Introduction
An oscillator is a device that generates an alternating signal from direct current without any input signal. An oscillator can be thought of as an amplifier that supplies its own (feedback) input signal. The operating range of frequency of oscillators generally varies from less than 1 Hz to GHz. A practical oscillator uses a frequency determining network or device to make it oscillate at a predetermined frequency. The frequency-determining network could be a resonant LC network or a RC phase shift network.
There are many applications of oscillators. Sine wave oscillators are basic to modern communication because they provide the carriers for transmission of sound and picture information and they play an important role in radio and TV receivers for retrieving the transmitted information. The square wave and saw tooth oscillators are used for timing, triggering and getting of events in instruments, including computers and other digital devices.
Functional Description
The basic principle of an oscillator implies that it receives d.c. energy and changes it into a.c. energy of desired frequency and wave shape .This action is represented with the block diagram as shown in Fig. 1.
INPUT OUTPUT
OUTPUT
Fig. 1: Block diagram of the general form of an oscillator.
An oscillator is designed with a wave forming network that produces the desired signal. Three basic waveforms extensively generated are the sinusoidal (frequency generation), square wave (clock function) and ramp (time base generation) .
Tank Circuit: This capacitor C1, (=0.001µf, C2 (=0.01µF) and inductor L (~15 mH) form the tank circuit where the frequency of oscillation is generated. Here C1, and C2 are in series and L is in parallel with them.
Feedback Circuit: It values the positive feedback across the capacitor C2 is feedback to the base of the amplifier to maintain the self-sustain oscillation.
Condition of Oscillation
The starting condition for any oscillation is, AB>1 at the resonant frequency of the tank circuit. Barkhausen criterion, where A is the voltage gain and B is the feedback factor
Taking, A> 1/ B
Therefore, the starting condition is,
A> 1/(C1/C2)
i.e. A> C2/ C1— (approx.)
Results and Discussion
The RF oscillator as designed and constructed in the laboratory is quite capable of producing high frequencies. We have designed a Collpilts type circuit to implement the RF oscillator. Experimental data are listed in Table-1 and Table-2. The frequency of oscillation was 1.33 MHz.
The theoretical value of the resonance frequency of oscillation is obtained by the following equation:
fr =1/2лГLC
In the experimental circuit the value of L = 15 µH= 15 x 10-6 H.
Conclusion
A radio frequency (RF) oscillator was designed and implemented in the laboratory in a cost effective way. The designed circuit was put under a series of tests and its performance was very satisfactory. This device is reliable in operation and can be used in data communication especially in radio and television receivers. The overall cost of our designed RF oscillator is relatively 40 % low compared to the available one in the local market.
No comments:
Post a Comment