Modulation
Physics: Cambridge International A Level, topic 10.1.2
Modulation
Radiowaves can be altered by a process called modulation in order to transmit information.
Amplitude modulation (AM)
- Amplitude modulation or AM radio waves are used to carry commercial radio signals in the frequency range from 540 to 1600 kHz.
- A carrier wave which has the frequency used to tune in to the radio station is varied or modulated in amplitude by an audio signal.
- The resulting wave has a constant frequency, but a varying amplitude.
Properties of AM
- AM waves can go over and around rather large obstacles.
- A very large antenna is needed to efficiently broadcast typical AM radio with its carrier wavelengths on the order of hundreds of meters.
- AM waves are susceptible to noise as the addition of other waves adds to the amplitude of the wave.
Frequency modulation (FM)
- Frequency modulation or FM radio waves are also used for commercial radio transmission but in the frequency range of 88 to 108 MHz.
- A carrier wave which has the frequency used to tune in to the radio station is modulated in frequency by the audio signal.
- This produces a wave of constant amplitude but with varying frequency.
Properties of FM
- FM radio is less subject to noise from stray radio sources than AM radio.
- FM antennas are much smaller than AM antennas.
- However, FM is best received when there is a line of sight between the broadcast antenna and receiver, so they are often sent from very tall structures.
Sideband frequencies
- If a carrier wave of frequency is amplitude modulated by a simple sine wave of frequency , the resulting wave is found to be composed of three frequencies:
- and
- The two frequencies and are known as sideband frequencies.
Bandwidth
- The bandwidth of this wave is equal to the range of the frequencies carried by the wave: