Odd order 60Hz Harmonics = EMI?

[cuibono]

Hi all,

I'm trying to improve the noise performance of a DAC and preamp I've built recently, and looking at an FFT of either output shows tons of odd order 60Hz harmonics (like F49 is 20dB above the noise floor). Is this due to electromagnetic pickup from transformers and power supply cords? Or is it a ground loop issue?

I remember reading somewhere that odd order harmonics are a sign of electromagnetic pickup. What should I do about this?

[Gerrit Boers]

There are two main sources for these harmonics:
1. The mains itself. This mains sine is a little topped of, this introduces odd harmonics.
See : AC supply noise
2. Switching noise from rectifiers.
I found that voltage regulators have very little effect on both. The first gets into the circuit directly, the second as common-mode noise. The switching noise can be reduced by using soft recovery diodes and, more importantly, by applying snubber caps across the rectifier.

Jensen 4-pole capacitors are good at reducing the higher harmonics: Capacitor comparison

[jcx]

actually you need to think about the narrow, nonlinear current pulses that are present with perfect sinewave line V and ideal rectification in a typical C input power supply

over sized C makes the charging current pulses even higher and sharper

the charging current pulses can result in radiated/magnetic coupling - think about loop areas/return path loops and twist wires to reduce areas

LC power supply filters require big chokes to keep current continuous, for low power like DAC/preamp it is possible to add R instead of L - for some filtering/slowing/spreading out of the charging current pulses - at low power the efficency hit (and extra V drop) may be tolerable


the cap charging current pulses also cause V drop in common impedances in the power wiring system - again think about where current is flowing, where the V drops in wire resistance/inductance appears and what star grounding is trying to accomplish

[underwurlde]

^ agreed. Concentrate on reducing Delta I's.

You have mixed (analogue & digital) layout ?

Post picture of layout / PCB design to spot issues for you?

Andy