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:
But what I got:
I then pulled out my hackrf and checked the frequencies and found emissions every 20MHz @20, 40, 60… MHz as below:
At 120 MHz and a bit at 100MHz though not as much.
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:
And a nice clean signal emerged
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.