[SI-LIST] Re: Ground plane voids under Tip/Ring
- From: Chris Padilla <cpad@xxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Thu, 10 Jan 2002 07:45:18 -0800
Having worked quite a bit with 10/100/1000 ethernet, I've found that
voiding the planes (we are talking the secondary side of the isolation
transformers now: magnetics to connectors) and then refilling with "chassis
ground" planes to help quite a bit. In other words, one simply cuts all
the planes underneath the magnetics thus "isolating" those planes in the
"secondary to connector" region from the "primary to rest of the board"
section on the PCB. On my systems, this is often the entire breadth (~
front 1/4) of the board (48-port, high density stuff). However, it isn't
truly isolated as my boards are mounted to a metal tray which through the
stand-offs on the tray and the metal shield around the connectors, the gnd
and "chassis gnd" are more or less reconnected. The main goal here is to
block any noise developed from the planes and force what's left through the
isolation magnetics which usually have common-mode chokes,
autotransformers, and other filters that hopefully knock the common-mode
energy that still manages to get on the cables down as much as possible to
meet your spec limits.
However, this get way away from the original question which was correctly
answered. It is for safety reasons that the Tip/Ring require so much
spacing on a PCB. I forget the exact voltages the phone systems are on (48
or 60 V or so??) but these levels are considered HIGH by the safety
engineers and the must take preventative action. I've had many a safety
engineer simply shred up my planes to garnish spacing and this makes
controlling EMI all the more tougher in some cases.
----->Chris
>Bob,
>
>There are a few reasons for voiding the planes (signal reference planes)
>around E1, T1, 10-Base-T, 100 Base-T, etc.
>
>The first one is for EMI. These traces are generally high impedance 75, 100,
>122, 150 Ohms, and it is easy for digital planes noise to couple to the
>traces (after all, the reference plane is part of the impedance of the
>trace). This digital noise is coupled via the plane to the traces and exits
>your cabinet as common mode noise (not differential which could be cancelled
>out by the receiver). Most, but not all, of these interfaces are unshielded
>twisted pair cables. The common mode noise present on the cable pair becomes
>radiated emissions which may cause you to fail EMI/EMC compliance testing.
>If the planes are voided then there is much less likelihood of the common
>mode noise coupling to the traces and causing the raditated emissions.
>*<snip>*
>There probably other reasons for voiding the planes, if there are, I would
>be interested in your comments.
>
>
>I hope this helps.
>
>Philip Ross Wellington
>Mgr. Signal Integrity & EMI
>L-3 Communications CSW
>
>
>
>-----Original Message-----
>From: Bob Patel [mailto:whizplayer@xxxxxxxxx]
>Sent: Wednesday, January 09, 2002 5:55 PM
>To: si_list
>Subject: [SI-LIST] Ground plane voids under Tip/Ring
>
>
>
>Hi! I had a question regarding the design of T1, T3
>interfaces i.e. in all the designs the portion from
>the TIP & Ring(outside world interface) upto the
>primary side of transformer is void of any ground
>planes.
>Is this to meet any immunity, safety, EMC requirement?
>Thanks in advance
>Bob
>
>
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