Amplitude (AM) and frequency modulation (FM) techniques (AQA A-Level Physics): Revision Notes

13.6.4 Amplitude (AM) and frequency modulation (FM) techniques

Introduction to Modulation

• Modulation is the process of making a signal suitable for its transmission medium.
• It's essential when transmitting data via electromagnetic waves (radio or light) through free space.
• There are two primary types of modulation:
  1. Amplitude Modulation (AM)
  2. Frequency Modulation (FM)

Amplitude Modulation (AM)

• In Amplitude Modulation, the shape of the information signal dictates the amplitude of the carrier wave.
  • The information signal is the data being sent (e.g., audio signal).
  • The carrier wave is the medium wave on which this data is "carried" (e.g., radio wave).
• Process:
  • The information signal and the carrier wave enter a modulator.
  • The information signal's amplitude imprints on the carrier wave, producing an AM signal.
  • This AM signal, with the amplitude variations corresponding to the information, is sent for further transmission.

Key Concepts in AM

1. Sidebands:

• Amplitude modulation generates two sidebands around the carrier frequency.
• Upper sideband (USB): Frequency $f_c + f_m$ (carrier frequency + maximum information frequency).
• Lower sideband (LSB): Frequency $f_c - f_m$ (carrier frequency - maximum information frequency).

2. Bandwidth:

• The AM bandwidth is the difference between the upper and lower sidebands.
• Formula:

$$\text{AM bandwidth} = 2f_m$$

• This bandwidth must be available in the transmission medium to effectively carry the AM signal.

Frequency Modulation (FM)

• In Frequency Modulation, the amplitude of the information signal defines the frequency of the carrier wave. Higher amplitude in the information signal results in higher frequency in the carrier wave.
• Characteristics of FM:
  • The amplitude of the FM signal remains constant, only the frequency changes based on the information signal.
  • Unlike AM, FM produces an infinite series of side frequencies around the carrier frequency due to frequency variations.

Key Concepts in FM

1. Frequency Deviation ($$\Delta f):

• Frequency deviation is the amount by which the carrier frequency can vary according to the information signal.

2. Bandwidth:

• Narrowband FM: Small frequency deviation relative to $f_m$ ; requires lower bandwidth.

• Wideband FM: Larger frequency deviation, needs more bandwidth.

• Carson's Rule approximates FM bandwidth for wideband signals:

$$\text{FM bandwidth} = 2(\Delta f + f_m)$$

Advantages of FM over AM

• Noise Resistance: FM is less susceptible to noise because noise typically affects amplitude, which remains constant in FM.
• Reduced Fading: FM avoids issues from wave reflections which may interfere with amplitude; this is beneficial in environments with many reflective surfaces.

Disadvantages of FM

• Higher Bandwidth Requirement: FM needs more bandwidth than AM, especially for wideband applications.
• Line-of-Sight Requirement: FM signals do not refract well and need direct paths, limiting long-distance or non-line-of-sight applications.

Data Capacity of a Transmission Channel

• The data capacity (maximum data rate) is calculated as:

$$\text{Data capacity} = 2 \times \text{maximum available bandwidth}$$

• This formula helps determine the highest achievable data transfer rate in various transmission media.

Comparison of Bandwidth in Different Transmission Media:

Transmission Media	Bandwidth
Coaxial Cable	$10^{10}$ Hz
Twisted-Pair Cable	$10^9$ Hz
Optical Fibre	$10^{16}$ Hz