Computing stuff tied to the physical world

Noisy workbench!

In Hardware on Dec 9, 2011 at 00:01

It never occurred to me that RFI would be an issue, but it sure is. There’s lots of electromagnetic interference around my workbench here at JeeLabs. And it’s messing up my scope experiments – here’s a baseline:


That’s the signal, picked up by an unconnected probe lying on the table. The pane at the top is the raw signal, synchronized to the AC mains line frequency. The pane at the bottom is a Fast Fourier Transform – which has always fascinated me as a kid (yeah, I was born a geek) – the X axis is frequency instead of time. This analyzes a signal and decomposes it into a set of sine waves (every periodic signal can be treated as a sum of sine waves).

The peak at the left is “zero”, or rather, 50 Hz. There’s a weak peak at 128 KHz and a surprisingly strong peak at 675 KHz (update: probably a nearby 100 kW AM transmitter). That’s the noise the probe is picking up.

This was with my LED light strip powered from the (linear) 12V lab power supply. With a switched 12V supply, still powered on 100%, i.e. no PWM, I get a couple of extra spikes:


Looks like this SMPS is switching at about 55 KHz. So I think I’m going to drive these LEDs with a conventional linear power supply – unregulated, to keep losses low.

I’m averaging the FFT results to get a clean readout. This is possible because the signals I’m after are repetitive and constantly present. Now let’s change the scale a bit and look again at the probe’s pickup with no lights on:


Pretty clean – but watch what happens with the little fluorescent light of the magnifying glass I use for soldering:


It’s a pretty hefty 30 KHz transmitter! (the strong odd harmonics indicate a square wave, i.e. on/off switching)

The moral of this story: if you want to measure weak signals, clean up your desk – electromagnetically, that is!

  1. It’s your neighbour radio maria

    • Oh, wow – a 100 kW AM transmitter @ 675 KHz, 7 km away. Never thought of it – thanks!

      And maybe the 1.58 MHz peak is “Radio Paradijs”, 1 kW and 3..4 km away…

  2. I know what you are talking about. When playing with DCF77 receiver, I had to move the kit at least 2m away of my desk, to see at least some DCF77 pulses. I was too lazy to bring my oscilloscope home, though, so I have no idea what was causing the problem.

  3. An unconnected probe has a very high impedance, most likely 1000 or more that of the circuit you are usually measuring. Try connecting a 10kOhm from the tip of the probe to ground and then see what is left of the noise.

    • Yes, I know – but I was demonstrating that you can see “what’s in the air” that way.

  4. hmm, I remember there was a chap in england way back who lived within the near field of a bbc lw transmitter and he was able to heat his glasshouses by coupling into the field with a large coil. they couldn’t do anything at the time but he made the mistake of not renewing his licence on time one year and they went in the next day and confiscated everything. so maybe you don’t even need a battery for your jeenode where you live.

    • Heh – the thought had crossed my mind. I have no idea what sort of energy levels one can get from this, but it’d be worth trying one day. All it needs, I suppose, is an L-C pair tuned to 675 KHz, and an efficient rectifier. I know very little about such circuits, for example how to get a nice high voltage out of them. It’d be neat if this works here at JeeLabs, even if not generally useful.

    • The battery-less JeeNode, the modern version of the crystal set :-)

      I do get amusement from travelling through Crystal Palace in South London, they have an enormous TV transmitter mast there. I’m so used to seeing TV antennas pointing horizontal, it’s rather amusing to see them all pointing up at 20 degrees, and the angle changing noticeably as you move down the road.

      Most be a doddle of a job being a TV antenna installer round there, you can line them up by eye!

  5. Putting RF energy where you do want it: You probably need an efficient coil to boost the voltage so that a conventional rectifier’s forward drop doesn’t waste much.

    Keeping RF out of where you don’t want it: Ferrite is your friend. :)

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