Ultrasonic transducer for e.g. rangefinder - has bias reversed after each pulse and with pulse and bias on both electrodes which are earthed alternately

摘要

The ultrasonic transducer (K), e.g. for a rangefinder, has two electrodes (K1,K2), one fixed, one movable. The electrodes are driven by a h.f. pulse (E1) and 250V DC bias is applied between them. To avoid loss of sensitivity due to the bias leaking away, the polarity is reversed immediately before each h.f. pulse. A voltage of +250V DC is applied to both electrodes via separate resistors (R1,R2). The HF pulse (E1) is applied to both electrodes by two transistors (T1,T3) with the resistors (R1,R2) as the collector loads. Two further transistors (T2,T4) alternately short circuit one of the electrodes to earth. They are driven separately by square pulses (E2,E3) whose leading edges coincide with the start of alternate h.f. pulses (E1).