Matching Shielded Loops for Cryptographic Analysis

Many sensors used for side channel analysis are simple coils, obtained without much design effort. The sensor is however a significant element in the measurement chain and should therefore be carefully designed. In this paper matching of some possible sensor topologies is investigated. Matching is needed to avoid reflections that would spoil the signal. This matching is however slightly different from what matching stands for in every day antenna practice. For our purpose it is best to match the sensor, but use a scope that is deliberately mismatched to the cable. Of the loops investigated, none had acceptable matching over the entire frequency range of interest. A combination of two loop types was suggested as a possible solution.