Module 18: Interference and Noise
Every electronic system has two enemies that limit what it can do: noise and interference. Noise is built into the physics — every resistor generates it, every amplifier adds to it, and it sets the smallest signal any circuit can detect. Interference comes from outside — switching power supplies, motors, lighting, digital circuits and radio transmitters — and reaches sensitive circuits along cables and through the air. This module explains both: how to calculate and minimize noise, how strong signals create distortion, and how to find and cure interference with filtering, ferrites, shielding, bonding and grounding.
- Calculate thermal noise, noise figure and the noise floor of an amplifier, receiver or instrument
- Calculate SNR and read SNR, ENOB, SINAD and noise density on datasheets
- Explain dynamic range, compression, intermodulation and the third-order intercept point
- Identify common interference sources by their signatures and locate them
- Distinguish common-mode from differential-mode noise and apply the right cure
- Choose and wind ferrite chokes, and use ferrite beads on circuit boards
- Apply shielding, bonding, star grounding and surge protection
Module Overview
The first part of the module covers noise. Thermal noise sets the fundamental floor; noise figure measures how much each stage adds; the noise floor formula combines them; and signal-to-noise ratio tells you whether a signal is usable. These lessons use examples from audio, sensors, ADCs and test instruments as well as receivers.
The middle lessons cover what happens at the top of the range: dynamic range and compression, intermodulation distortion, and the third-order intercept point that predicts it — essential for understanding amplifiers, ADCs, receivers and spectrum analyzers when strong signals are present.
The final five lessons are about interference in practice: identifying EMI sources and fixing them, understanding common-mode and differential-mode currents, choosing and using ferrites, shielding and bonding enclosures and benches, and star grounding and surge protection for circuits, equipment and buildings.
Lessons
Thermal Noise
Thermal Noise
Johnson-Nyquist noise, P = kTB, noise voltage and temperature, the -174 dBm/Hz reference, and source resistance and bandwidth in real circuits.
Noise Figure
Noise Figure
Noise factor and noise figure, noise temperature, the Friis formula for cascaded stages, and preamplifiers and cable loss, with calculators.
Noise Floor
Noise Floor
N = -174 + 10log(BW) + NF, resolution bandwidth, minimum discernible signal, and the noise floor of ADCs, oscilloscopes and FFTs.
Signal to Noise Ratio
Signal to Noise Ratio
Calculating SNR, how much SNR different applications need, processing gain, ENOB, noise density, SINAD and THD+N.
Dynamic Range
Dynamic Range
Blocking and spurious-free dynamic range, the 1 dB compression point, attenuators, and dynamic range in audio, ADCs and analyzers.
Intermodulation Distortion
Intermodulation Distortion
Second- and third-order products, the 3:1 slope, the attenuator test for IMD, and two-tone testing of RF and audio amplifiers.
Third Order Intercept
Third Order Intercept
IIP3 and OIP3, cascaded IP3, predicting IMD levels, ADC linearity specifications, and the IP3 versus noise figure trade-off.
EMI Sources and Fixes
EMI Sources and Fixes
Switching supplies, solar inverters, LED lighting, power lines, motors and computers, circuits as victims, locating sources, and EMC rules.
Common Mode and Differential Mode
Common Mode and Differential Mode
The two current modes, how common-mode current arises and radiates, current probes, entry paths, and chokes and filters.
Ferrite Chokes and Cores
Ferrite Chokes and Cores
Ferrite mixes, core shapes, turns and impedance, cable chokes, snap-on clamps, ferrite beads, saturation, and VNA measurement.
Shielding and Bonding
Shielding and Bonding
Skin depth, magnetic shielding, apertures and cable penetrations, bonding, ground loops, safety earth, and bonding benches and products.
Single Point Ground and Surge Protection
Single Point Ground and Surge Protection
Star grounding in circuits, single-point earthing, surge sources, GDTs, MOVs and TVS diodes, mains protection, and protecting circuits.