On 10-Apr-04 07:44:05 Kbalasubramanian wrote: >>... I do not have a high precision current probe, I used 2 digital multi-meters one at the d.c +ve line & the other at the d.c. ?ve line of the EUT to measure the current and the current is found to be exactly equal in magnitude in both the wires. Same EUT when directly connected to the laptop computer without the cable, the emission @ 72 MHz is below the floor level noise of the analyzer. Is it still a common mode problem? <<
It definitely sounds like a common mode problem *on the 1 meter cable*, which is a good enough antenna at that frequency, with equipment connected to it, to do the job. As for finding sum and difference currents, your digital meters are not up to the task. First, they need to be RF meters able to measure at 72 MHz, where the problem is. Second, the power radiated is very low and even if you had the right voltmeter, and it didn't upset the cable balance simply by connecting it, you might still miss a difference of less than .01 volt -- more than enough enough to create the signal you see. At RF, one usually finds common mode current using a suitable current probe and a receiver or spectrum analyzer. Have you tried this? You may have assumed that because the laptop's main processor is stopped, no oscillators are running in it and nothing is running. But if the suspend/resume status is being polled, SOME oscillator has to run in order for the command to resume processing to seen and acted upon. It would be worthwhile to probe the equipment on the table to find the source of that 72 MHz emission. I suspect you'll find it is the computer. If it is, you'll need to swap computers. If not... you need to fix the EUT. However, things are often not that simple; an emission is often carried from one piece of equipment into another over a shielded cable, there to be radiated, either >from the "innocent" equipment, or its power or peripheral cables. Good luck! Cortland This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ieee-pses.org/ To cancel your subscription, send mail to: [email protected] with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: [email protected] Dave Heald: [email protected] For policy questions, send mail to: Richard Nute: [email protected] Jim Bacher: [email protected] All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc
