| 摘要 |
<p>Fetal heart rate monitoring using ultrasound by means of a transducer in contact with the maternal abdomen. The transducer (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 received signal by the local oscillator signal. The sum of the frequencies is removed by low-pass filter (15), while the difference of the frequencies is the Doppler frequency of the received signal which passes through the filter to be digitised by ADC (16), the Rx gate opening 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 bin in intervals during the Rx gate-open interval. One or two of the Doppler audio signals, aperiodic or periodic, will contain the signal from the fetal heart. When a periodic signal is found, its rate is tested to see if it lies within or outside the typical range of a fetal heart.</p> |
