When probing a digital circuit, it is best to avoid long wires as may they form accidental antennas. I had a run in with this one when I quickly wanted to check a toggling signal on an Intel Edison. I connected it as below:

 

The waveform I expected:

correct

But what I got:

incorrect

I then pulled out my hackrf and checked the frequencies and found emissions every 20MHz @20, 40, 60… MHz as below:

screenshot-from-2017-01-30-22-05-17

At 60MHz

screenshot-from-2017-01-30-22-12-32

At 80MHz

screenshot-from-2017-01-30-22-13-27

At 120 MHz and a bit at 100MHz though not as much.

screenshot-from-2017-01-30-22-18-02

It then became very noticeable in the 400MHz range and greatest at 440MHz.

When I removed the offending wire, the RF noise stopped as below:

screenshot-from-2017-01-30-22-31-56

whatsapp-image-2017-01-30-at-22-51-28

And a nice clean signal emerged

correct

The offending wire’s length was 33.5cm, I then used an amateur vertical antenna calculator tool to calculate the length for the emission frequency of 440MHz and check if it would match and for sure closely did correspond at 32.4cm.

screenshot-from-2017-01-30-23-41-41

4 thoughts on “Minimising RF emmisions during circuit probing

  1. It is an antenna, but that is irrelevant. You should start by understanding the Fourier series of a square wave, in particular what defines the highest frequency present in a digital signal. Then you should learn about transmission lines, and how the capacitance of a probe tip and inductance of a ground lead will affect the displayed signal.
    See
    http://www.sigcon.com/Pubs/straight/probes.htm
    and
    https://entertaininghacks.wordpress.com/2015/04/23/scope-probe-accessory-improves-signal-fidelity/

    Liked by 1 person

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