LIMITATIONS OF LC AND RC OSCILLATORS

LIMITATIONS OF LC AND RC OSCILLATORS

By Engr. Aneel Kumar   March 28, 2018   No comments:

The LC and RC oscillators have their own limitations. The major problem in such circuits is that their operating frequency does not remain strictly constant. There are two principal reasons for it viz., 

1. As the circuit operates, it will warm up. Consequently, the values of resistors and inductors, which are the frequency determining factors in these circuits, will change with temperature. This causes the change in frequency of the oscillator. 
2. If any component in the feedback network is changed, it will shift the operating frequency of the oscillator. 

However, in many applications, it is desirable and necessary to maintain the frequency constant with extreme low tolerances. For example, the frequency tolerance for a broadcasting station should not exceed 0.002% i.e. change in frequency due to any reason should not be more than 0.002% of the specified frequency. The broadcasting stations have frequencies which are quite close to each other. In fact, the frequency difference between two broadcasting stations is less than 1%. It is apparent that if we employ LC or RC circuits, a change of temperature may cause the frequencies of adjacent broadcasting stations to overlap. In order to maintain constant frequency, piezoelectric crystals are used in place of LC or RC circuits. Oscillators of this type are called crystal oscillators. The frequency of a crystal oscillator changes by less than 0.1% due to temperature and other changes. Therefore, such oscillators offer the most satisfactory method of stabilizing the frequency and are used in great majority of electronic applications.

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SINUSOIDAL OSCILLATOR

SINUSOIDAL OSCILLATOR

By Engr. Aneel Kumar   March 28, 2018   No comments:

An electronic device that generates sinusoidal oscillations of desired frequency is known as a sinusoidal oscillator. Oscillator does not create energy, but merely acts as an energy converter. It receives DC energy and changes it into AC energy of desired frequency. The frequency of oscillations depends upon the constants of the device. Although an alternator produces sinusoidal oscillations of 50Hz, it cannot be called an oscillator. Firstly, an alternator is a mechanical device having rotating parts whereas an oscillator is a non-rotating electronic device. Secondly, an alternator converts mechanical energy into AC energy while an oscillator converts DC energy into AC energy. Thirdly, an alternator cannot produce high frequency oscillations whereas an oscillator can produce oscillations ranging from a few Hz to several MHz. 

Advantages

• Although oscillations can be produced by mechanical devices (e.g. alternators), but electronic oscillators have the following advantages: 
• An oscillator is a non-rotating device. Consequently, there is little wear and tear and hence longer life. 
• Due to the absence of moving parts, the operation of an oscillator is quite silent. 
• An oscillator can produce waves from small (20 Hz) to extremely high frequencies (> 100 MHz). 
• The frequency of oscillations can be easily changed when desired. 
• It has good frequency stability i.e. frequency once set remains constant for a considerable period of time. 
• It has very high efficiency

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