I ran into that exact problem (a floating power source) a few years ago. The conducted emissions test results on a power supply at an outside lab were 4-5dB higher than our precompliance bench in-house. I sent the LISN's back to the manufacturer for performance verification, and they were fine. I took the spectrum analyzer to the outside lab to compare its performance to their equipment, and the results matched.
I finally realized the only difference between the test setups was the source of power: labs fed their LISNs directly from a wall outlet (with grounded neutral), while we fed ours from a variac followed by an isolation tranformer. Out of frustration, I connected our LISNs directly to a wall outlet. The conducted emissions readings finally agreed! It turns out the filter in the switching power supply had differing values of EMI capacitors from primary to chassis (after the rectifier), and using a grounded source at the LISN input degraded its performance. ---- Patrick Lawler [email protected] On Thu, 29 Nov 2001 17:43:35 +0000, John Woodgate <[email protected]> wrote: >I read in !emc-pstc that Peters, Michael <[email protected]> wrote >(in <[email protected]>) >about 'LISN Calibration', on Thu, 29 Nov 2001: >>If you have not already done so, I would recommend an isolation transformer. >>Many factories and office buildings have a great deal of ambient noise that >>will interfere with your measurements. > >That could be a bit problematic. The LISN was designed to work with >normal mains supplies, in which the neutral is earthed or earthy. You >may not get the same results with a floating supply. > >My LISN has a mains filter on the incoming supply. CISPR16-1 mentions >that this may be necessary and, by implication, allows it. ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: [email protected] with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Michael Garretson: [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: No longer online until our new server is brought online and the old messages are imported into the new server.
