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Power System Interference Spectrum

power system interference spectrum

Take time out to understand the source and complexity of power system interference spectrum. My video explains where a lot of RFI comes from around your house.

Did you ever wonder why tiny electromagnetic signals at very low frequencies can cause radio frequency interference across all of shortwave, and beyond? And why does power system RFI have that distinctive 120 Hz buzz, even at 12 to 30 MHz? Let’s answer these questions.

It all starts with an impulse, or a very short spark of electric current. You may not know this, but an ideal impulse has three interesting properties. First, it has instantaneous rise and fall times. Second, you will find an ideal impulse has infinitely small width and extreme amplitude. Third and most interesting, you ideal impulse blankets the entire frequency domain equally.

Now, I have to admit that ideal impulses don’t really exist. But I can tell you about lots of impulses that come close. These include lightning strokes, power line sparking, and switching power supply components. You will discover these have really fast impulses at 60 μsec for lightning, and 2-5 nanoseconds for the others.

These impulses spread radio interference over wide frequencies. Typically, they are triggered twice power system cycle at 120 Hz. You get a mixing of sparking impulses modulated every 8.3 milliseconds or 120 Hz, as shown above.

Switch mode power supplies also oscillate at higher frequencies (50 kHz to 1 MHz) creating current spikes from fast switching FET, adding further to the complex confusion we call RFI.

Power System Interference Spectrum Visualized

If you hear strange buzzes across HF, you can easily visualize what’s happening. Just tune in your buzz in AM mode and hook the audio into a sound card oscilloscope. You will be able to see these multiple impulses (rapid current changes) modulated by power system frequencies.

If you switch between time and frequency domain, you will also see the spectrum of 120 Hz and higher harmonics of RFI.

Recently I posted a video which demonstrates and explains these phenomena in great detail. I also explain why noise blankers, so effective on ignition noise, really cannot work on more complex RFI from power supplies.

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