Why do laptops’ chargers have coaxial cords?

  IEC61000-4-6 is the EMC spec for conducted RF testing (immunity testing via injection) and it clearly states that all “ports” should be tested. “All ports” I take to mean the following: –

  Power AC input

  DC output to “whatever”

  Any other port such as USB, audio, external control, really anything

  No distinction is made between screened and unscreened cable. Life is hard for the unscreened! For a laptop charger, I don’t see any way of not testing the DC power lead because it is sold as a separate product in its own right. An outer earth screen on the cable will make it a lot more resilient to the transients they can inject in the testing.

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  Shown above are typical examples of how coupling is made to cables – the cable is placed in the box and the lid closed. Inside the box (voltage injection), and surrounding the cable is a conducting hinged plate that touches the outside of cable thus capacitively coupling itself to the cores of the cable. Other types use current injection and therefore have split ferrites to make surrounding the cable relatively easy – this is called current injection.

  What do they inject? This is a good document to explain what is done but in short: –

  Continuous conducted RF immunity testing involves injecting RF

  voltages or currents into each of the cables associated with the

  equipment under test (EUT). The purpose of the test is to simulate

  the proximity of the EUT and its connected cables to radio

  transmitters and RF manufacturing equipment operating at low

  frequencies. These frequencies are not easy to test using the radiated

  RF immunity techniques described in 2. It is hard to generate

  uniform fields in typical test facilities at frequencies much below

  80MHz, but for typical sizes of apparatus the immunity problems at

  frequencies below 80MHz are normally associated with cable coupling,

  so conducted testing of the cables is seen as a reasonable alternative

  to radiated methods at such frequencies.

  A good document that has helped me is this from Laplace Instruments.

  So, do you use a screened cable whose outer screen is earthed or do you trust to probably more expensive solutions at either end of the cable?