Load-insensitive active quasi-circulator for continuous-wave Doppler ultrasound applications

Circulators are often used in RF systems; however, their size, cost, and weight increase greatly at lower frequencies (<300 MHz). The development of a load-insensitive active quasi-circulator is presented as an alternative for the traditional circulators to be used in low-frequency systems such as Doppler ultrasound. Major challenges to these active devices are limited power and low isolation between non-consecutive ports. The use of current-feedback operational amplifiers allowed higher-power delivery to the load. Simulations using Monte Carlo method showed an isolation >19 dB for 20% of variation of the load impedance. The circuit was implemented and tested with an ultrasound probe in a water tank. The results show that the circuit was able to send and receive signals simultaneously from a single ultrasound transducer element.