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Author Topic: TIP! Home-Made EMI Probe  (Read 4799 times)

Posts: 2575

« on: December 29, 2013, 05:23:40 AM »

Here is a TIP on how to make your own short-range EMI probe to identify and isolate the source of low frequency interference.

Find an old tape player and remove the "play" head. This will be a tiny cube, usually in the center of where the tape runs across the captains. There will be some number of wires connected to the back of this cube. If there are more than two wires you need to identify what wires are for "play". The tape head may be a stereo head that can receive more than one audio channel at a time.

Looking at the face of a tape head there should be a bar or bars on it. If there is more than one bar then that tape head may be stereo or multi-track (I have seen one that recorded eight tracks at a time).

If there are multiple tape heads then there may be different ones for play, record and erase. You only want the play head. I cannot give you a good tip to identify which head is for play.

Wire up the tape head to the microphone input on your laptop computer or even to a small audio amplifier that has good input sensitivity.

This is a good probe for finding a source of interference a few inches away from the probe. If you have a bunch of cables in a bundle you can just touch the probe to a selected cable to find the one with the most EMI interference.

Once you identify a problem cable then you can make a corrective action like adding capacitors, filter or ferrites and then go back and measure the difference to get a relative comparison of how your fix worked.

The reason I use a computer is that many sources of EMI interference may be "supersonic", meaning they are above the frequency range of the human ear. On my computer I use "Spectrum Lab" by DL4YHF so I can see the interference on a waterfall display. This works even if it is above the range of my hearing. Usually a tape head is good up to a few tens of KHz. Unless you are a bat you are not going to hear frequencies that high!

Once you make one of these and get a chance to poke around cables with it then it will become self-evident of how this device works. It is sensitive enough that if you held the probe to the face of a digital watch you would be able to hear the oscillator buzzing away inside of the watch.

If using this around bare wires or connectors you will want to put insulation on the tape head so it does not come in contact with anything conductive.

Have Fun,
Tisha, AA4HA

Ms. Tisha Hayes, AA4HA
Lookout Mountain, Alabama
Free space loss (dB) = 32.4 + 20 × log10d + 20 × log10 f
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