I've built myself a crude EMI probe:

crude EMI probe

I then pointed it at the top part of my phone when the phone was powered on, and I saw the following waveforms on my scope.

First off, there are spikes every 525ms:

picture of scope with regular spikes

These spikes consist of exactly 6ms of some kind of waveform: rectangle made up of waves on scope, with a tiny gap approx. 9/10 through the rectagle

Zooming in on the gap shows the following short-long-short-long-long-long ...thing. Also interesting to note is that the gaps are exactly 3us.

picture of the (data?) waveform

And then if I zoom further in on the rectangular part of the wave, I see the following 13.55MHz sine wave:

13.55MHz sine wave and measurements

What's going on here?

  • 2
    \$\begingroup\$ What's the big deal? Cellphones emit electromagnetic radiation, scope probes also function as antennas (and any other piece of metal in the universe). What you are seeing is bandwidth limited cellphone packets. Interestingly enough mic's also pick up cellphone radiation. \$\endgroup\$
    – Voltage Spike
    Sep 26, 2019 at 2:15
  • \$\begingroup\$ I'm not saying this is a problem, just that I'd like to know what it is. 13.55MHz is clearly quite far away from the dangerous-for-pale-people 750THz rays that the sun puts out :) \$\endgroup\$
    – flaviut
    Sep 26, 2019 at 2:22
  • 6
    \$\begingroup\$ en.wikipedia.org/wiki/Near-field_communication \$\endgroup\$ Sep 26, 2019 at 2:33
  • 1
    \$\begingroup\$ Looks like digital code is being transmitted and received. This looks like OOK (on-off keying) digital modulation. \$\endgroup\$
    – user103380
    Sep 26, 2019 at 3:59

1 Answer 1


Your phone has NFC turned on so it is trying to read any NFC devices that might come into range.

13.56 MHz is a standard RFID frequency.

  • 2
    \$\begingroup\$ This is it! 13.56 MHz is a standard RFID frequency. \$\endgroup\$ Sep 26, 2019 at 7:47

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