Electrical Engineering

Electrical Engineering

Electrical Engineering

Task 1

1.a)Draw an amplitude modulated waveform and describe which part of it contains the information to be conveyed.

When amplitude modulated signal is created, the amplitude of the signal is varied in line with the variations in intensity of the sound wave. In this way the overall amplitude or envelope of the carrier is modulated to carry the audio signal. Here the envelope of the carrier can be seen to change in line with the modulating signal. (Leland 2009)

Amplitude modulation, AM is the most straightforward way of modulating a signal. Demodulation, or the process where the radio frequency signal is converted into an audio frequency signal is also very simple. It only requires a simple diode detector circuit. The circuit that is commonly used has a diode that rectifies the signal, only allowing the one half of the alternating radio frequency waveform through. A capacitor is used to remove the radio frequency parts of the signal, leaving the audio waveform. This can be fed into an amplifier after which it can be used to drive a loudspeaker. As the circuit used for demodulating AM is very cheap, it enables the cost of radio receivers for AM to be kept low. (Ryder 1984)

Amplitude modulation efficiency

Amplitude modulation, AM has advantages of simplicity, but it is not the most efficient mode to use, both in terms of the amount of spectrum it takes up, and the usage of the power. It is for this reason that it only has limited applications for broadcast and two way radio communications systems. (Leland 2009)

The reason for its inefficiency occurs as a result of the composition of the radio signal. When a radio frequency signal is modulated by an audio signal the envelope will vary. The level of modulation can be increased to a level where the envelope falls to zero and then rises to twice the un-modulated level. Any increase on this will cause distortion because the envelope cannot fall below zero. As this is the maximum amount of modulation possible it is called 100% modulation.

Even with 100% modulation the utilisation of power by an amplitude modulated signal is very poor. When the carrier is modulated sidebands appear at either side of the carrier in its frequency spectrum. Each sideband contains the information about the audio modulation. To look at how the signal is made up and the relative powers take the simplified case where the 1 kHz tone is modulating the carrier. In this case two signals will be found 1 kHz either side of the main carrier. When the carrier is fully modulated i.e. 100% the amplitude of the modulation is equal to half that of the main carrier, i.e. the sum of the powers of the sidebands is equal to half that of the carrier. This means that each sideband is just a quarter of the total power. In other words for a transmitter with a 100 watt carrier, the total sideband power would be 50 watts and each individual sideband ...