Understanding Noise: Sources and Types

Noise is an unwanted electrical signal present in a circuit or system, distinct from the desired, useful signal. It unpredictably alters the frequency, amplitude, or phase of the desired signal.

When noise voltage leads to improper functioning of the system or circuit, it’s referred to as interference. Noise cannot be completely eliminated, but its magnitude can be reduced to minimize its impact on system performance.

Noise is almost always present in communication signals due to various factors. Engineers constantly explore ways to minimize the effects of these different types of noise on systems.

Manmade Noise

Manmade noise is prevalent in urban and industrial areas. Examples include noise generated by automobiles, electric motors in aircraft, leakage from high-voltage power lines, and fluorescent lights.

These noise sources typically affect signals in the frequency range of 1 to 600 MHz. As a general rule, the wider the receiver bandwidth, the more noise is received along with the desired signal.

Manmade noise can be mitigated through proper shielding and by eliminating coupling channels.

Atmospheric Noise

Atmospheric noise results from spurious radio frequency waves caused by lightning and other natural disturbances in the atmosphere. Other sources include rain, snow, and dust storms near the RF antenna.

This type of noise is significant in the frequency range of 10 MHz to 20 MHz. It’s often observed while listening to radio programs using shortwave receivers.

Atmospheric noise can be reduced by eliminating sharp points around the antenna.

Extra-Terrestrial Noise

Solar noise and cosmic noise are examples of extra-terrestrial noise.

Solar Noise

The sun radiates a broad range of frequencies at a temperature of approximately 6000 degrees Celsius. These frequencies fall within the bands used for various communication purposes, resulting in what is known as solar noise.

Cosmic Noise

When a directional antenna is pointed towards the sky to receive a signal, it also receives random noise from the galaxy, in addition to the desired signal. The intensity of this noise varies widely and is referred to as cosmic noise.

A major source of cosmic noise is ionized intersteller gas clouds in our galaxy. It can be categorized into thermal and non-thermal noise. Cosmic noise is predominant when the antenna is pointed towards the sky to receive satellite signals and exists in the frequency band ranging from 15 MHz to 100 GHz.

Internal Noise

Internal noise is generated by active and passive devices within radio receivers and supporting electronic circuits.

This noise is randomly distributed across the entire acquisition range and is directly related to bandwidth. Shot noise, quantum noise, and thermal noise are examples of internal noise.

Shot Noise

Shot noise is present in all active devices, including amplifiers. It’s generated due to random variations in the arrival of electrons and holes at the collector terminal of a transistor.

The shot noise equation is:

$I_{sh} = \sqrt{2 \cdot q \cdot I_{dc} \cdot B}$

Where:

• $I_{sh}$ is the RMS value of the shot current.
• $q$ is the electron charge (approximately $1.602 \times 10^{-19}$ coulombs).
• $I_{dc}$ is the diode DC current.
• $B$ is the bandwidth in Hz.

Quantum Noise

Thermal noise typically has a flat noise density spectrum that extends well into the infrared at room temperature. As the frequency increases, the spectrum starts to fall. At this point, another type of noise, known as quantum noise, emerges.

Quantum noise has a power spectral density defined by the following equation:

$W(f) = h \cdot f \text{ watts/Hz}$

Where:

• $h$ is Planck’s constant (approximately $6.626 \times 10^{-34}$ J·s).
• $f$ is the frequency in Hz.

Quantum noise is significant at optical frequency bands and negligible at commonly used lower frequencies.

Thermal Noise

Thermal noise is generated in a resistance or any resistive component of impedance. It’s caused by the rapid and random motion of electrons.

The temperature of a particle expresses its kinetic energy. At 0 degrees Kelvin, the kinetic energy of a particle becomes zero. The noise power produced in a resistor is proportional to the absolute temperature and measurement bandwidth.

Thermal noise power is given by:

$P = K \cdot T \cdot B$

Where:

• $P$ is the thermal noise power.
• $K$ is Boltzmann’s constant (approximately $1.38 \times 10^{-23}$ J/K).
• $T$ is the temperature in Kelvin.
• $B$ is the bandwidth.