We report the design and construction of a non-invasive radio frequency instrument used for real-time measurement of plasma-generated harmonics of the fundamental drive frequency of 13.56 MHz. The design allows the selection of a single harmonic for analysis. A digital oscilloscope is employed to display one harmonic (n = 4 to 7) with respect to the fundamental as a Lissajous figure. This method offers an immediate and graphic real-time visualisation that allows an operator to very quickly diagnose the state of the system, without the need of any further analysis. The instrument achieves very high sensitivity in process discrimination by utilizing an amplifier with very low noise-figure (NF = 2dB), by rejecting spurious FM broadcast and co-channel signals and by tuning to the plasma and plasma-tool frequency response to changes in plasma state. Information about the amplitude and relative phase of the two signals can be readily extracted, permitting the identification and classification of a unique plasma state as a real-time evolving 2-dimensional signature. This can be used to calibrate the plasma as a function of pump-down time, pressure and radio frequency power, and also to monitor plasma processes in real-time. The reconstruction of the time evolving signature of the plasma can be used to develop an automated monitoring and controlling system for the plasma-tool, which would be of great advantage when employed in the manufacturing of semiconductors and opto-electronic devices of less then 1 cm2 in total area.