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Noise Voltage Spectral Density

Key Takeaways

  • Noise voltage spectral density measures the amount of random electrical noise present in a circuit over a range of frequencies.

  • Different types of noise have different spectral density profiles, which describe how the noise energy is distributed across frequencies

  • The spectral density profile influences PCB decisions related to partitioning noisy and sensitive sections, grounding strategies, the placement of decoupling capacitors, and other considerations. 

llustration of white noise, which has a unique noise voltage spectral density

White noise (illustrated above) has a unique noise voltage spectral density.

Noise voltage spectral density, sometimes called noise density or noise voltage density, measures the amount of random electrical noise present in an electronic component or circuit over a range of frequencies. It significantly impacts PCB design, which will be discussed below. But first, let’s look at the different types of noise and their spectral density profiles. 

Common Noise Types and Their Spectral Density Profiles

Noise Types, Causes, and Spectral Density Profiles

Noise Type


Spectral Density

White Noise

Thermal agitation of electrons in a conductor

Constant across all frequencies

Shot Noise

Discrete nature of the electrical current flow

Relatively flat across frequencies

Flicker Noise

Defects or irregularities in semiconductors

Inverse relationship with frequency

Pink Noise

Slow fluctuations of condensed-matter materials in devices

Decreases by 3 dB per octave

Gaussian Noise

Thermal vibration of atoms

Depends on the standard deviation of the noise amplitude

Impulse Noise

Rapid release of compressed gases or the collision of solid objects

Varies depending on the pattern and frequency of the impulse

There are different types of noise, and each type has a different spectral density profile, which describes how the noise energy is distributed across frequencies. Here are some common noise types and their spectral density profiles:

  • White noise (thermal or Johnson-Nyquist noise) has a constant spectral density across all frequencies, which means that its energy is distributed uniformly across the frequency spectrum. It is characterized by a flat noise density plot. White noise is primarily caused by the thermal agitation of electrons in a conductor and is present in all electronic components and circuits.

  • Shot noise is caused by the discrete nature of the electrical current flow, arising from the statistical variation in the arrival times of individual charge carriers. Its spectral density is also relatively flat across frequencies. In electronic devices, shot noise is more noticeable at low currents.

  • Flicker noise (also known as 1/f noise or low-frequency noise) increases as frequency decreases, which means that its energy is concentrated at lower frequencies. The spectral density of flicker noise follows an inverse relationship with frequency, often leading to a curved slope on a logarithmic frequency axis. Flicker noise is often associated with defects or irregularities in semiconductor materials and can significantly impact low-frequency electronic systems.

  • Pink noise is a type of noise that has equal energy in octave-wide frequency bands. In other words, its spectral density decreases by 3 dB per octave. This type of noise is often considered more balanced in frequency distribution than white noise. It is caused by slow fluctuations of condensed-matter materials in devices

  • Gaussian noise, also known as normal noise, has a random distribution following the Gaussian probability distribution. Its spectral density depends on the standard deviation of the noise amplitude and doesn't exhibit specific frequency characteristics like other types of noise. Gaussian noise is often used as a theoretical or modeling assumption. It is caused by the thermal vibration of atoms.

  • Impulse noise consists of sudden and brief spikes in voltage or current. Its spectral density can vary depending on the pattern and frequency of the impulses, but in many cases, it's localized to specific frequencies corresponding to the timing of the impulses. It is often caused by the rapid release of compressed gases or the collision of solid objects.

Understanding a component or circuit's noise voltage spectral density is crucial in various applications, such as designing low-noise amplifiers, communication systems, and other sensitive electronic systems where minimizing noise interference is essential for accurate signal processing and communication.

Noise Voltage Spectral Density and PCB Design

PCB design strategies are influenced by noise voltage spectral density. By characterizing the distribution of noise energy across different frequency ranges, engineers can arrange components, traces, and ground planes to ensure optimal signal integrity. The spectral density profile influences decisions related to partitioning noisy and sensitive sections, grounding strategies, and the placement of decoupling capacitors. 

 Here's how noise voltage spectral density considerations affect PCB design:

  • Signal Integrity and Crosstalk: Noise can degrade the integrity of signals on a PCB, leading to signal distortions, jitter, and timing issues. Understanding the noise voltage spectral density when designing high-speed circuits is crucial for managing signal integrity. Proper placement and routing of traces, ground planes, and power planes can help mitigate noise-induced signal integrity problems. Crosstalk between traces can also lead to noise coupling, so careful layout practices are essential.

  • Ground Plane Design: A well-designed ground plane helps provide a low-impedance return path for currents and reduces electromagnetic interference (EMI). Noise from various sources can couple into sensitive traces through shared ground paths. By designing a solid ground plane and using proper grounding techniques, the impact of noise on the circuit can be minimized.

  • Partitioning and Isolation: Separating noisy components from sensitive components is a common technique to mitigate noise-related issues. Noise voltage spectral density considerations can help guide the placement of components to minimize noise coupling. High-frequency components and noise sources should be isolated from sensitive analog or digital sections to prevent unwanted interference.

  • Decoupling and Bypass Capacitors: Noise can be introduced through power supply lines due to voltage fluctuations. Placing decoupling and bypass capacitors strategically on the PCB helps stabilize power supplies and reduce noise-induced voltage variations. The choice of capacitor type, value, and placement depends on the noise characteristics in different frequency ranges.

  • Component Selection: Components themselves can contribute to noise generation and propagation. When selecting PCB components, it's essential to consider their noise characteristics and how they might interact with other components. Low-noise components, such as low-noise amplifiers, are preferred for sensitive applications.

  • EMI and ESD Considerations: Noise on a PCB can lead to EMI issues, affecting the board's performance and causing problems for other nearby electronics. Careful design practices that reduce noise can help minimize EMI. Additionally, designing the PCB with proper electrostatic discharge (ESD) protection can prevent transient noise.

  • Shielding and Filtering: For high-frequency applications, noise can be effectively mitigated through shielding and filtering techniques. Shielding can prevent external electromagnetic fields from coupling into the circuit, and filters can attenuate noise in specific frequency ranges.

  • Thermal Considerations: Some noise sources, like thermal noise, are affected by temperature. Proper thermal management strategies are essential to prevent temperature-related noise fluctuations that can affect the performance of sensitive components.

Understanding the noise voltage spectral density and its distribution across frequency ranges helps guide various aspects of PCB design. Careful consideration of noise-related factors allows engineers to create PCB layouts that maximize signal integrity, minimize interference, and ensure the overall functionality of the electronic system. Addressing noise voltage spectral density effectively during PCB design protects against unwanted signal distortions, EMI, and other noise-induced challenges, ensuring the functionality and reliability of electronic systems.

Use Cadence PCB Design and Analysis Software to Reduce Noise

Choosing the right PCB design and analysis software will give you access to the right analysis features you need to create circuits that are barely susceptible to noise. Cadence's OrCAD PCB Designer includes analysis tools for optimizing power and signal integrity to reduce noise. This toolset is ideal for those who need to design PCBs for advanced and sensitive applications.

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