The RF-powered surface wave sensor oscillator - a successful alternative to passive wireless sensing

This paper describes a novel passive wireless SAW sensor providing a highly coherent measurand proportional frequency, FM modulated with identification (ID) data and immune to interference with multiple path signals. The sensor comprises a highly stable low-power oscillator, stabilized with the sensing SAW resonator and powered by the rectified RF power of the interrogating signal which is received by a λ/4 whip antenna on the sensor part. A few hundred μW of dc power are enough to power the sensor oscillator and ID modulation circuit and achieve stable operation at 1.0 and 2.49 GHz. Reliable sensor interrogation was achieved over a distance of 0.45 m from a SAW based interrogation unit providing 50 mW of continuous RF power at 915 MHz. The -30 to -35 dBm of returned sensor power was enough to receive the sensor signal over a large distance and through several walls with a simple superheterodyne FM receiver converting the sensor signal to a low measurand proportional intermediate frequency and retrieving the ID data through FM detection. Different sensor implementations including continuous and pulsed power versions and the possibility of transmitting data from several measurands with a single sensor are discussed.