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. The second problem is that you may induce an EMI/EMC susceptibility problem. During immunity testing, the product is subjected to intentional radiation (in Volts/Meter) to induce failures. The severity of the immunity testing may range from 3 Volts/Meter to 1000's of Volts/Meter depending on what standard you must meet. The frequency sweep range and dwell time is also a function of the test standard (may be from 10 KHz to 40 GHz). Your cable connected to the E1, T1, etc, port will see this as common mode noise again. If the common mode noise is referenced to a digital (or sensitive analog reference plane), that common mode noise will be impressed on that plane(s). The noise is always trying to return to the source (it tries to get there from ground), and will distribute over your PWB planes until it finds it through the path of least inductance. It may (and very likely will), pass along several sensitive digital chips with PLLs, clocks, analog mixers, modulators, demodulators, etc, and cause problems. If the planes are voided then there is much less likelihood of the common mode noise coupling to the planes and causing the susceptibility problems. Using Shielded cabling helps with both of these problems because you contain the radiated emissions from the board and provide a local preferred return path (the short shield connection to chassis ground) to minimize the loop inductance. A third reason is that some regulator agencies (FCC, PTT, etc) require them to be voided and for all exposed copper to be voided by a certain distance away from the traces. This is to prevent noise coupling to the PBX and phone network and improve immunity. There was an application note from Dallas Semiconductor a few years ago the gave that distance. It is always better to prevent problems than fix them later. At 3:00 A.M., the lab creates a humble engineer, submissive to a great teacher - experience. 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 __________________________________________________ Do You Yahoo!? Send FREE video emails in Yahoo! Mail! http://promo.yahoo.com/videomail/ ------------------------------------------------------------------ To unsubscribe from si-list: si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field or to administer your membership from a web page, go to: //www.freelists.org/webpage/si-list For help: si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field List archives are viewable at: //www.freelists.org/archives/si-list or at our remote archives: http://groups.yahoo.com/group/si-list/messages Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu ------------------------------------------------------------------ To unsubscribe from si-list: si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field or to administer your membership from a web page, go to: //www.freelists.org/webpage/si-list For help: si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field List archives are viewable at: //www.freelists.org/archives/si-list or at our remote archives: http://groups.yahoo.com/group/si-list/messages Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu