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RF probes for measuring spurious emissions from 30 MHz to 18 GHz.

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Apr 17, 2015
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I am trying to find the source of certain radiated spurious emissions from a device. I am looking for some sharp edge probes if available which can easily help me find the source point accurately. The frequency range I am interested is 30 MHz to 18 GHz. Any suggestions on this?


I am afraid that you won't be able to isolate the problem on a device using probe ...
Moreover, the frequency range from MHz to GHz - thats too much.
Better way is to carry out analysis and isolate the issue on the device.

Miniature electrical and magnetical probes are available from various manufacturers. The idea of finding a source of radiated emission with a "sharp edge probe" involves however a simplistic understanding of EM phenomena, I fear. It probably works at low frequencies where relevant device dimensions are far below wavelength. At least above a few 100 MHz, emissions are radiated by the distributed structures, e.g. a ground plane or case parts. The field maxima may be observed at other locations than the primary source.

  • Spurious emissions are calibrated with a special antenna such as dodecahedral dipole at 10m in an open field.
  • Although you can locate spurious sources using any Spectrum Analyzer and a stub antenna ( short piece of wire ) and realize that the gain is proportional to length up to 1/10λ and use a 1/2λ length at the observed frequency. Usually LO leakage is dominant or any fast rise time pulses on SMPS or clocks on long cables.
  • Near field levels will be much stronger and easier to locate by waving loop or stub over the interfering unit under test.
  • Ambient noise can be stored in memory for nulling (A-B) in digital RF analyzers.
  • Faraday Cages (e.g. Lingren) work better for ambient suppression but then interfere from standing waves.
  • Determine if it is radiated out of slots or gaps or transmission lines.
  • To get rough absolute levels, use an RF generator and similar stub on coax end parallel to Rx stub on SA for comparison at a known distance and path loss.
  • A small loop or stub works well as hemispherical omni-directional antenna with a null spot looking at the end.
  • Get a unit that passes and compare.

We used to do this by just waving a bare 50 Ohm semi rigid cable and our spectrum analyzer. We were only doing up to 3GHz though.

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