have different frequencies the situation can be as illustrated in fig7 in the diagram waveforms 59 and 60 may represent an incidental electromagnetic radiation signal v 1 t and a label interrogation signal v 2 t when the signals are of an equal amplitude but of different frequencies an interference pattern with total excitation v t t as shown in waveform 61 occurs from which it is observed that peaks are at some times twice the amplitude of individual excitation signals when this waveform is presented within an electronic label to a peak detector rectifier the output waveform v r t can be as shown in waveform 62 whereas the output voltage from the rectifier when only a single one of the two interfering signals is presented to the rectifier is the lower amplitude signal 63 when two signals are present the rectifier will also have a perceptible output at the difference frequency between the two signals in one embodiment of the present invention that difference frequency output signal may be usefully employed within the label as a timing signal regulating operation of circuits within the label such timing regulation is of advantage in being able to accurately predict the sub carrier frequency commonly used to generate reply signals from modulated rf backscatter labels the issues of signalling command signals to such a label in the presence of such.