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Course code: 10CS32. 6.Sinosoidal Oscillators and Wave shaping Circuits. Engineered for Tomorrow. Prepared by : Supriya.V.Sullad Department of Computer Science. Date : 05.10.2014. Course code: 10CS32. 1. Introduction and Overview of Operating Systems.
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Course code: 10CS32 6.Sinosoidal Oscillators and Wave shaping Circuits Engineered for Tomorrow Prepared by : Supriya.V.Sullad Department of Computer Science Date : 05.10.2014
Course code: 10CS32 1.Introduction and Overview of Operating Systems Engineered for Tomorrow Engineered for Tomorrow
Chapter 1: Introduction • Types of Oscillators • Condition of Oscillators : Barkhausen Criterion • Types of Oscillators • Crystal Oscillator • Voltage Controlled Oscillator • Frequency Stability • Basic RC Low Pass Circuit • Basic RC Low Pass Circuit as Integrator • Basic RC High Pass Circuit • Basic RC High Pass Circuit as Differentiator • Basic RL Circuit as Integrator • Basic RL Circuit as Differentiator • Multivibrators • Integrated Circuit Multivibrator
Types of Oscillators • Oscillators are Classified as : Sinosoidal Oscillator and Non Sinosoidal Oscillator. • Depending on the nature of feedback, Oscillators are Classified as: RC,LC and Crystal Oscillators.
Conditions for Oscillations :Barkhausen Criteria • An Oscillator circuit is essentially an amplifier with a frequency-selective feedback network. • A feedback network feeds a fraction of amplifier output back to its input so as to satisfy two fundamental requirements for occurrence of sustained oscillations, commonly known as Barkhausen Criteria. • One of the basic building blocks of electronics • Input is a direct current (DC) power source • Output is alternating current (AC) • Can generate sub-audible frequencies or very high frequencies • Most oscillators are amplifiers with feedback
Integrant of Linear Oscillators For sinusoidal input is connected “Linear” because the output is approximately sinusoidal A linear oscillator contains: - a frequency selection feedback network - an amplifier to maintain the loop gain at unity
Barkhausen Criteria and If Vs = 0, the only way that Vo can be nonzero is that loop gain A=1 which implies that (Barkhausen Criterion)
In a Crystal Oscillator, a quartz crystal with the desired value of resonant frequency forms the part of the frequency – selective feedback network. • Crystal Oscillator is basically choosen when the accuracy and stability of Oscillation frequency is of paramount importance • In the above diagram,R,L and Cs are Resistance,Inductance and Capacitance of piezoelectric crystal.,while Cm represents mounting capacitor. • The Quality factor of the crystal is given by : Q=ῲL/R=1/ῲCsR. • The Crystal exhibits resonant frequency fs.It is the frequency at which the inductive reactance of inductor L equals capacitive reactance Cs .
It is expressed as Cp=CmxCs/(Cm+Cs) The parallel Resonence is given by :
The LC tank circuit is tuned to the series resonant frequency of the crystal. • The crystal offers minimum impedance at series resonent frequency and allows feedback signal to reach the input with no additional attenuation. • The oscillator can oscillate only at the resonent frequency of the crystal ,thus improving the frequency stability of the oscillator.
Another popular Colpitts Oscillator configuration is found, which is popularly known as Pierce Oscillator. • Pierce Oscillator works well in very high frequency(VHF) and ultra high frequency (UHF) ranges. • Purpose of putting Radio Frequency Chock (RFC) is to prevent the High – frequency output from getting grounded through drain power supply.
A voltage-controlled Oscillator is the one in which the frequency of oscillations can be varied by an applied DC control voltage. • This is achieved by having a voltage-dependent capacitor known as varicap /varactor diode as a part of frequency – determining tank circuit. • Usually back to back connected varactor diodes are entertained
A Voltage controlled oscillator is an oscillator with an output signal whose output can be varied over a range, which is controlled by the input DC voltage. It is an oscillator whose output frequency is directly related to the voltage at its input. The oscillation frequency varies from few hertz to hundreds of GHz. By varying the input DC voltage, the output frequency of the signal produced is adjusted.
For a Voltage controlled oscillator generating a saw tooth waveform, the main component is the capacitor who’s charging and discharging actually decides the formation of the output waveform. The input is given in form a voltage which can be controlled. This voltage is converted to a current signal and is applied to the capacitor. As the current passes through the capacitor, it starts charging and a voltage starts building across it. As the capacitor charges and the voltage across it increases gradually, the voltage is compared with a reference voltage using a comparator. • When the capacitor voltage exceeds the reference voltage, the comparator generates a high logic output which triggers the transistor and the capacitor is connected to ground and starts discharging. Thus the output waveform generated is the representation of the charging and discharging of the capacitor and the frequency is controlled by the input dc voltage.
Frequency Stability Criteria • If there exists a small set of elements which introduces a large phase change (dθ) for for a given change in frequency (dω),then higher the value of dθ/dω,more will be the dependence of ω . • When dθ/dω approaches infinity, ω becomes independent of the oscillator and a higher of all other features. • dθ/dω is the measure of frequency stability of oscillator and a higher value of dθ/dω means higher frequency stability.
