What is the difference between a SMPS (switch-mode power supply) and linear supply in terms of RF noise?

• A. Linear supplies are quieter and generate less RF noise than SMPS.
• B. SMPS produces no RF noise.
• C. SMPS can generate more conducted and radiated RF noise due to switching, requiring filtering; linear supplies are quieter but heavier.
• D. RF noise is unrelated to power supply type.

Answer: (C)

The main idea is how the regulation method affects RF emissions. Switch-mode power supplies regulate by rapidly turning the output on and off at a high switching frequency. Those fast edges create lots of harmonics and EMI that can couple into the circuit as conducted or radiated RF noise. Because of that, SMPS designs rely on input and output filtering, proper shielding, and careful layout to stay within EMI limits. Linear supplies, on the other hand, regulate by dropping voltage across a pass element without high-frequency switching, so they don’t generate the same switching harmonics. Their RF noise is typically much lower, though they tend to be heavier due to the larger transformers and greater heat dissipation. So SMPS can generate more RF noise and require filtering, while linear supplies are quieter but heavier.