Peter, After first making sure the RES BW and VIDEO BW are 9-10 kHz throughout the range (more than once the BW on my analyzer has been higher than what I set it to because I increased sweep or span and had BW in the AUTO setting), you might consider the following:
Look for broken cables in the chain. Try making the measurements inside the screen room using a known "good" short cable. If the readings are low the way you expect them to be, there may be an open shield or bad connection in one of the cable/cable adapters that gets the LISN signal to outside the screen room. If you connect outside cable via a feedthrough BNC or N connector mounted on the screen room wall, make sure the feedthrough is not the kind that has an insulating washer between shell and where the mounting nut screws on. When the analyzer is outside the screen room and connected to the LISN signal, try connecting the instrument ground to the outside wall of the screen room using a 20-50 cm length of wide braid. I remember this being helpful once in the 10kHz - 150 kHz region, can't remember if it helped at higher frequencies but it's easy to try. Your LISN and analyzer may be on entirely different circuits at different ground potentials and this test should let you know. If your screen room supply is not filtered and your AC line is noisy you should expect low frequency emissions. Install a commercial sample line filter between inside screen room mains and LISN AC feed, making sure you place filter can on metal surface of wall or floor and that you use short leads (<.5m) between wall and filter and LISN and filter. If the readings improve you should consider getting a screen room filter. If you have a screen room filter and the inside filter helps the situation, your screen room filter may need service or replacement - but first check all connections for looseness or wear. Do you use an in-line spike or surge protector at the analyzer input? If so, try making your measurement without it - the diodes fail in these devices and as I recall we had a noise problem in the past with one of these failures. That's all I can remember about similar problems in various labs over the years. Hope it helps, and please share the solution when you find it. Best regards Tom Cokenias EMC and RF Approvals Consultant >Hello group, > >I recently carried out a conducted emission in our unlined screen room with no >EUT connected. I.e just the LISN inside and measuring receiver outside the >room. I was expecting the plot to be a horizontal line and very low perhaps in >the region of 10 or 20dB. Instead I noticed that the plot starts at about 40dB >at 150kHzand decreases to about 20dB at around 1MHz and then after that is a >horizontal line all the way to 30MHz. Bearing in mind that the room is very >well grounded and I use a reputable measuring device. My questions are: > >1-Is this what one would expect to see? >2-Why the noise floor so high in an screen room? >3-What can we do to reduce the noise floor? >4-What is the best method to ensure that the room is O.K? > >Any help and advice is appreciated. > >With regards >Peter > >E-mail: peterh...@aol.com > >--------- >This message is coming from the emc-pstc discussion list. >To cancel your subscription, send mail to majord...@ieee.org >with the single line: "unsubscribe emc-pstc" (without the >quotes). For help, send mail to ed.pr...@cubic.com, >j...@gwmail.monarch.com, ri...@sdd.hp.com, or >roger.volgst...@compaq.com (the list administrators). --------- This message is coming from the emc-pstc discussion list. To cancel your subscription, send mail to majord...@ieee.org with the single line: "unsubscribe emc-pstc" (without the quotes). For help, send mail to ed.pr...@cubic.com, j...@gwmail.monarch.com, ri...@sdd.hp.com, or roger.volgst...@compaq.com (the list administrators).
