Modulation of Waves(FM Radio, AM Radio and Television)

Year 11 Physics

Transmitting information with waves

• All waves carry energy from one place to another.• However, a wave that carries exactly the same amount

of energy continuously does not carry information.• For a wave to carry useful information, the wave must

vary.• There are two simple ways to vary a wave to add

information to it:– Vary the Frequency (wavelength and energy changes)

– Vary the Amplitude (energy changes)

Modulation• When you speak you add information to

the sound waves that are your voice by varying both the amplitude and frequency of the sound waves.

• These variations give the signal you know as language.

• Electromagnetic waves can also be used to carry information (like your voice) but the information must be added to the waves.

• The process of adding signal information to a carrier wave is called modulation.

Amplitude Modulation (AM)

• In Amplitude Modulation (AM) the signal remains constant in frequency but the amplitude of the wave varies according to the signal information.

• Amplitude modulation is an example of superposition of the modulating signal that carries the message onto the carrier wave.

Frequency Modulation (FM)

• In Frequency Modulation (FM) the signal remains constant in amplitude but the frequency of the wave varies according to the signal information.

• Notice at the ‘peaks’ of the modulating signal the corresponding FM modulated carrier wave appears compressed – but it is still a transverse wave!!

Producing Radio Waves• Radio waves are emitted by stars but can also be produced

artificially.• Radio waves can be produced when electrons in a metal rod, called

a transmitting antenna or transmitter, are made to vibrate rapidly.

• Radio stations use transmitters to transmit their own frequency of radio wave (carrier wave).

• The voice or music from the radio station forms an audio signal.• The audio signal is then mixed with the carrier wave to produce the

modulated wave.• This modulated wave is what is transmitted by the radio station.

Detecting Radio Waves• The radio waves produced by the radio station can be detected using a receiving antenna (a metal rod just like the

transmitter).• In the receiving antenna the radio waves cause electrons in the metal to vibrate rapidly and produce an electrical signal.• The receiving antenna of your radio detects the modulated wave.• Your radio then subtracts the carrier wave from the signal, leaving just the audio signal to be amplified by an audio

amplifier and sent to the speakers.

AM and FM Radio• In AM (Amplitude Modulation) Radio the audio

signal changes the amplitude of the carrier wave. • In FM (Frequency Modulation) Radio the audio signal

changes the frequency of the carrier wave.

AM versus FM for broadcasting

• AM radio waves have longer wavelengths than FM and can be received at greater distances.

• FM radio waves are less affected by electrical interference and hence provide a higher quality transmission of sound

• FM radio waves are less able to travel around obstacles such as hills and large buildings. Better suited to line of sight transmission.

• FM radio signals require a larger bandwidth of the electromagnetic spectrum

Television• Television signals are transmitted on two separate carrier

waves– Visual signal is added onto one carrier wave using Amplitude

Modulation (AM)– Audio signal is carried on a separate carrier wave using Frequency

Modulation (FM)

• When you select a particular channel, you are selecting the respective visual and audio carrier waves for that channel.

• Your TV then completes the task of ‘stripping’ the carrier waves to produce the desired picture and sound.

• NOTE: In Digital Television visual and audio information is transmitted as a binary code using a variety of modulation techniques such as; pulse code modulation (PCM), pulse amplitude modulation (PAM), pulse duration modulation (PDM), and pulse position modulation (PPM).