---
title: "Spread spectrum"
question: "What does spread spectrum do to a buck’s EMI test?"
url: "https://nearfieldnotes.com/videos/spread-spectrum/"
site: "Near Field Notes"
video: "https://nearfieldnotes.com/media/spread-spectrum/full.mp4"
duration: "1:19"
date: "2026-11-03"
note: "https://nearfieldnotes.com/notes/spread-spectrum/"
---

# Spread spectrum (video)

What does spread spectrum do to a buck’s EMI test?

1:19, with sound. [Read Note 6](https://nearfieldnotes.com/notes/spread-spectrum/) for the full answer, with figures, conditions and sources.

## Transcript

This laptop's 5 V supply changes its own switching frequency on purpose.

It's called spread spectrum. Here's what it is, and what it's for.

The MNT Reform's 5 V buck switches at 400 kHz, from its 25.6 V battery.

With spread spectrum on, the chip sweeps that frequency up and down 13,500 times a second.

It adds a small random jump every cycle, for about 15 kHz either way.

Every switching edge makes noise at 400 kHz and every multiple of it.

An EMI test reads that noise through a 9 kHz window, about one AM radio channel.

At a fixed frequency, each spike sits in that window the whole time.

With spread spectrum, the 10th harmonic moves 10 times as far, so the window catches less of it.

At 4 MHz, that reads 10 dB lower.

But the 400 kHz spike only moves 15 kHz, too little to spread much, so it drops just 1 dB.

Up at 30 MHz, the harmonics smear into each other, and it reads 12 dB lower.

The energy is all still there, just spread out, so no one radio channel gets all of it.

That's why clocks for PCIe, USB and displays do it too.

---

Source: https://nearfieldnotes.com/videos/spread-spectrum/