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Switched mode power supplies, LDOs and RF circuitry

Dear Sir

  1. Are LDOs after a Switched mode power supply a good idea to filter noise (glitches due to switching)? Mostly when supplying sensitive circuitry, such as Radio Frequency Blocks (LNAs, VCOs, PLLs, etc…).
  2. Or simply it is not a good idea, as they behave as a High Pass Filter. They filter out the low frequency components of noise coming out of the Switched mode power supply, but the noise due to the switching inside the switched power supply is above their cut-off frequency, and therefore it will pass through the LDO. Do you agree?
  3. Are ferrite beads a better approach to remove the noise coming out of the Switched mode power supply?



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