Potential common mode currents on the ISDN S and T-interface caused by cable unbalance

Although the effect is ignored in emission measurements with impedance stabilization networks (ISNs), it´s well recognized that the unbalance of twisted pairs is an important factor in the conversion from a differential mode voltage into a common mode (cm) current and vice versa. Another factor to take into account is the common mode impedance of the cable, which is standardized to be 150 Ω in emission measurements with ISNs. Longitudinal conversion loss (LCL) and CM-impedance measurement results in situ quad telephone cables are presented to show that the cable unbalance can´t be ignored and that the CM-impedance can be much lower than 150 Ω. An expression is derived for the CM-current as a function of transmission signal, LCL and CM-impedance. The derived expression is used to calculate potential CM-currents for both 192 kbit/s (basic access) and 2 Mbit/s (primary rate) ISDN systems. The results are compared with the most rigid CM-current limits on signal ports proposed in CISPR/G