Computing stuff tied to the physical world

Unwanted RF

In Hardware on Mar 15, 2013 at 00:01

Houten is not a good place to be if you want to analyse signals in the 100 MHz range …

After looking into some switching regulator artefacts for a while, I’ve concluded that there is nothing wrong with my circuit after all. It’s all due to a nearby FM broadcasting tower!

(367 m high and about 10 km away: the Gerbrandytoren in Lopik – for the Dutch readers)

Getting rid of RF electromagnetic waves is hard. You need a Faraday cage, but you also have to prevent all unwanted RF from entering via measurement and power cables – and it has to work at all the frequencies you’re examining!

I doubt I’ll find a way to get rid of this, but the least I can do is to measure it:


Whoa – nice FM antenna!


Now let’s do the right thing and use a coax cable with internal 50 Ω termination in the scope turned on, and the input sensitivity increased to 1 mV per division:


Again, nice antenna. The “noise floor” is now 20 dB (100x) lower – a much cleaner signal:


And finally, just to make sure it’s really the loop that is picking up the signal, let’s replace that loop by a 50 Ω terminator on the input side as well:


Ah, now we’re getting to the bottom of this – a 30 .. 35 dB reduction:


Note that we’re still picking up very weak FM signals (no cable shielding is perfect). But it’s clear that the incoming signals are coming from antenna-type pick-up of electric fields, not some fault in the measuring setup, or that switcher I was investigating.

Let’s just hope I never have to really chase and analyse signals in this frequency range!

  1. Toren van lopik / gerbrandytoren is in IJsselstein. Not lopik actual…

  2. You’ll need a screened room for any serious RF investigations or a TEM cell.

  3. arduino DUE : 84Mhz clock…. when (if ?) the new generation chips gets a foothold, then that feared frequency range is getting closer.

  4. Can any oscilloscope look at the FM or AM band or do you need something else?

  5. Think so. For FM, the scope must have a bandwidth over 100 MHz, for AM, any sensitive one should be ok. oh, and you need built-in FFT to see the frequency spectrum.

    • Thanks, I have a Tektronix TDS 320 that does 100Mhz and 500MS/s but I don’t think my probe is rated for 100Mhz.

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