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Broadband RFI Spectrum and Transmission Paths

broadband rfi spectrum

We listen to our radios in a broadband RFI spectrum conducted and radiated all around us. 

Most of our radio frequency interference effecting reception is broadband. You will find it spreads across a broad range of frequencies because of harmonics. You will encounter broad peaks, such as from SMPS and narrow peaks, such as from Ethernet routers.

When your interfering signals are not pure sine waves, everything gets worse. Sine waves have gentle rise and fall times. On the other hand, all other types of interfering signals with irregular shapes – such as impulses or square waves – can have very sharp and fast transitions. Rapid changes are caused by very large and sudden changes in current or voltage at the source.

A faulty power line can exhibit arcs and sparks at 120 Hz. All switched mode power supplies contain free running oscillators from 50 kHz up to several MHz. Clocks in digital devices can run at tens or hundreds of MHz. You will find that all three of these sources across the spectrum have one thing in common: they create square waves.

Low frequency RFI is typically conducted by wires. Really high frequency RFI tends to be radiated directly by its source. In between, and unfortunately stretching across LF, MF and HF, you get a nasty mixture of conducted and radiated EMI. Often, different sources connect to wires by near field coupling but this is a short range phenomena. Radiated and conducted RFI can spread hundreds of meters from the source.

Pretty much all of our broadband RFI starts off being conducted by power and signal cables. You will find it riding along the outside of coaxial transmission lines. Even gets carried along the ground. EMI running along power lines or common-mode along coax tends to get radiated.

When current flows in a conductor, EM fields are created. When conductors are more than perhaps 10% of a wavelength at the EMI frequency, conductors become pretty good antennas, creating EM waves. More on that later.

Broadband RFI Spectrum – Sharp Transitions Breed Harmonics

Take a look at the difference between harmonic content from sine versus square waves. In the picture below, you will see the output from two 100 kHz signal generators. The sine wave is in red, and the square wave in green.

broadband rfi spectrum

The oscillator at 100 kHz is pretty typical of a SMPS. You can see how the green square wave harmonics, with their 1 ns rise and fall times, are generated all the way through HF and are maybe 80 dB stronger than from sine waves.

The EMI generated by the green FFT is typically generated by every switched mode power supply. The only question is how well the manufacturer filtered it out of the AC line and DC power cables.

Using your SDR, you can measure the base frequency of SMPS by the gap between harmonics on the spectrum display, divided by two as the offenders are 3rd multiples.

And, remember, most of the broadband RFI is spread by a combination of conduction along cables/wires, and then radiation from those cables/wires when they act as antennas.

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