sentences:  Initially this was limited to the use of thyristors for generating the 'MF range of frequencies using discrete electronic control systems.State of the art units now employ SCR (silicon-controlled rectifier), IGBT or MOSFET technologies for generating the 'MF' and 'RF' current.The modern control system is typically a digital microprocessor based system utilising PIC, PLC (programmable logic controller) technology and surface mount manufacturing techniques for production of the printed circuit boards.