations produced by the rotor in the AC generator are similar to the variations of the sinusoidal signal. Oscillators are widely used in electronic devices, such as sequential circuits, clock generators, radio transmitters, quartz clocks, and video games.

A device that converts AC to DC is commonly known as converter. But, after the invention of diodes and rectifiers, the oscillators can also convert DC to AC. The device that converts DC to AC is known as inverter. Thus, we can say that converts work oppositely as inverters.

The types of oscillators are Wien bridge oscillator, LC oscillator, Crystal oscillator, etc. Among these types, crystal oscillators are also called as modern oscillators operating in the high frequency range.

History of electronic oscillators

• The first oscillators were developed in the 19^th century. It was capable of producing electric arcs used for lighting. It was first noticed by a Cornish chemist named Sir Humphry Davy. He also discovered the arc lamp.
• In 1888, an Austrian physicist named Ernst Lecher proved that the current produced by the electric arc oscillators could be oscillatory (to produce motion forward and backward).
• In 1892, an engineer named Elihu Thomson developed a magnetic blowout in the oscillators by placing an LC tuned circuit (combination of the inductor (L) and the capacitor (C)) in parallel with the electric arc.
• In 1892, the same year, an Irish physicist named George FitzGerald built a parametric oscillator or a negative resistance oscillator by varying damping resistance in the resonance circuit.
• In 1900, an electrical engineer named William Duddell rediscovered the negative resistance oscillator. He created a 'singing arc' using the sound produced by the oscillator by attaching a tuned LC circuit with it. But, it could not generate frequencies above than the audio range.
• When the negative resistance electric arc oscillator is placed in a hydrogen atmosphere with the magnetic field, it starts behaving like a continuous wave radio transmitter. In 1902, Danish engineer V Poulsen and a Danish physicist named P.O Pederson increased the frequency range using the electric arc in the hydrogen atmosphere. It was further named a Poulsen radio transmitter. It was quite popular during the 1920s.
• In 1913, an Austrian physicist named Alexander Meissner discovered the oscillator with positive feedback.
• In 1917, the first astable multivibrator was discovered by a group of two French engineers named H. Abraham and E. Bloch.
• In 1920, a German physicist named H G Barkhausen discovered the first tube Barkhausen K oscillator that could produce oscillations in the Ultra High-Frequency range. Other popular vacuum tubes were magnetron and klystron developed around 1930s.
• In 1969, a Japanese architect named K Kurokawa introduced the necessary conditions for oscillations. His study today forms the basis of modern microwave oscillator designs.

Oscillating frequencies

The frequency at the output of the oscillator is measured in Hertz (Hz). It is defined as the cycles per second. The different type of oscillators generates a different range of frequencies.

For example,

LC Oscillators - High-frequency signals

RC Oscillator - Low-frequency signals

The main types of oscillators operating at different frequencies are RF (radio-frequency), audio, and low-frequency oscillators.

• The RF frequencies range is from 100 kHz to 100 GHz.
• The audio frequency range is from 10 Hz to 20 Hz. It is the frequency at which a human ear can recognize the sound.
• A low frequencies range lies below 20 Hz. Animals, such as an elephant, can detect it.

Let's discuss the types of electronic waveforms produced by the oscillators.

Electronic Waveforms

The waveforms produced by the oscillators are sinusoidal, triangular, square, and sawtooth. All these waveforms can operate in various ranges, such as [0. 1], [-1, 1], etc.

The sinusoidal curve defines the smooth continuous wave. It depicts the natural representation of various processes and their states.

The square waveform has fixed amplitude and length.

The triangular waveform reaches its peak at fixed points. It is known for its triangular shape.

The sawtooth waveform represents the deflections in the form of teeth. A single sawtooth wave is known as ramp function wave. The wave rises upwards and sharply drops and again rises upwards.

Types of Oscillators

There are two types of oscillators, linear and non-linear.

Linear Oscillators

The linear oscillator produces sinusoidal waves at the output. It is also known as harmonic oscillator. The linear oscillators are further categorized as feedback oscillators and negative resistance oscillators.

