Spectrum refers to the entire range of frequencies right from the starting frequency (the lowest frequency) to the ending frequency (the highest frequency). Spectrum basically refers to the entire group of frequencies.
The electromagnetic spectrum is one good example. The electromagnetic (EM) spectrum covers frequencies right from zero (DC) to gamma band frequencies. This spectrum includes the human voice frequency band (audio band), ISM (Industrial Scientific Medical) band and optical frequency bands.
Microwave radiations span in frequency from 300 MHz to 300 GHz. What is the spectrum of microwave radiation?
Spectrum refers to the entire range of frequencies that exist in the microwave band. Therefore, the spectrum of microwave radiation is (300 MHz to 300 GHz).
The difference between spectrum and bandwidth is that spectrum refers to the ‘entirety’ while bandwidth is a ‘sub-section’ of the spectrum. Spectrum refers to the wholesome of the quantity while bandwidth, on the other hand, is a portion of the entire spectrum.
Bandwidth is a sub-section of a portion of spectrum.
If frequencies from 12 MHz up to 40 MHz are allocated for an application, the spectrum refers to the entire range of frequencies right from 12 MHz to 40 MHz Therefore, the spectrum is (12 to 40) MHz. In some cases, the entire allocated
12 MHz to 40 MHz = Spectrum 17 MHz to 20 MHz = Bandwidth
frequencies may not be used by the application. So, if only 17 MHz to 20 MHz is used by an application, then, those range of frequencies used is called the ‘bandwidth’.
The commercial FM radio is one good example that explains the difference between the spectrum and the bandwidth. Spectrum here refers to the entire range of frequencies allocated for the FM radio application which is usually around 20 MHz (108 MHz to 87.5 MHz). An FM radio station doesn’t occupy the entire frequency band but just a fraction of it. Let us say that an FM station operates over the center frequency 93.5 MHz FM radio stations are usually allocated a bandwidth of 200 kHz. So, the range of frequencies over which the FM radio station operates is 93.4 MHz to 93.6 MHz.
Let us look at one more example to understand the difference between spectrum and the bandwidth. Grapes come in different colours, sizes and tastes depending on the growing conditions and methods. The entire family is still called grapes even though there are differences in colour, size and taste. Bandwidth, however, refers to a particular group- a group of grapes grown in area X might have black colour, slightly sour taste Spectrum = 20 MHz Bandwidth of the radio station = 200 kHz and oval figure while a group of grapes grown in area Y might have green colour, sweet taste and oval figure.
Entire family of grapes = Spectrum Different groups among grapes = Bandwidth