OSCILATTORSPRESENTED BY: MUHAMMAD MUJTABA KHAN 01-133112-047 MUHAMMAD MURTAZA KHAN 01-133112-048

OBJECTIVES•Define the basic oscillator circuit

•Identify elements of feedback in the oscillator

•Identify the conditions for oscillation to occur

•Identify input and output characteristics

•Identify the difference between VFO and VCO oscillators

•Identify common uses of the oscillator

WORD WALL

•Oscillation: an effect that repeatedly and regularly fluctuates about a mean value

•Oscillator: circuit that produces oscillation

•Characteristics: frequency, amplitude, distortion, wave-shape, stability

OVERVIEW OF THE OSCILLATOR•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

AMPLIFIERS AS OSCILLATORS?

•Most amplifiers will oscillate when conditions are correct

•Example: Too high of a volume on public address system = loud noise and squeals that are the result of acoustic waves traveling from the speakers to the microphone

•The result is oscillation

ELECTRICAL FEEDBACK

•Practical oscillators use electrical feedback to produce oscillation

•Feedback circuits use resistors, capacitors, coils, or transformers to connect a portion of the output signal back to the input of the amplifier

FEEDBACK AND AMPLIFIER GAINCONDITIONS FOR OSCILLATION

Feedback alone does not promise oscillation

There is always some loss in the feedback circuit

Amplifier gain must be greater than this loss

Feedback must be in-phase

FEEDBACK OSCILLATOR

•The most common form of linear oscillator is an electronic amplifier such as a transistor or op amp connected in a feedback loop with its output fed back into its input through a frequency selective electronic filter to provide positive feedback.

•When the power supply to the amplifier is first switched on, electronic noise in the circuit provides a signal to get oscillations started. The noise travels around the loop and is amplified and filtered until very quickly it becomes a sine wave at a single frequency

FEEDBACK OSCILLATOR

•Feedback oscillator circuits can be classified according to the type of frequency selective filter they use in the feedback loop:

1. RC OSCILLATOR

2. LC OSCILLATOR

3. CRYSTALL OSCILATOR

RC OSCILLATOR

•In an RC oscillator circuit, the filter is a network of resistors and capacitors.

•RC oscillators are mostly used to generate lower frequencies, for example in the audio range.

•Common types of RC oscillator circuits are the phase shift oscillator and the Wien bridge oscillator

LC OSCILLATOR

•In an LC oscillator circuit, the filter is a tuned circuit (often called a tank circuit) consisting of an inductor (L) and capacitor (C) connected together.

•Charge flows back and forth between the capacitor's plates through the inductor, so the tuned circuit can store electrical energy oscillating at its resonant frequency. There are small losses in the tank circuit, but the amplifier compensates for those losses and supplies the power for the output signal.

•LC oscillators are often used at radio frequencies, when a tunable frequency source is necessary, such as in signal generators, tunable radio transmitters and the local oscillators in radio receivers. Typical LC oscillator circuits are the Hartley, Colpitts and Clapp circuits

CRYSTAL OSCILLATOR

•In a crystal oscillator circuit the filter is a piezoelectric crystal (commonly a quartz crystal).The crystal mechanically vibrates as a resonator, and its frequency of vibration determines the oscillation frequency. Crystals have very high Q-factor and also better temperature stability than tuned circuits, so crystal oscillators have much better frequency stability than LC or RC oscillators.

•Crystal oscillators are the most common type of linear oscillator, used to stabilize the frequency of most radio transmitters, and to generate the clock signal in computers and quartz clocks.

CRYSTAL OSCILLTOR

• Crystal oscillators often use the same circuits as LC oscillators, with the crystal replacing the tuned circuit; the Pierce oscillator circuit is also commonly used. Quartz crystals are generally limited to frequencies of 30 MHz or below. •Surface acoustic wave (SAW) devices are another kind of piezoelectric resonator used in crystal oscillators, which can achieve much higher frequencies. They are used in specialized applications which require a high frequency reference, for example, in cellular telephones

INPUT CHARACTERISTICS

•Steady source of direct current (DC)

•In many applications, the DC source requires a filter

OUTPUT CHARACTERISTICS

•Amplitude

•Frequency

•Waveform type

•Stability

•On some oscillators, the capability to change frequency

OSCILLATOR AMPLITUDE

•Usually determined by the gain available from the amplifier

•Supply voltage and circuit losses affect amplitude

OSCILLATOR FREQUENCY

•Frequency of operation is normally determined by the feedback circuit

•Common feedback circuits used to determine oscillator frequency include:• crystals• resistor and capacitor networks (RC)• coil and capacitor networks (LC) {tank circuit}

OSCILLATOR STABILITY

•Sometimes referred to as a stable oscillator

•Source of a signal with consistent amplitude

•Source of a signal with consistent frequency

ABILITY TO CHANGE FREQUENCY

•Oscillators sometimes have the ability to change frequencies

•Crystal oscillator frequency is controlled by changing the crystal•Crystals are usually cut from quartz to generate a specified frequency when operating

VOLAGE FREQUENCY OSCILLATOR (VFO)

•Variable frequency oscillators (VFO) generally use a variable resistor, coil, or capacitor in the feedback circuit

•Varying the value of resistance, impedance, or reactance in the circuit results in a change of the oscillator output

VOLTAGE CONTROLLED OSCILLATOR (VCO)

•Voltage controlled oscillators (VCO) utilize changes in voltage to generate changes in the oscillator output

•An oscillator can be designed so that the oscillation frequency can be varied over some range by an input voltage or current. These voltage controlled oscillators are widely used in phase-locked loops, in which the oscillator's frequency can be locked to the frequency of another oscillator.

•These are ubiquitous in modern communications circuits, used in filters, modulators, demodulators, and forming the basis of frequency synthesizer circuits which are used to tune radios and televisions.

VOLTAGE CONTROLLED OSCILLATOR (VCO)

•Radio frequency VCOs are usually made by adding a varactor diode to the tuned circuit or resonator in an oscillator circuit. Changing the DC voltage across the varacter changes its capacitance, which changes the resonant frequency of the tuned circuit.

•Voltage controlled relaxation oscillators can be constructed by charging and discharging the energy storage capacitor with a voltage controlled current source. Increasing the input voltage increases the rate of charging the capacitor, decreasing the time between switching events.

TYPICAL USES OF THE OSCILLATOR

•Radio and television stations require oscillators to develop the basic signal to transmit their information

TYPICAL USES OF THE OSCILLATOR

•Cell phones, electronic keyboards, and remote controls use oscillators to produce the required frequencies for operation

TYPICAL USES OF THE OSCILLATOR

•Digital devices such as computers, watches, calculators, and iPods all require oscillators to generate the rectangular waveform required for operation

TYPICAL USES OF THE OSCILLATOR•Variable oscillators, known as signal generators, are used to generate frequencies and waveforms needed for troubleshooting and the testing of electronic equipment

THANK YOU !!!

ANY QUESTION & ANSWERS ?