each transducer band [3]. When broad-band procedures for optimum impedance coupling [4] are applied to a
piezoelectric load, the network design turns very complex. Moreover, they assume linear and resistive generators
and so are not applicable with high-voltage spike generators used in an NDE context. The standard emitters in NDE, based on HV step generators, contain series and parallel coupling networks [5], with non-linear behaviour during the pulse emission and also under some specific receiver conditions. Fig. 1 shows a block diagram of such a pulsed ultrasonic emitter. It generates spikes across the probe by discharging the capacitor C through a HV step generator of low onimpedance (based on a fast switching device: thyristor or Power Mos-Fet [6]) and the series circuit ZL—rectifier
RII C. The spike shaper network is completed by a resistance RSh in parallel with an inductive component LSh.
The rectifier set RI C allows the direct charge of C. From a functional point of view, for descriptive
purposes, output VT could be represented by a temporal convolution between an exponentially descendent ramp
function and the impulsive responses of two blocks of Fig. 1: charge circuit and spike shaper responses (IrCH,
IrSS).