Modulation of small-scale wind waves by long waves, for example, by swell or dominant waves, is a phenomenon which is central for the radar image formation at the sea surface. Recent theoretical models suggest that the modulation is dependent not only on direct wave straining by long-wave orbital motions, but also on variation of the windexerted stress along long waves. Owing to the complexity of long wave-short wave-air flow interaction processes at the origin of these phenomena, well-controlled observations made in a wide range of wind and wave parameters are needed to validate the models. An experiment was then designed in the large IRPHE-Luminy wind-wave tank aiming at simulating two-scale wave fields composed of mechanical long waves and narrow-band wind-generated short waves. Measurements of water surface heights and mean air flow parameters were made simultaneously by high-resolution capacitance wave gauges and hot X-wire probes. To estimate the modulation transfer function, the long wave signal and the short wave energy component modulated by the long wave are extracted from the wave signal by means of a continuous wavelet decomposition. The potentiality of this method to analyze two-scale wave fields as well as more complex wind wave fields is examined in detail and compared with the classical methods based on the phase average or the windowed Fourier transform. The phase and the amplitude of the modulation transfer function are thus determined for the different wind speed, wave steepness and frequency range conditions, and the results obtained are analyzed within the framework of the recent coupled air-water flow theories.