Hello, I finally passed rf conducted immunity on my telephone cards! As it turned out, not only the telephone card using the Legerity 79R70 was demodulating but also, the card towards the network using a Mitel/Zarlink MH88437AD-P.
To each card I added a common mode choke (TDK ZJYS51R5-2PB), two caps from tip and ring to ground, relayout of the pcbs to reduce loop areas and trace lengths, and added ground plane and power plane layer on the telephone card, but we didn't do the network card relayout yet. The network card with no ground plane was sensitive in the higher frequencies. I am sure with a new pcb with ground and power planes we can pass with a much better margin. I took old telephone card and modified it the same way, but without the ground plane it still failing. The network card however did pass without the ground plane. I tried doing each modification one by one to see the impact and these are my conclusions, which concur with what everyone is saying. At low frequencies caps are most effective, at higher frequencies the common mode choke is most effective. Trace lengths to auxiallary components should be as short as possible with no loop areas. Ground planes help at higher frequencies. Caps to ground can not be greater than .022 uF for FCC impedance to ground during ringing limit of 100k ohms, it will still pass FCC leakage current test at 6.8 mV @ 1000V. Have I missed anything about impact of caps to ground for European requirements? Alot of happy people around here, especially me! Thanks to all for your help. Best regards, David. David Gelfand Regulatory Approvals Memotec Communications Inc. ------------------------------------------- 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: http://www.rcic.com/ click on "Virtual Conference Hall,"
