Noise is the unwanted disturbance in an electrical signal. It is considered to be a source of error.

• 60Hz AC supply. This is almost always present.
• Inductors.
• Motors and varying magnetic field.
• RF, Wifi, and Microwave.
• Anything that causes abrupt change in current (switching regulators and switches).

• It affects the operation of the system.
• It could introduce error when measuring or using very low signals (in mV range). The noise could appear on both the signal and power line.

• Use a filter (band-pass or band-stop).
• Keep inductor’s traces short and fat.
• Battery power generates less noise compared to regulated power from main source.
• Use filter/decoupling capacitor. Maybe use a combination of 0.1uf, 1uf and 10uf to handle multiple noise frequencies. Low capacitance handles high frequency noise very well.
• Add ground planes and Vcc planes.
• Avoid right angle traces on board design. Especially for high frequency signals, use 45 or curved traces.
• Keep like components close to each other.
• Use a split Vcc (analog Vcc and Digital Vcc)
• Do not cram components too close.
• Zener can help handle spikes in the circuit.
• Keep radios away from inductors as much as possible.