An eighth-order CMOS continuous-time transconductor-capacitor (Gm-C) Butterworth filter with a tunable 2.5- to 7-MHz center frequency for the ultrasound medical system is presented. For this filter, the transconductor having a very wide tunable range is designed. The master-slave automatic tuning circuit uses a voltage-controlled oscillator(VCO). This circuit has the newly designed tuning controller which change the center frequency adaptable to the real-time ultrasound imaging system.
