Clock Harmonic Interference Guide

What is RF Desensitization?

RF desensitization (or "desense") occurs when unwanted signals couple into a receiver, raising its effective noise floor and reducing its ability to "hear" weak signals. In HaLow (IEEE 802.11ah) designs, this often manifests as reduced range that appears to be a radio limitation but is actually self-inflicted interference.

Key insight: Morse Micro radios have exceptional receiver sensitivity. This is a strength, but it means your board's RF cleanliness is under a microscope. Signals that would be invisible to less sensitive receivers can cause measurable range degradation in HaLow systems.

Common Sources of On-Board Interference

SPI clock at 25 MHz (37th harmonic lands at 925 MHz)
SDIO clock at 48 MHz (19th harmonic lands at 912 MHz)
MIPI clock at 27 MHz from camera modules (34th harmonic lands at 918 MHz)
Processor clocks and their harmonics
Switching regulators with harmonic content extending into sub-GHz

How Harmonics Cause Problems

Digital clock signals are not pure sine waves. Square waves, trapezoidal clocks, and other non-sinusoidal signals contain energy at integer multiples of the fundamental frequency. These harmonics can extend far beyond the original clock frequency.

The diagram shows how a 25 MHz clock generates harmonics that extend into the HaLow spectrum. The 37th harmonic lands at 925 MHz, directly in the US HaLow band.

The Mathematics

// Harmonic frequencies
f_harmonic = n × f_fundamental

// For a square wave, odd harmonics dominate:
// n = 1, 3, 5, 7, 9, 11, ...

// Harmonic amplitude (ideal square wave):
V_n = V_peak × (4 / (n × π))

// Power in dBm (50Ω):
P_dBm = 10 × log₁₀(V² / 50 / 0.001)

HaLow Frequency Bands

HaLow (IEEE 802.11ah) operates in sub-GHz ISM bands. Different regions use different frequency allocations:

United States (FCC)

902 - 928 MHz

Europe (ETSI)

863 - 868 MHz

Australia (ACMA)

915 - 928 MHz

Any harmonic landing within these bands can cause receiver desensitization. The calculator below helps you identify which of your clock frequencies pose a risk.

Harmonic Interference Calculator

Identify clock harmonics that land in Wi-Fi HaLow bands (863 - 868 MHz, 902 - 928 MHz)

Clock Voltage 3.3 V

Clock Waveform Square

PMOS/NMOS Balance Balanced

Time Domain Waveform

Clock Frequency 25 MHz

Frequency Spectrum (850 - 950 MHz)

Harmonics in Range

No harmonics in range

Required Isolation

When a harmonic does fall within the HaLow band, the solution is to provide sufficient isolation between the interference source and the receiver. The required isolation depends on the harmonic power level and your sensitivity requirements.

Harmonic Power	Required Isolation	Typical Approach
> -30 dBm	60+ dB	Shielding, filtering, frequency change
-30 to -60 dBm	30-60 dB	Board layout, ground planes, filtering
< -60 dBm	0-30 dB	Standard layout practices

Mitigation Strategies

Choose clean frequencies - Select clock frequencies whose harmonics fall outside HaLow bands. The calculator above can help identify safe alternatives.

Reduce harmonic content - Use sinusoidal clocks or add filtering at the source. Slower edge rates reduce high-frequency harmonic energy.

Physical separation - Maximize distance between interference sources and the HaLow antenna/RF front-end.

Shielding - Use shielded clock modules and ensure proper grounding around sensitive RF sections.

This tool is provided for general guidance only. Although care has been taken in preparing it, no representation is made that the results will reflect all real-world conditions. Results should be treated as indicative only and verified against measured performance where required.