08-06-2012, 12:56 PM
Lecture 3 Oscillator
3 Oscillator.ppt (Size: 314 KB / Downloads: 9)
Oscillators
Oscillation: an effect that repeatedly and regularly fluctuates about the mean value
Oscillator: circuit that produces oscillation
Characteristics: wave-shape, frequency, amplitude, distortion, stability
Application of Oscillators
Oscillators are used to generate signals, e.g.
Used as a local oscillator to transform the RF signals to IF signals in a receiver;
Used to generate RF carrier in a transmitter
Used to generate clocks in digital systems;
Used as sweep circuits in TV sets and CRO.
LC Oscillators
The frequency selection network (Z1, Z2 and Z3) provides a phase shift of 180o
The amplifier provides an addition shift of 180o
Two well-known Oscillators:
Colpitts Oscillator
Harley Oscillator
Amplitude Stability
In order to start the oscillation, the loop gain is usually slightly greater than unity.
LC oscillators in general do not require amplitude stabilization circuits because of the selectivity of the LC circuits.
In RC oscillators, some non-linear devices, e.g. NTC/PTC resistors, FET or zener diodes can be used to stabilized the amplitude