Hi! I have a fast ethernet design that does not pass EMI from 30 to 100mhz and based on the scan it is common mode noise. In my design i have the phy connected to xfmr1--choke1---connector---I/O card---xfmr2----choke2---rj21---cables---patch panel---rj45. Now, if I use shielded cables from RJ21 to patch panel then I pass with lowest margin of 2db at 66mhz but want to pass with unshielded cables. Now, grounding scheme, our chasis has mesh ground scheme so we isolated the gnd from secondary of xfmr1 and this gnd is same upto the rj21 on i/o card. Xfmr2, choke2 and RJ21 is on I/O card. Since the differential mode currents are not cancelling out they give rise to common mode current which need path to gnd and since this gnd extends to RJ21 the I/o cables radiate. Because if we remove the patch panel and RJ21 then we are OK. Hence I was thinking to isolate the GND from motherboard and I/O card i.e. the gnd on I/O card should be connected to chassis gnd and the gnd on motherboard from secondary of xfmr1 to connector should be connected to DGND through a choke. Any ideas or suggestions with this approach. Any feedback will be appreciated. Thanks Bob
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