13. Introduction to Signals

a. What is a signal?

A signal is the transmission of data. We deal with signals constantly during the span of our lives. We interact with signals from music, power lines, telephones, and cellular devices. This means the use of antennas, satellites, and of course wires. In "computer land" signals are very important. Anyone that uses a computer should know how the machine transforms data into signals that other computers and devices can understand. In many cases, knowing how signals work will help you solve some kind of technical problem over the span of your life.

b. How is data transmitted?

Data is transmitted through a transmission medium. Transmission media come in two forms: wired and wireless.

Wired transmission media include: twisted pairs, coaxial cables and optical fibres. They involve the use of a physical connection from point to point and are generally tedious to adopt in systems which require many communication terminals to be set up.

Wireless transmission media offer the advantage of mobility at the cost of compromising speed and security. Examples of wireless media are: RF waves, microwaves and infrared.

c. Analog vs Digital Signals

The two main types of electrical signals are analog and digital signals. An analog signal generally takes on a continuous waveform, taking a possible infinite set of values across its range. A digital signal on the other hand, is discrete, with the amount of possible values being finite.

The following diagram depicts the differences between an analog and digital waveform:

d. What is RF?

Radio frequency (RF) is a type of transmission which involves pushing a oscillation of analog signals across a medium which corresponds to the frequency of radio waves. While RF waves take the form of analog signals, digital signals can be transmitted through RF as long as it is first properly modulated onto an analog carrier. It is subsequently demodulated at the receiving end to extract the original signal.

The following two tables outline the various nomenclatures for the frequency bands. The third table outlines some of the applications at each of the various frequency bands.

 

Table 1: Frequency Band Designations

Table 1 shows a relationship between frequency (f) and wavelength (λ). A wave or sinusoid can be completely described by either its frequency or its wavelength. They are inversely proportional to each other and related to the speed of light through a particular medium. The relationship in a vacuum is shown in the following equation:

where c is the speed of light. As frequency increases, wavelength decreases. For reference, a 1 GHz wave has a wavelength of roughly 1 foot, and a 100 MHz wave has a wavelength of roughly 10 feet.

 

Table 2: Microwave Letter Band Designations

 

e. Why Operate at Higher Frequencies?

Reasons accounting for this push into higher frequencies include efficiency in propagation, immunity to some forms of noise and impairments as well as the size of the antenna required. The antenna size is typically related to the wavelength of the signal and in practice is usually ¼ wavelength.

Next topic will be Modulation