Let’s go one step deeper into the subject of signals and talk about the electromagnetic spectrum and bandwidth. As you know from your experience in listening to a radio, RTHK (Radio Television Hong Kong) and other radio stations broadcast on different frequency ranges. In addition, some stations send out Amplitude Modulation (AM) radio broadcasts and others send out Frequency Modulation (FM) radio broadcasts. The details of both AM and FM schemes will be covered in Unit 4 of ELEC S211. Here, we’ve just used them as examples to demonstrate the idea of different frequency allocations.
The total span of frequencies used in communication systems is called the electromagnetic spectrum, or simply the spectrum. This total spectrum is allocated into different bands.
Since many users, services and countries share the electromagnetic spectrum, some rules are necessary to govern its use. The International Telecommunications Union (ITU) is the international organization that performs this regulatory role. Individual countries also have local organizations that do similar jobs. In Hong Kong, the Office of the Telecommunications Authority (OFTA) is responsible for the assignment, licensing and monitoring of the electromagnetic spectrum. The OFTA is an organization of the Hong Kong Government that was established in 1993.
You may readily understand the purposes of frequency allocation. But the question is, even if allocation rules are in place, how can we ensure that every signal remains precisely within the frequency boundary that has been assigned to it? The answer to this is bandwidth.
Bandwidth is the span of frequencies within the spectrum that is occupied by a given signal.
For example, let’s say a signal transmitted at 300 kHz has a bandwidth of 20 kHz. The frequency range it
occupies, therefore, is from 290 kHz to 310 kHz, as shown in Figure 4 below. So, in this case, frequencies below 290 kHz and above 310 kHz are clear for allocation to other signals.
The next question is how much bandwidth is required to convey a given signal. This depends on the information being transmitted. For example, the human voice has significant amplitude in the frequency range of 300 Hz to 3.4 kHz. So the bandwidth required for analogue voice signals is 3.1 kHz (3.4 kHz – 300 Hz = 3.1 kHz).
Let's take a look at Example 2, in which we calculate the bandwidth of a signal with a given frequency range.