6.7 Power-over-Coax (PoC)

The Power-over-Coax (PoC) capability requires the use of circuitry following certain specifications connected to both ends of the cable. These specifications include impedance versus frequency characteristics and return loss. The PoC circuit must have low impedance at DC, but to not interfere with the data path, the characteristic impedance must be large over the band of the forward channel and back channel (10 MHz to 2 GHz) compared to the 50-Ω impedance of the coax line.

The critical consideration is the return loss profile after adding the PoC network. Limits on return loss make sure that the impedance of the PoC network is high enough that high-speed path signal integrity is maintained. The PoC network offers low impedance at DC to allow an efficient power transfer to the DC-DC powering the serializer and the camera, but also offers high impedance at the frequencies used for data transfer. Depending upon the current consumption of the camera and serializer board, there could be 200 to 500 mA of current going through the network. As a result, the engineer must check the inductor current limits before finalizing components to ensure that the PoC network components do not saturate at these current levels. A good starting point for the PoC Network to provide high impedance at the high frequencies is to use the recommended network in the EVM schematic.

For more information, see the Power-over-Coax design guidelines for the DS90UB953-Q1 application note (SNLA272).