The authors describe an experimental set-up for the graduate teaching laboratory on the propagation of surface waves on water and on the determination of the normal density of superfluid 4He. The sensing elements are either the capacitor of the inductor comprising the tank circuit of a tunnel diode oscillator. Change of these components modulate the frequency of the oscillator and are detected by an FM detector. This technique is useful in introducing the student to the subject of parametric conversion of low frequency signals. Measurements of the surface waves yield the surface tension of water and demonstrate its sensitivity to the presence of surfactants, such as soap. The sensing parts of the experiments are simple enough to be constructed by the students with the minimum of help.