This type of sensor may be remotely interrogated by connecting the input of the transducer to a radio frequency (RF) antenna. When the antenna receives an electromagnetic signal, the latter gives rise to waves on the surface of the substrate which are themselves reconverted into electromagnetic energy in the antenna. Thus, the device, consisting of a set of resonators connected to an antenna, has a response to the resonant frequencies of the resonators that it is possible to measure remotely. It is thus possible to produce remotely interrogable sensors. This possibility is a major advantage of surface acoustic waves and may be used notably in the context of pneumatic pressure sensors. This is because it is advantageous in this type of application to be able to place the sensor in the pneumatics whereas the interrogation electronics are stowed onboard the vehicle.
According to the prior art, remote interrogation systems use interrogation signals in the form of pulses (typically with periods of about 25 μs) which are transmitted via an emitting antenna in the direction of a receiving antenna connected to the surface-wave sensor (referred to below, in the description, as a SAW sensor).
A preferred frequency band for this type of system is the ISM (industrial, scientific and medical) band having a central frequency of 433.9 megahertz, the associated band width being 1.7 megahertz (MHz).
Generally, a remotely interrogable SAW sensor and its interrogation system may comprise, as illustrated in FIG. 1, in the simplified case of a single transducer:                an interrogation system 2; and        at least one resonator 1 comprising;                    an antenna 100; and            a comb transducer having interdigitated electrodes 11 and an SAW resonant cavity 13 characterized by its central frequency F and its quality factor Q (corresponding to the ratio between the central frequency and the width of the pass band). The cavity 13 comprises two series of reflectors that are uniformly spaced apart by a distance d. The transducer is connected to the antenna 100.                        
The interrogator 2 sends a long radio-frequency pulse so as to charge the resonator 1. After the emission has been stopped, the resonator discharges at its resonant eigenfrequency with a time constant τ equal to Q/πF. This discharge of the resonator forms the return echo detected by the receiver of the interrogator. Spectral analysis then allows the resonator frequency to be calculated and identified. This analysis may be carried out by algorithms based on Fourier transforms, for example an FFT (fast Fourier transform). This type of spectral-analysis treatment is particularly complex.
A method for remotely interrogating passive SAW-type sensors has been proposed, in patent application WO 2008/015129, based on a frequency-modulation method. More precisely, this patent application discloses a method of measuring the resonant frequency of a resonator comprising the following steps:                emission, in succession, of radio-frequency signals of known carrier and modulation frequency, including the resonant frequency;        reception, via a receiving system, of response waves from the sensor; and        spectral analysis of the response waves from the sensor.        
The RF emission is frequency modulated with a modulation ωm and an amplitude modulation typically of about a kHz.
The response signal of the sensor is amplitude modulated with a modulation frequency ωm.
The measurement principle described above is based on the conversion of an (emitted) frequency modulation into an amplitude modulation via the transfer function of the resonator, as illustrated in FIG. 2. An emission modulated at an angular frequency ωm (corresponding frequency fm) results in the power detector receiving a signal modulated at the frequency ωm but more importantly, in a possible phase inversion of the modulated signal depending on whether it is below or above the resonant frequency.
The sinusoid, injected in the signal, at the frequency ωm, representing the modulating signal, is either directly converted into an amplitude modulation by the transfer function of the resonator (positive slope beneath the resonant frequency, amplitude modulation in phase with the frequency modulation), or inverted (negative slope above the resonant frequency, amplitude modulation in antiphase with the frequency modulation). The intermediate point corresponds to a null contribution to the received signal at the modulation frequency fm.
Around this frequency position, the amplitude of the component of the signal at the frequency fm varies linearly.
The applicant observed that when implementing an algorithm providing the function described above, namely transformation of the contribution to the modulation frequency into a signed datum around the null contribution to the modulation frequency fm, and when detecting the amplitude, although a sinusoid is emitted to generate the frequency modulation, it is possible to use only two components of the received signal, respectively at the maximum and minimum modulation frequencies, this observation allowing interrogation times to be very substantially reduced.