Abstract: The present invention is directed towards an apparatus and method to detect electromagnetic radiation or signals emitted by plasma comprising a sensor unit, the sensing unit comprising a first sensor for detecting electromagnetic radiation or signals; an electromagnetic barrier wherein the electromagnetic barrier is configured to surround the first sensor to prevent ambient electromagnetic radiation reaching the first sensor; a first port through which electromagnetic radiation can pass, the port configured for coupling to a port of a plasma chamber such that electromagnetic radiation or signals emitted from the plasma can reach the first sensor; and a first output coupled with a balun and a dual coaxial system whereby a signal sensed is grounded and a phase reversed version of the signal is grounded. A signal acquisition system (SAS) for detecting electromagnetic radiation emitted by a plasma is also described.