A common mode termination helps to reduce EMI emissions and susceptibility to EMI by properly terminating common mode energy that can exist on a twisted pair cable. This technique is used for unshielded twisted pair (UTP) which, due to lack of shielding, is susceptible to common mode noise.
The impedance of a common mode channel within the UTP can be calculated to be approximately 75 ohms and is connected between the primary of the transformer center tap and the capacitor. The other side of the capacitor is connected to chassis ground. This capacitor is rated at 2 kV to protect against high energy transients coupled on to the LAN cable system, TP-PMD Twisted Pair Physical Media Dependent (TP-PMD) specification establishes the voltage rating of the capacitor. This capacitor also improves circuit performance by providing a solid common mode reference and reducing common mode noise.
The 10/100Base-TX transmit transformer uses the center tap to access to common mode noise present on the cable. The 10/100Base-Tx receive transformer uses the center tap to attenuate common mode noise. Unused pairs 4-5 and 7-8 are tied together and connected to the same 2 kV capacitor through a 75 ohm resistor. This provides direct termination for potential common mode noise on the unused pairs. Because of cross talk within the cable, common mode noise exists on the unused pairs and can have an adverse effect.
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