Data Communication in Read-Out Systems: How Fast Can We Go Over Copper Wires?

In a digital X-ray imaging system, data has to be transmitted from the detector to the storage system. In future digital X-ray imaging systems, higher data rates will be needed. For some applications, e.g. protein crystallography at synchrotron beams, data rates in the order of gigabits per second are expected. Present trend for such systems is to move from a parallel data bus towards a high-speed serial readout. For high speed signaling over short distances (up to 10 m) the attenuation of copper cables is low enough to permit multi-gigabit per second speeds. In this article, an overview will be given of problems encountered in high speed data transmission over copper cable and techniques will be shown to overcome these problems. The bandwidth bottleneck in short distance communication is in the IC-technology and not in the channel. The cable transfer function results in inter-symbol interference (ISI). The skin-effect is the most significant cause of ISI for short length (10 m) coaxial copper cables. Fortunately, equalization can compensate for these effects. An equalizer has a transfer function that is the inverse of the channel transfer function. With the correct equalizer, a very low Bit Error Ratio (BER) can be achieved. The measured RG-58U cable (τ1=0.12 ns) could transmit at a bit rate of 8.3 Gbps, with a BER of 10−12. Multi-gigabit speeds are possible over short length coaxial copper cables.