Basic RC Low Pass Filter A low-pass filter is a filter that passes signals with a frequency lower than a certain cutoff frequency and attenuates signals with frequencies higher than the cutoff frequency. The amount of attenuation for each frequency depends on the filter design. The filter is sometimes called a high-cut filter, or treble cut filter in audio applications. A low-pass filter is the opposite of a high-pass filter. A band-pass filter is a combination of a low-pass and a high-pass filter.
RC Low Pass Filter working as a Integrator • The same circuit can be treated as an Integrator. • An integrator does the mathematical operation ‘Integration‘ of the input signal. • When pulse waveform is applied to the circuit, capacitor charges through the resistor and hence the output voltage builds up. • When the input is removed (ground potential), capacitor starts to discharge. • Thus we get a triangular waveform.
Since the reactance of they capacitor is inversely proportional to the frequency ,it would increase with decrease in frequency. • The frequency falls to 0.707 of its maximum value known as a lower db cut-off frequency.
Basic RC filter as differentiator • Again we have an AC source with voltage vin(t), input to an RC series circuit. This time the output is the voltage across the resistor. This time, we consider only low frequencies ω << 1/RC, so that the capacitor has time to charge up until its voltage almost equals that of the source.
Assume anyone of the transistors Q1 or Q2 turns ON due to parameter variation or due to some switching transients, let it be Q1. • Then the collector voltage of Q1=Vce(sat)=0.2V, it is cross coupled to base terminal of Q2 through C1, then Q2 remain in OFF state. • During Q1 ON, the current path through R1 charges the capacitor C1, the capacitor C1 voltage is coupled to base of transistor Q2. • While charging of C1, when the capacitor voltage exceeds 0.7V, Q2 become turns ON. • As soon as Q2 ON, its collector voltage falls to Vce(sat)=0.2V, it is coupled to base terminal of Q1 then Q1 become OFF.
At the same time capacitor C2 starts charging through R2, when the C2 voltage exceeds 0.7 V, Q1 turns ON due to cross coupling. This process continues.
The Astable Multivibrator is another type of cross-coupled transistor switching circuit that has NO stable output states as it changes from one state to the other all the time. The astable circuit consists of two switching transistors, a cross-coupled feedback network, and two time delay capacitors which allows oscillation between the two states with no external trigger signal to produce the change in state. • In Electronic Circuits, astable multivibrators are also known as Free-running Multivibrator as they do not require any additional inputs or external assistance to oscillate. Astable’s produce a continuous square wave from its output or outputs, (two outputs no inputs) which can then be used to flash lights or produce a sound in a loudspeaker.
The basic transistor circuit for an Astable Multivibrator produces a square wave output from a pair of grounded emitter cross-coupled transistors. Both transistors either NPN or PNP, in the multivibrator are biased for linear operation and are operated as Common Emitter Amplifiers with 100% positive feedback. • This configuration satisfies the condition for oscillation when: ( βA = 1∠ 0o ). This results in one stage conducting “fully-ON” (Saturation) while the other is switched “fully-OFF” (cut-off) giving a very high level of mutual amplification between the two transistors. Conduction is transferred from one stage to the other by the discharging action of a capacitor through a resistor
Monostable Multivibrator Circuit • By giving an input trigger the monostable/ single shot multivibrator generates an output voltage corresponding to the time predetermined by the circuit elements (RC network). ie; The output wave extends as long as the time constant determined by RC network and come back to its reference position. So Monostable Multivibrator produce a Single Shot of output voltage for a trigger pulse. If their is no trigger the output voltage will be zero. In monostable multivibrator, the HIGH state is called QUASI STABLE state because, it is not stable in output waveform. ie; the output will returns to LOW state after the time t. - See more at: http://www.mycircuits9.com/2012/05/theory-about-monostable-multivibrator.html#sthash.FyZmbj9E.dpuf
An Astable Multivibrator is an oscillator circuit that continuously produces rectangular wave without the aid of external triggering. So Astable Multivibrator is also known as Free Running Multivibrator. I have already posted about Astable Multivibrator using Transistors. Astable Multivibrator using 555 Timer is very simple, easy to design, very stable and low cost. It can be used for timing from microseconds to hours. Due to these reasons 555 has a large number of applications and it is a popular IC among electronics hobbyists. • Astable Multivibrator using 555 – Circuit
A monostable multivibrator (MMV) often called a one-shot multivibrator, is a pulse generator circuit in which the duration of the pulse is determined by the R-C network,connected externally to the 555 timer. In such a vibrator, one state of output is stable while the other is quasi-stable (unstable). For auto-triggering of output from quasi-stable state to stable state energy is stored by an externally connected capacitor C to a reference level. The time taken in storage determines the pulse width. The transition of output from stable state to quasi-stable state is accomplished by external triggering.
Operation • The operation of the circuit is explained below: • Initially, when the output at pin 3 is low i.e. the circuit is in a stable state, the transistor is on and capacitor- C is shorted to ground. When a negative pulse is applied to pin 2, the trigger input falls below +1/3 VCC, the output of comparator goes high which resets the flip-flop and consequently the transistor turns off and the output at pin 3 goes high. This is the transition of the output from stable to quasi-stable state, as shown in figure. As the discharge transistor is cutoff, the capacitor C begins charging toward +VCC through resistance RA with a time constant equal to RAC. When the increasing capacitor voltage becomes slightly greater than +2/3 VCC, the output of comparator 1 goes high, which sets the flip-flop. The transistor goes to saturation, thereby discharging the capacitor C and the output of the timer goes low
