The Fundamentals of Oscillators in Communication Systems

A communication system consists of an input device, a transmitter, a transmission medium, receiver and output device. The input device can be a computer, sensor or oscillator, depending on the application of the system, while the output device could be a speaker or computer.

The input device and transmitter makes up the source section. The receiver and the output device make up the destination section.

The source section produces two types of signals namely the informational signal, which may be speech, video or data, and a signal of constant frequency and constant amplitude called the carrier. The information signal mixes with the carrier to produce a complex signal which is transmitted.

The destination section must be able to reproduce the original information, and the receiver block does by separating the information from the carrier. The information is then fed to the output device.

The transmission medium may be a copper, cable, such as a co-axial cable, a fiber-optic cable or a waveguide. These are all guided systems in which the signal from the transmitter is directed a long a solid medium. However, it is often the case with telecommunication systems that the signal is unguided. This occurs when an antenna system is used at the output of the transmitter block and at the input of the receiver block.

Both the transmitter block and the receiver incorporate many amplifiers and processing stages, and one of the most important is the oscillator stage. The oscillator in the transmitter is generally referred as the master oscillator as it determines the channel at which the transmitter functions. The receiver oscillator is called the local oscillator as it produces a local carrier within the receiver which allows the incoming carrier from the transmitter to be modified for easier processing within the receiver.

The master oscillator generates a constant-amplitude, constant-frequency signal which is used to carry the audio or intelligence signal. These two signals are combined in the modulator, and this stage produces an output carrier which varies in harmony with the audio signal or signals. This signal is low-level and must be amplified before transmission.

The receiver amplifies the incoming signal, extracts the intelligence and passes it on to an output transducer such as a speaker. The local oscillator in this case causes the incoming radio frequency (RF) signals to be translated to a fixed lower frequency, called the intermediate frequency (IF), which is then passed onto the following stages. This common IF means that all the subsequent stages can be set up for optimum conditions and do not need to be readjusted for different incoming RF channels. Without the local oscillator this would not be possible. To conclude, we can define an oscillator as a form of frequency generator which must produce a constant frequency and amplitude.

You can also read: The Basic Principle of Operation of an Oscillator