radiated emission from power cable
i have radiated emission problem in one of our product. the 3th harmonic of the clock frequency radiate from the power cable. the cable is shielded with foil and braid.
when we test it in EMC lab we used 15 meter long power cable, and because it was too long, the lab coiled it (like a loop with), with diameter of 15 cm or so, and we passed CISPR 11 limit. now when we use shorter 1.5 cable with straight orientation, the radiation is 20dB above what we measured with the long coiled cable. any one know why we have such a different radiation when the length and configuration of the cable change? the 3th harmonic of the clock frequency is 120 MHz, where we have radiated emission problem.
At 120 MHz, the wavelength is 2.5 meters. The 15 meter cable is six wavelengths long. I would expect the longer cable to radiate more. However, you coiled it which acts like a big inductor or choke to keep that from radiating. I would expect if you uncoiled that loop, it would increase in emissions.
I have also seen test cables built a lot better than production cables. It could also be how they grounded the shields at the connectors. A pigtail ground would really increase the radiation.
I guess your shorter cable, in combination with height above ground plane and LISN termination is a more or less perfect radiator for 120 MHz. How is the shielding of the cable terminated at the DUT's end and at the LISN ? Is the shield open at the LISN, the shield itself may become a resonator.
And, as Tom Fagan wrote, a pigtail grounding of the shield may act as a good injection point, especially if connected to signal GND on a PCB inside DUT.
The shielding shall be a extension of the faradays cage that encloses the DUT, connected to the shell of the shielded connector.
I find it often a lot easier to filter the RF away within the DUT than try to prevent the cable from radiating.
120 MHz is not that hard to kill with a CM + DM filter. Got a current probe ? Make a breakout cable and measure the RF current in the cables at the DUTs end, measure DM and CM and try to figure out what filter type you need.
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