Limitations Of Lc And Rc Oscillators

What is an LC circuit?

LC circuit is a key component in many electronic devices, such as oscillators and filters that help to generate oscillations. The capacitor stores energy between its two plates in the electric field, while the inductor stores it in its magnetic field.

The operating frequency in Hertz of the LC oscillator is given by:

Where,

L is the inductance measured in Henry.

C is the capacitance measured in Farads

The unit of frequency is Hertz.

Here, we will discuss the types of LC oscillators, and their advantages and disadvantages.

There are three types of LC oscillators, which are as follows:

Hartley Oscillator
Colpitts Oscillator
Clapp Oscillator

Advantages of LC oscillators

The advantages of LC oscillators are as follows:

High phase stability
The LC oscillator produces good stability at high frequencies. It is because the operating frequency of the oscillator does not change much with temperature change.
Low noise
It is due to the inductors and capacitors in the feedback network.
High Q-factor
The LC oscillators have a high Quality-factor as compared to other oscillators.

Disadvantages of LC oscillators

The disadvantages of LC oscillators are as follows:

The change in temperature affects the components, such as transistors, capacitors, resistors, supply voltages, and the inductor of the circuit.
The operating frequency of the oscillator is not constant. It is due to the various components involved in the circuit.
The operating frequency can shift if any component in the feedback circuit is changed.
It is not suitable at low frequencies. At low frequencies, the capacitor and inductor do not work well and produce instability in the circuit.
High cost.

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 this viz.,

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 a change in the frequency of the oscillator.
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 extremely 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 that are quite close to each other. In fact, the frequency difference between the two broadcasting stations is less than 1%.

It is apparent that if we employ LC or RC circuits, a change in temperature may cause the frequencies of adjacent broadcasting stations to overlap. In order to maintain a 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 the great majority of electronic applications.