Fetal heart rate monitoring using ultrasound by means of a transducer incontact with the maternal abdomen. Thetransducer (11) is driven by a transmit amplifier (12), a receive amplifier(13) amplifies the echoes detected by the transducer.While the receive gate is open, demodulator (14) multiplies the receivedsignal by the local oscillator signal. The sum of thefrequencies is removed by low-pass filter (15), while the difference of thefrequencies is the Doppler frequency of the received signalwhich passes through the filter to be digitised by ADC (16), the Rx gateopening a fixed delay after the end of the transmit pulse.A number of range bins are chosen and two ADC readings are made for each binin intervals during the Rx gate-open interval.One or two of the Doppler audio signals, aperiodic or periodic, will containthe signal from the fetal heart. When a periodic signalis found, its rate is tested to see if it lies within or outside the typicalrange of a fetal heart.
