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I would need to use a 15 meters BNC Cable (RG58) in order to measure an audio signal that goes directly to a pair of earbuds. This means, the test point would be exactly before the balanced armature speaker located in the earpiece. The idea is that I would leave a BNC jack in the DUT a connect this directly to an oscilloscope. I need this setup in order to check signal integrity during EMC Testing and we would prefer to place the oscilloscope outside the chamber.

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    \$\begingroup\$ A bigger question would be, would it be OK to do EMC testing with an extra, "non standard for operation of the unit", BNC cable attached to the device under test, exiting the room. Answer to that one would be. No! \$\endgroup\$ – Trevor_G Oct 26 '17 at 14:29
  • \$\begingroup\$ Oh, I did not know that. We thought that as a way to check the integrity of the audio while the Radiated Immunity test takes place and we also thought that it was better to leave the oscilloscope out of the chamber. Should we place the oscilloscope inside the chamber then? \$\endgroup\$ – the_moon Oct 26 '17 at 14:35
  • \$\begingroup\$ For immunity testing you may be ok with the scope in there. But you need to remember, the scope leads add a path for the radiation to enter your system. For radiated from your unit, not so much. They can subtract whatever the scope is radiating, but it's cleaner without it. But you should check with the LAB/tester to check what they allow. \$\endgroup\$ – Trevor_G Oct 26 '17 at 14:54
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    \$\begingroup\$ I think perhaps you are trying to kill 2 stones with one test. EMI testing is to ensure your device does not croak while being radiated or emit so much noise as to interfere with other equipment. The testing has nothing to do with how well the thing works. If the earpiece buzzes like crazy, that won't stop it from passing the radiation test as long as it continues to function. \$\endgroup\$ – Trevor_G Oct 26 '17 at 14:58
  • \$\begingroup\$ For radiated emissions, we will not monitor anything with the oscilloscope. We wanted to be able to check the sounds when radiation is entering the DUT. Would there be even a path if we use just BNC plugs and jacks? That´s why we decided to use a BNC Cable instead of the regular probes. \$\endgroup\$ – the_moon Oct 26 '17 at 15:06
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For audio frequencies it would most likely be fine. You can see the attenuation from this chart - it would probably be less than a dB: http://www.w4rp.com/ref/coax.html

What about trying an audio recorder? You could record the audio and playback the audio later into an oscilloscope.

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  • \$\begingroup\$ Thanks for the chart. We thought that the microphone might be affected by the testing during Immunity testing. That's also why we think about leaving the oscilloscope outside the chamber \$\endgroup\$ – the_moon Oct 30 '17 at 15:01

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