The system uses a tunable inductor in series with the piezoelectric crystal excitation transducer in the probe which has a flux modulation coil. The bias current through this flux modulation coil is controlled by the system. It is controlled such that the inductance of the tunable inductor cancels out...http://www.google.de/patents/US4970656?utm_source=gb-gplus-sharePatent US4970656 - Analog drive for ultrasonic probe with tunable phase angle
