High Power Signal Path PoE Magnetics

The system comprises an isolation transformer, a common mode choke and six capacitors. The capacitor across the primary of the isolation transformer on the TX side tunes the circuit by matching the output capacitance of the chipset (see schematic). This design allows these components to be used with virtually all chipsets.

For this power level, the magnetic components do not fit into a conventional Ethernet jack. The Coilcraft solution places two isolation transformers in a single package (HPX2126L) and both common mode chokes in an even smaller package (HPF2187L). Both packages fit easily behind a standard Ethernet jack.

A matching network is used to perform the testing for return loss, insertion loss, crosstalk, common mode to common mode rejection and differential to common mode rejection. The network matches the 50 Ohm single ended connection of network analyzers to 100 Ohm differential balanced lines. A 1:1 balun is used with a resistive network to transform the impedance. The network is then calibrated out of the measurement by using open, short, load calibration techniques. The circuit is then placed behind the matching network for each of the tests. All tests are performed on an Agilent/HP 8753ES network analyzer.

For return loss, the chip side of the component is loaded with 100 Ohms and the cable side of the system is tested using the matching network.

For insertion loss, the fixture used for the return loss is altered to replace the 100 Ohm load with a matching network for the output port of the network analyzer.

For common mode to common mode rejection, TX+ and TX– (or RX+ & RX–, depending on side of the system) is tied together and an S21 measurement is taken from chip side to cable side.

For differential to common mode rejection, the chip side of the system is excited by the network analyzer through the matching network. The measurement is taken by looking at the balance between two 50 Ohm loads across the cable side of the system.

For crosstalk, the chip side TX is excited by the network analyzer through the matching network. The center-tap on the chip side of each isolation transformer is grounded, and the chip side RX is measured. The cable side of both RX and TX is loaded with 100 Ohms outside of the system.