OscillatorOscillators are circuits that producespecific, periodic waveforms such assquare, triangular, saw tooth, andsinusoidal. They use some form ofactive device and passive device likeresistor capacitor and inductor.
OscillationOscillation is the repetitivevariation, typically in time,of some measure about a centralvalue (often a point of equilibrium)or between two or more differentstates.
Classification of OscillatorThere are two main classes of oscillators. Harmonic Oscillator Relaxation Oscillator
Sinusoidal OscillatorThe oscillator which gives a sinusoidalwave form is called a sinusoidal oscillator.
Basic PrincipleA sinusoidal oscillator generallyconsists of an amplifier havingpart of its output returned tothe input by means of a feedbackloop.
Barkhausen CriterionFor oscillation the BarkhausenCriterion must be equal to 1. Aβ = 1we can write a complex phasor Aβ(jω) ‹which can be also written as |Aβ| θ where |Aβ| is the loop gain magnitude and θis the phase shift.
Barkhausen criterion requires that |Aβ|= 1and θ=±360⁰n where n is any integer.So in polar and rectangular form , theBarkhausen criterion is expressed as Aβ(jω)=1‹±360⁰n = 1+j0
Harmonic OscillatorHarmonic Oscillators generate complexwaveforms, such as square, rectangularand saw tooth.
Basic PrincipleIt contains an energy storingelement(capacitor or an inductor) and anonlinear trigger circuit that periodicallycharges and discharges the energy storedin the storage element thus causing abruptchanges in the output waveform.
The Wien Bridge Oscillator is socalled: because the circuit is based on a frequency-selective form of the Whetstone bridge circuit. The Wien Bridge oscillator is a two-stage RCcoupled amplifier circuit that has good stabilityat its resonant frequency, low distortion and isvery easy to tune making it a popular circuit asan audio frequency oscillator.
It can be seen that at very low frequencies the phase angle between theinput and output signals is "Positive" (Phase Advanced), while at very highfrequencies the phase angle becomes "Negative" (Phase Delay). In themiddle of these two points the circuit is at its resonant frequency, (ƒr) withthe two signals being "in-phase" or 0o.
Wien Bridge Oscillator SummaryThen for oscillations to occur in a Wien Bridge Oscillator circuit thefollowing conditions must apply.1. With no input signal the Wien Bridge Oscillator produces outputoscillations.2. The Wien Bridge Oscillator can produce a large range of frequencies.3. The Voltage gain of the amplifier must be at least 3.4. The network can be used with a Non-inverting amplifier.5. The input resistance of the amplifier must be high compared to R sothat the RC network is not overloaded and alter the required conditions.
The RC OscillatorThe phase angle between the input and output signals of anetwork or system is called phase shift. In a RC Oscillator the input is shifted 180o through theamplifier stage and 180o again through a second invertingstage giving us "180o + 180o = 360o" of phase shift which is thesame as 0o thereby giving us the required positive feedback. Inother words, the phase shift of the feedback loop should be"0".
In a Resistance-Capacitance Oscillator or simply an RCOscillator, we make use of the fact that a phase shiftoccurs between the input to a RC network and the outputfrom the same network by using RC elements in thefeedback branch, for example.
Then by connecting together three such RC networks inseries we can produce a total phase shift in the circuit of180o at the chosen frequency and this forms the bases of a"phase shift oscillator" otherwise known as a RC Oscillatorcircuit.
The Quartz Crystal OscillatorsOne of the most important features of anyoscillator is its frequency stability, or in other words itsability to provide a constant frequency output undervarying load conditions. Some of the factors that affectthe frequency stability of an oscillator include:temperature, variations in the load and changes in theDC power supply. Frequency stability of the outputsignal can be improved by the proper selection of thecomponents used for the resonant feedback circuitincluding the amplifier but there is a limit to the stabilitythat can be obtained from normal LC and RC tankcircuits. To obtain a very high level of oscillator stability aQuartz Crystal is generally used as the frequencydetermining device to produce another types of oscillatorcircuit known generally as a Quartz CrystalOscillator, (XO).
The quartz crystal used in a Quartz Crystal Oscillator is a very small, thinpiece or wafer of cut quartz with the two parallel surfaces metallised to makethe required electrical connections. The physical size and thickness of a pieceof quartz crystal is tightly controlled since it affects the final frequency ofoscillations and is called the crystals "characteristic frequency". Then once cutand shaped, the crystal can not be used at any other frequency. In otherwords, its size and shape determines its frequency.The crystals characteristic or resonant frequency is inversely proportional toits physical thickness between the two metallised surfaces. A mechanicallyvibrating crystal can be represented by an equivalent electrical circuitconsisting of low resistance, large inductance and small capacitance asshown below.
The equivalent circuit for the quartz crystal shows an RLC seriescircuit, which represents the mechanical vibrations of the crystal, inparallel with a capacitance, Cp which represents the electricalconnections to the crystal. Quartz crystal oscillators operate at "parallelresonance", and the equivalent impedance of the crystal has a seriesresonance where Cs resonates with inductance, L and a parallelresonance where L resonates with the series combination of Cs and Cpas shown. Crystal reactance:
The slope of the reactance against frequency above, shows that the seriesreactance at frequency ƒs is inversely proportional to Cs because below ƒsand above ƒp the crystal appears capacitive, i.e. dX/dƒ, where X is thereactance. Between frequencies ƒs and ƒp, the crystal appears inductive asthe two parallel capacitances cancel out. The point where the reactancevalues of the capacitances and inductance cancel each other out Xc = XL isthe fundamental frequency of the crystal.
Relaxation oscillator• The oscillator which shows square wave, saw tooth wave, triangular wave and pulse wave in his o/p instead of sine wave is called relaxation oscillator.
Shockley diode relaxation oscillator• The oscillator circuit which consists of Shockley diode and we have sawtooth wave in its o/p is called Shockley diode relaxation oscillator.• As shown in Fig A and B
Circuit diagram of Shockley diode relaxation oscillatorA B