Frequency Modulation vs Amplitude Modulation

Frequency Modulation:

Frequency Modulation is the encoding of information in a carrier wave by varying the frequency of the carrier wave at that instant corresponding to the variation of the base waveform to be transmitted.

Amplitude Modulation:

Amplitude Modulation is an electronic communication systems technique wherein the base band signal is superimposed with the amplitude of the carrier wave i.e the amplitude of the carrier wave is varying with proportion to the base waveform being transmitted.

frequency modulation

Advantages of Frequency Modulation over Amplitude Modulation:

  • The amplitude of an FM wave remains constant over time. This allows the encoders and decoders the freedom to remove the noise from the received signal. This is done with the assistance of a filter that removes signal of wavelength greater than that of the transmitted signal thereby removing the noise. FM systems are noise immune systems. Since the AM signal transferred information using the amplitude, the envelope of the waveform cannot be altered.
  • In AM systems, the power consumption for signal transmission is higher when compared to FM systems. In AM systems, the power depends on the modulation index also called ‘MA’. When MA reaches unity, the power consumption is a 100%. In FM systems, the power of the transmitted signal is proportional to the amplitude of the unmodulated carrier signal and it is constant. Therefore FM is usually more power efficient than AM systems.
  • In FM systems, the frequency deviation of the signal is related to the noise ratio. A higher frequency deviation means that the baseband signal can be easily retrieved from the FM signals, whereas less deviation means it is harder to separate the data from the noise. In AM systems, the only method of reducing noise in the transmission is the increase the transmitted power of the signal. This increases the cost of operation of the AM system.
  • In an AM system, there is no guard band in between two adjacent channels. This seriously increases the occurrence of interference of AM radio station unless one signal is strong enough to overpower the other. Whereas in the case of FM signal, the adjacent FM channels are separated by guard bands which results in very little interference between adjacent FM channels.

Disadvantages of Frequency Modulation over Amplitude Modulation:

  • The equipment needed for FM and AM systems are different. The equipment cost of an FM channel is more since the equipment is much more complex and involves complicated circuitry. As a result FM systems are costlier than AM systems.
  • FM systems work using line of sight propagation whereas AM systems use sky wave propagation. Consequently the receiving area of an FM system is much smaller than that of an AM system. The antennae for FM systems need to be close by whereas AM systems can communicate with other systems throughout the world by reflecting signals off the ionosphere.
  • In a FM system, there are an infinite number of side bands resulting in a theoretical bandwidth of an FM signal being infinite. This bandwidth is limited by Carson’s rule but it is still much larger than that of an AM system. In an AM system, the bandwidth is only twice the modulation frequency. This is another reason why FM systems are costlier than AM systems.

Stay tuned with byju’s to understand through the comparative study, how the usage of either FM or AM system is decided based on the characteristics of each system and it’s feasibility for the need at hand.

Practise This Question

A man wants to swim across a river from A to B and back from B to A always following line AB. The distance S between points A and B is S. The velocity of the river current v is constant over the entire width of the river. The line AB makes an angle α with the direction of current. Let the man travel with velocity u and at angle β to the line AB. Find time 't' by which he will be back.