Dear Patrick, All cables exiting the product should have 150 Ohm common mode terminations. Otherwise, as you pointed out, there is no current path. I suppose an EUT which is a single lump at the end of an AC cable with NO exiting cables could be tested as you described, but the lab should still have set up the AC cable with a 150 Ohm common mode impedance.
Properly performed, the product is set up 10 cm above a ground plane on an insulating support. The lab identifies 2-5 cables exiting the product. Other cables are removed or high common mode impedance terminated with ferrite sleeves. Each of the unshielded cables should be terminated with a CDN (coupling device network). Shielded cables have their shields terminated to the ground plane with 150 OHms. Any grounding studs or connections are also terminated to the ground plane with 150 Ohms (don't connect them directly!). Then each cable in turn is used as the injecting point. If you are injecting directly, the termination is replaced by the signal source. If you are using a current probe to inject, the termination remains in place. Because of the slow sweep time required (.0015 decades/second) the test takes 40 minutes per cable, thus the standard allows us to limit the stimulation to 2-5 cables on a particular EUT. For coupling clamps it is particularly important that the common mode impedance be set up as 150 Ohms on BOTH the cable away from the coupling clamp and the EUT and on the other cables exiting the EUT. Otherwise, currents other than those developed in the calibration setup could flow. Sometimes it is difficult because of high cable wire count or high speed signal content to provide the termination on unshielded cables. The standard currently doesn't provide a solution in all cases. I have taken to wrapping the cable in one meter of aluminum foil and terminating the foil wrap to the ground plane with 150 Ohms in these cases. I know that at the lower frequencies this much foil doesn't have enough capacitance to provide a 150 Ohm termination, but I feel it's better than not doing anything. Anybody else have any bright ideas? Jon D. Curtis, PE Curtis-Straus LLC [email protected] One-Stop Laboratory for EMC, Product Safety and Telecom 527 Great Road voice (508) 486-8880 Littleton, MA 01460 fax (508) 486-8828 http://world.std.com/~csweb On Sat, 1 Nov 1997, Patrick Lawler wrote: > I recently had power supplies tested for conducted immunity per > IEC1000-4-6, 'Conducted Immunity to RF Fields'. This test calls for > injecting an RF signal in the range of 0.15-80MHz on the AC input cord > to see how the system responds. > > The power supplies had resistive loads attached, and were > approximately 24" above the metal floor. There were no other leads or > additional grounding wires attached to the unit. > The technician then clamped a current transformer around the _entire_ > AC power cord (line, neutral, and ground), and performed the test. > The power supply passed without problem. > > Although I've never seen this test before, it appears that there would > be no EMC stress on the power supply at all. The current transformer > would simply be trying to induce a current into an open circuit. > > When I questioned the lab manager, he said it was a common-mode test. > I asked him to identify the path the induced current was flowing in, > but he couldn't. > > Furthermore, the power supply was fed from a simple EMI filter with a > low RF impedance. Should a controlled impedance source (LISN) have > been used? > > Was this test performed correctly? > > -- > Patrick Lawler > [email protected] >
