A SAW probe for time variable high voltage measurements

A biasing electric field applied to the propagation path of a surface acoustic wave (SAW) delay line produ- ces a change n the time delay of the line due to the frac- tional change of the propagation path length and to the relative variation of the wave velocity. This effects has been used to electrically control the time delay of SAW delay lines, to improve the performances of SAW devices and to realize. d.c. high voltage acoustic sensors. In the present communication, results concerning a SAW sensor for a.c. high voltage measurements are re- ported. The sensor uses a SAW-delay line oscillator configuration. When an a.c. voltage is applied, the fre- quency of the SAW oscillator changes its value according to the instantaneous amplitude of the voltage, provided the period of the voltage is longer than the time respon- se OF the device. The signal at the output of the oscilla- tor is frequency modulated by the applied voltage, whose amplitude can be simply recovered by monitoring the mo- dulation index of the signal. The sensitivity of the devi- ce has been experimentally investigated as a function of the temperature for different piezoelectric substrates, and a compensating technique is proposed in order to correct the sensor response for emperature variations. Results of the transient response of the device to a step voltage are given as obtained on YZ and YX LiNbOl;. Finally the SAW probe has been used for probing elec- tric fields.