The IEEE 802.3ab defines the new Gigabit Ethernet Standard (i.e. 1000Base-T). This Local Area Network (LAN) can use 4 twisted pairs of unshielded copper cable (Category-5) at 100m maximum operating at 250Mb/s per pair in full-duplex bi-directional mode. This standard is designed to utilize existing LAN cables such as older 10Base-T and 100Base-T networks. The signaling (i.e. baud) rate is 125MHz per pair using 5-level Pulse Amplitude Modulation (PAM) . This probably results in the fundmental energy at 62.5MHz given that the signal must be bipolar to be compatible with transformers. Vendors such as Broadcom Corp. have developed single chip copper cable interfaces for this new standard.
In the past, standards using unshielded cables, such as 100Base-T, 10Base-T, T1, E1, etc, have had signaling rates less than 30MHz, so that the fundamental frequency was below the 30MHz FCC and CISPR starting frequency for radiated emissions. However, 1000Base-T has a 125MHz signaling rate. A common mode current of less than 10uA at 30MHz at 1/2 of wavelength can yield an emission level equal to or greater than the Class B level. In addition, the 4 twisted pairs are all phase locked to each other. The intentional differential mode current is about 10mA, so the trans hybrid balance needs to be better than 60dB. This is achievable with the hybrid at the component level, but not at the system level with 100m of Cat 5 cable attached. Conclusion, I think that 1000Base-T (IEEE 802.3ab) on unshielded Category 5 cable is doomed to fail EMI. Anybody ever try a test? Any other opinions? Don Kimball --------- This message is coming from the emc-pstc discussion list. To cancel your subscription, send mail to majord...@ieee.org with the single line: "unsubscribe emc-pstc" (without the quotes). For help, send mail to ed.pr...@cubic.com, j...@gwmail.monarch.com, ri...@sdd.hp.com, or roger.volgst...@compaq.com (the list administrators).
