# Oscillator

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## Presentation Description

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## Presentation Transcript

### Oscillators :

Oscillators DIGITAL ELECTRONICS

### Zaid Mahmood Farhat :

Zaid Mahmood Farhat BEE-FA06-099 Department of Electrical Engineering

### Types of Oscillator :

Types of Oscillator Wien-bridge Oscillator Phase Shift Oscillator Twin-T Oscillator Crystal-control Oscillator Colpitts Oscillator Clapp Oscillator Hartley Oscillator Armstrong Oscillator 3

### RC Phase Shift Oscillator :

RC Phase Shift Oscillator 4

5

### Phase Shift :

Phase Shift The displacement of a waveform in time. For example, if a waveform is displaced by a complete waveform. it is described as having a phase-shift of 360°. If it is displaced by half a wavelength (i.e. 180°) one wave will peak where the other is in a trough state and complete cancellation will result. If they are at any other angle of phase-shift, partial cancellation will result. 6

### Phase shift Oscillator :

Phase shift Oscillator A phase shift oscillator is a simple sine wave electronic oscillator. It contains an inverting amplifier, and a feedback filter which 'shifts' the phase by 180 degrees at the oscillation frequency. 7

RC Phase shift 8

### Cont. :

Cont. The most common way of achieving this kind of filter is using 3 cascaded resistor-capacitor filters, at the oscillation frequency each filter produces a phase shift of 60 degrees and the whole filter circuit produces a phase shift of 180 degrees. Thus the total phase shift produced by the three RC networks is 180°. Therefore at the specific frequency fo the total phase shift from the base of the transistor around the circuit and back to the base is 360° 9

### Cont. :

Cont. The mathematics for calculating the oscillation frequency and oscillation criteria for this circuit are surprisingly complex, due to each R-C stage loading the previous ones. The calculations are greatly simplified by setting all the resistors (except the -ve feedback resistor) and all the capacitors to the same values. In the diagram, if R1 = R2 = R3 = R, and C1 = C2 = C3 = C, then: oscillation criteria is: 10

### Hartley oscillator :

Hartley oscillator The Hartley oscillator was invented by Ralph. V.L.Hartley while he was working for the Research Laboratory of the Western Electric Company. Hartley invented the design while overseeing Bell System's transatlantic radiotelephone tests of 1915. who filed for a patent on June 1, 1915 and was awarded patent number 1,356,763 on October 26, 1920. 11

### Cont. :

Cont. The Hartley oscillator is an LC electronic oscillator that derives its feedback from a tapped coil in parallel with a capacitor (the tank circuit). Although there is no requirement for there to be mutual coupling between the two coil segments, the circuit is usually implemented as such. A Hartley oscillator is essentially any configuration that uses a pair of series-connected coils and a single capacitor. 12

### Cont. :

Cont. Hartley oscillators may be series or shunt fed. A Hartley oscillator is made up of the following: Two inductors which need not be mutual (may be a two-winding transformer) One tuning capacitor 13

### Slide 14:

Advantages of the Hartley oscillator include: The frequency is varied using a variable capacitor The output amplitude remains constant over the frequency range. The feedback ratio of the tapped inductor remains constant . Disadvantages include: Harmonic-rich content of the output. It is not suitable for a pure sine wave. The Hartley oscillator was extensively used on all broadcast bands including the FM 88-108MHz band. 14

### Clapp Oscillator :

Clapp Oscillator The Clapp oscillator is one of several types of electronic oscillator constructed from a transistor (or vacuum tube) and a positive feedback network. It was invented by James K. Clapp in 1948. 15

### Cont. :

Cont. the network is comprised of a single inductor and three capacitors, with two capacitors (C1 and C2) forming a voltage divider that determines the amount of feedback voltage applied to the transistor input. 16

### Slide 17:

The Clapp oscillator is a Colpitts oscillator with an additional capacitor placed in series with the inductor. The oscillation frequency in hertz (cycles per second) for the circuit in the figure, which uses a field-effect transistor (FET), is 17

### Reason for using Clapp Oscillator :

Reason for using Clapp Oscillator A Clapp circuit is often preferred over a Colpitts circuit for constructing a variable frequency oscillator (VFO). In a Colpitts VFO, the voltage divider contains the variable capacitor (either C1 or C2). This causes the feedback voltage to be variable as well, sometimes making the Colpitts circuit less likely to achieve oscillation over a portion of the desired frequency range. This problem is avoided in the Clapp circuit by using fixed capacitors in the voltage divider and a variable capacitor (C0) in series with the inductor. 18