1. Feedback oscillators

The examples of feedback oscillators include op-amps (operational amplifiers) and transistors. These devices are connected in a feedback loop with the output connected back to the input through a selective electronic filter. It provides a positive feedback to the connection circuit. When the power of the amplifiers or transistors is switched ON, the noise travels along with the current. When the noise travels through the loop, it gets amplified and filtered very quickly. The process goes on till the device gets the desired output, which becomes a sine wave at a single frequency. The positive feedback in oscillators has various advantages, such as sustained oscillations, stability, and cost effectiveness. Some of the oscillator uses selective frequency in their feedback loop, which is discussed as follows:

• RC oscillators
  As discussed, RC oscillators operate in the low-frequency range. The filter network in RC oscillators combines R (Resistors) and C (Capacitors). The resistor and capacitor combination provides the desired phase shift required by the signal. Examples of RC oscillators are the Wien Bridge oscillator and phase shift oscillator.
• LC Oscillators
  The LC oscillators operate in the high frequency range. The circuit consists of inductor and capacitor connected to form a tuned LC oscillator. The capacitor and the inductor are the energy storage devices. It allows the circuit to store electrical energy oscillating at its resonant frequency.
  The amplifier in the circuit compensates for any losses in the circuit and provides the desired output. The types of LC oscillators are Hartley, Colpitts, and Clapp circuits.
• Crystal Oscillator
  The circuit filter is a quartz crystal, which amplifies the voltage signal and sends it back to the resonator. Thus, a crystal oscillator is also known as a quartz crystal or piezoelectric crystal oscillator. The crystal oscillators have a high Q-factor, better temperature, and frequency stability than tuned circuits. The Q factor signifies fewer losses in the circuits. The higher the Q-factor, the fewer will be the number of losses. It can generate clock signals in computer and quartz clocks.

2. Negative resistance oscillators

The feedback oscillators generally used two-port elements, such as transistors and amplifiers. The linear oscillators can also use the one port networks with negative resistance. Examples of such devices are Gunn diodes and magnetron tubes. The negative resistance oscillators operate well at high frequencies. It can also amplify and increase the power of the signal. The feedback oscillators at high frequencies fail to produce the desires results.

The block diagram of the negative resistance oscillator is shown below:

Non-linear oscillators

The linear oscillator produces non-sinusoidal (square, triangular, etc.) waves at the output. It is also known as a relaxation oscillator. It consists of a non-linear switching device, such as Schmitt trigger and the energy storage elements (L or C). Both the devices are connected in the feedback loop of the oscillator. The function of the switching devices is to switch the energy storage level of the connected devices periodically. It further changes the output waveform. Non-linear oscillators are used: VCO (Voltage Controlled Oscillators), ADC (Analog to Digital Converters), function generators, etc.

The non-sinusoidal oscillators with their preferred operating devices are listed as follows:

Square wave oscillator - Timers and counters

Triangular wave oscillator - CRT (Cathode Ray Tubes), VCO, and oscilloscopes

Sawtooth wave oscillator - Switch mode power supplies

Requirements for oscillations

The simple oscillator with the negative feedback is shown below:

Where,

VI is the input voltage

Vo is the output voltage

A is the amplifier

B is the feedback factor of the negative feedback

E is the error term

E is equal to the summation of the input voltage and the feedback factor

E = B + VI

We can also represent error term as E = Vo/A

The feedback is fed back to the summation junction at the input. The use of positive and negative feedback in the oscillators improves its performance. The negative feedback reduces the distortion and gain of the amplifier.

The output voltage can be expressed as:

Vo = E x A

E = VI - BVo

Putting E = Vo/A

Vo/A = VI - BVo

Vo/A + BVo = VI

VI = Vo (1/A + B)

Thus, the feedback expression of the feedback oscillator is:

Vo/VI = A/(1 + AB)

The oscillators do not depend on the external signals as the input. Instead, it uses a fraction of the output through the feedback as the input signal.

The above system becomes unstable, when 1 + AB = 0

Or

AB = -1

It is defined as the Barkhausen criteria. The negative sign indicates a phase shift of 180 degrees. If the term is positive, the phase shift is zero degrees.

RC Phase shift Oscillators

Wien Bridge Oscillator

Crystal Oscillators

LC Oscillator

Voltage Controlled Oscillators