Analysis and optimisation of transmission characteristics of cables for down-hole tools and data acquisition systems

This paper examines the conditions required to optimise transmission of mixed signals (analogue, digital and power) for down-hole tools and data acquisition systems, when using coaxial cables. The performance of the cables was analysed using a finite element model. A special feature of the model is that it implicitly introduces the effects of temperature differentials to which the cable may be exposed through the primary constants of the cable, where, the temperature dependency of each constant is either known, or practically established. Using this model, various matching options were investigated and verified by experiments. From these investigations, it is found that within the low-to-medium frequency range (1-100 kHz) conventional method of matching (termination impedance equal to nominal characteristic impedance at high frequency) does not yield optimal performance. It is proposed that better matching conditions could be achieved as predicted by the model