FreeLists / si-list / [SI-LIST] Re: Ground plane voids under Tip/Ring

[Bob Patel <whizplayer@xxxxxxxxx>]

Hi! From all the responses I have received so far, I conclude: 
1) The interface from magnetics to connector need to be void of ground and 
power planes. 
2)Refill this void portion with chassis ground which is etched away from the 
board ground plane by at least 40mils. 
Question: 
This chassis ground which has to be etched, can I etch it on all the planes or 
should I just use TOP and Bottom layer of the board for this chassis ground. 

Thanks

Bob
--- Chris Padilla <cpad@xxxxxxxxx> wrote: 
> 
> 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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