Procedure for the recovery time estimation of the ultrasonic preamplifier

摘要

When it comes to the preamplifier, operating in a pulse-echo mode, input circuit has to protect the sensitive electronics from high voltage pulses used for excitation. Protection circuit has to introduce high impedance path when excitation signal is present and return to low impedance state when preamplifier is in reception. The recovery time is influencing the length of the blind zone. When using procedure described in EN 12668-1 the recovery time is not always properly estimated. For some preamplifier topologies recovery time is overestimated. In some cases recovery time is underestimated. Novel, yet simple recovery time estimation technique is suggested which accounts the transducer used. Technique is using the Butterworth-Wan-Dyke (BVD) electrical model of the ultrasonic transducer to simulate the load and signal source simultaneously. CW signal is supplied via BVD circuit into preamplifier input. Preamplifier output signal is filtered by the bandpass filter with bandwidth equal to 100°% of transducer center frequency. Then, Hilbert transform is used to take the envelope of signal. The start and the end of recovery process were registered at -6°dB (50°%), -3°dB (30°%), -1°dB (10°%) and -0.5°dB (5°%) level. Experimental investigation of suggested technique is presented and comparison with results obtained according to EN 12668-1 is given.