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From: Bailin Ma <[email protected]> Subject: RE: Grounding of screen room List-Post: [email protected] Date: Wed, 7 Oct 98 9:18:11 PDT To: [email protected], [email protected] Cc: Mekonen Buzuayene <[email protected]>, Mekonen Buzuayene <[email protected]> Hi Ed, Yes, I agree. The way you suggested is an ideal method to get Ambient noise in the Conducted emission test. Please allow me to make two more points about impedance vs. frequency: (1) It is inconvenient in practice to find "a resistive load sized to draw the same amount of power." We have to make the equivalent load first, connect it to a power cord, and then plug the power cord into EUT port of the LISN. If we consider the equivalent load should be not only at 60 Hz, also "equivalent" on 150 KHz - 30 MHz to the EUT, it would become much more troublesome. As a matter of fact, the frequency range of 150 KHz - 30 MHz is of our interest. (2) From the standpoint of spectrum analyzer, does it make lot of difference between using open circuit and equivalent load on EUT port of the LISN to detect the Ambient noise on 150 KHz - 30 MHz? Please correct me. Best Regards, Barry Ma ------------- (Original text snipped as it was getting too ragged) Barry: You don't have to provide a dummy load which is the precise analog of the equipment to be tested. All you want to do is draw some fundamental power. Since I do Mil testing, I see power busses of 28VDC, 120/208 60Hz and 120/208 400Hz. To keep it simple, I constructed a load bank consisting of 16 surface mount light bulb sockets, all wired in parallel. I just screw in an array of 25/60/75/100 Watt rated lamps until I get the necessary current. Sure, there's some unknown slight lead inductances and capacitances. But all I want to do is draw a few amps DC, and this setup works pretty well. Ideally, it shouldn't make any difference between no-load and full-load conditions on the LISN. Your power source should be clean and quiet, your shielded enclosure powerline filters should be very effective, and your LISN also provides some filtering. But, in the real world, your power source may be very noisy, with additional RF environment currents induced onto the powerlines. Some people may try to measure CE without a shielded enclosure; that should only be as a last resort. If you bought cheap powerline filters, or if you built them yourself, you can have two types of problems. First, your inductor design may not handle the DC current without going into magnetic saturation. The decreased incremental permeability will result in a dramatic drop in inductance, which will decrease the filter's filtering ability. Second, the inductor may have been physically damaged during filter assembly. I have seen cracked toroids and poorly stacked laminated cores which have resulted in the filter creating harmonic energy content that wasn't in the original AC power. I suppose that what I'm saying is that if you are creating a new measurement setup for each job you work on, I would measure the Ambient CE each time I do a CE test. I would be suspicious about the power source, filtering, etc. But, if you have a stable test setup, to which you bring product after product, I would suggest that you measure the ambient CE only periodically, maybe once a week, or at some other comfortable interval. Of course, this advice doesn't pertain to those doing Mil-Std-461/462D testing, since that spec REQUIRES an Ambient at the time of each emission test. Regards, Ed -------------------------- Ed Price [email protected] Electromagnetic Compatibility Lab Cubic Defense Systems San Diego, CA. USA 619-505-2780 List-Post: [email protected] Date: 10/07/98 Time: 12:18:32 -------------------------- --------- This message is coming from the emc-pstc discussion list. To cancel your subscription, send mail to [email protected] with the single line: "unsubscribe emc-pstc" (without the quotes). For help, send mail to [email protected], [email protected], [email protected], or [email protected] (the list administrators).
