T3/E3 protection circuit and solution

Interface parameters

Signal rate: 34.368Mbit/s±20ppm

Load impedance: 75Ω

Signal level: 1.0V

Empty number level: 0V±0.1V

The signal transmission rate of T3/E3 is relatively high, so it should be protected by a TVS with low junction capacitance. The device selected is LC03-6. There is no common-mode protection device in the figure. The main reason is that the interface circuit is isolated by a transformer. The insulation withstand voltage of the primary and secondary sides of the transformer can generally reach 1500VAC, which can effectively suppress external common-mode overvoltage.

The transmission distance of the T3/E3 interface is within 50 meters, and the cable is generally not pulled outdoors. However, in some low-end routers, the application environment is relatively harsh, and the cable may be pulled from one room along the outdoor wall to another room. In this case, the protection energy should be considered to be greater.

The shielding layer of the coaxial cable is reliably connected to the housing, that is, it is grounded. At this time, the external overvoltage and overcurrent are reflected in the form of differential mode, so only differential mode protection is performed. The front stage needs to add a diode gas discharge tube N81-A90, the back stage uses SLVU2.8-4, and the middle stage uses a surge protection resistor 2.2Ω/2W resistor for decoupling.