Crystal Oscillators

Under the guidance of,H V Ravish Aradhya,Asst. Prof,Dept. of ECE, RVCECrystal Oscillatorsmost important features Frequency Stability(its ability to provide a constant frequency output under varying conditions)factors that affect the frequency stability:1. temperature, 2. variations in the load and 3. changes in the power supply

How to improve Frequency stability of the output:1. proper selection of the components used for the resonant feedback circuitthere is a limit to the stability that can be obtained from normalLCandRCtank circuitsFor very high stability aquartz crystalis generally used as the frequency determining device to produce another types of oscillator circuit known generally asCrystal Oscillators

When a voltage source is applied to a small thin piece of crystal quartz, it begins to change shape producing a characteristic known as thePiezo-electric Effect

Piezo-electric Effect: piezo-electric effect is the property of a crystal by which an electrical charge produces a mechanical force by changing the shape of the crystal and vice versa.a mechanical force applied to the crystal produces an electrical charge.piezo-electric devices can be classed asTransducersas they convert energy of one kind into energy of anotherpiezo-electric effect produces mechanical vibrations or oscillationsThe quartz crystal used inCrystal Oscillatorsis a very small

The physical size and thickness of a piece of quartz crystal is tightly controlled since it affects the final frequency of oscillations and is called the crystals characteristic frequencyThen once cut and shaped the crystal cannot be used at any other frequencyThe crystals characteristic or resonant frequency is inversely proportional to its physical thickness between the two metallized surfaces.A mechanically vibrating crystal can be represented by an equivalent electrical circuit consisting of low Resistance, large Inductance and small Capacitance as shown

A quartz crystal has a resonant frequency similar to that of a electrically tuned tank circuit It has much higherQfactor due to its low resistance, with typical frequencies ranging from 4kHz to 10MHz.

The cut of the crystal also determines how it will behave as some crystals will vibrate at more than one frequency.

if the crystal is not of a parallel or uniform thickness it will have two or more resonant frequencies having both a fundamental frequency and harmonics such as second or third harmonics

Fundamental frequency is more stronger.

WKT, an amplifier circuit will oscillate if it has a loop gain greater or equal to 1 and it has positive feedbackThe equivalent circuit above has three reactive components and there are two resonant frequencies:Lower is a series type frequency and Higher a parallel type resonant frequency

In aCrystal Oscillatorcircuit, the oscillator will oscillate at the fundamental series resonant frequency of the crystal, as the crystal always wants to oscillate when a voltage source is applied to it

it is also possible to "tune" a crystal oscillator to any even harmonic of the fundamental frequency, (2nd, 4th, 8th etc.) Harmonic OscillatorsOvertone Oscillatorsvibrate at odd multiples of the fundamental frequency, 3rd, 5th, 11th etc

Generally, crystal oscillators that operate at overtone frequencies do so using their series resonant frequency

The frequency of oscillations for Crystal Oscillators is given as:

A series resonant crystal has the following values after being cut,R = 1k,C = 0.05pFandL = 3H. Calculate the fundamental frequency of oscillations of the crystalA series resonant crystal has the following values after being cut,R = 1k,C = 0.05pFandL = 3H. Calculate the fundamental frequency of oscillations of the crystal

The frequency of oscillations for Crystal Oscillators is given as:

Then the fundamental frequency of oscillations for the crystal is given as411 kHz

RC Phase-Shift Network

Phase Angle

Basic RC Oscillator CircuitIf all the resistors,Rand the capacitors,Care equal in value, then the frequency of oscillations produced by the oscillator is given as

fis the Output Frequency in HertzR is the Resistance in OhmsC is the Capacitance in FaradsN is the number ofRCstages and in our example N = 3

Determine the frequency of oscillations of aRC Oscillatorcircuit having 3-stages each with a resistor and capacitor of equal values.R = 10kandC = 500pFDetermine the frequency of oscillations of aRC Oscillatorcircuit having 3-stages each with a resistor and capacitor of equal values.R = 10kandC = 500pF

The frequency of oscillations for a RC Oscillator is given as:

The circuit is a 3-stage oscillator which consists of three 10k resistors and three 500pF capacitors therefore the frequency of oscillation is given as: