F9 Fetal & maternal Monitor Service Manual Principle Introduction
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Chapter 7 Principle Introduction
7.1 Modules Principle
7.1.1 Ultrasound Doppler
The fetal heart Doppler signal detecting is based on Ultrasound Doppler frequency shifting
principle. A sound source sends out constant frequency sound wave, the wave transmits in organs
and reflects when coming across interfaces. If the interface moves toward the sound source, the
frequency of the reflected wave will be different from that of the incident wave: the incident
wave’s approaching will be translated into an increase in frequency; its receding will cause the
frequency to decrease. This is the Doppler Effect. When the Doppler Effect happens, there is a
difference between the frequencies of the sent out sound wave and the received sound wave,
which is called Doppler frequency Offset (f
D
).
C
f
Vf
D
0
*2±=
In this equation 0f is for the frequency of the ultrasound, C denotes the speed of wave
transmitting in the organ, V denotes the moving speed of the interface.
For the fetal heart, the frequency offset of 1 MHz ultrasound source is about 60 ~ 400Hz (the
standard adapted by EDAN). After being demodulating, amplified and filtered, the received
ultrasound signals (the filter has a passband of 60 ~ 400Hz) are translated into fetal Doppler
frequency offset signals. For the convenience of calculating, we envelope these signals after that.
Then DSP does autocorrelation calculation to the A/D sampled results and figure out the fetal
heart rate.
7.1.2 Fetal movement Doppler Signal Detecting Principle
When the sound source is 1MHz ultrasonic, the frequency offset (f
D
) of fetal movement is about
10 ~ 40Hz, the flow of signal processing is the same as that of the fetal heart detecting. The
algorithm applied currently is squaring the A/D sampling results and taking the average value as
the fetal movement signal intensity.
7.1.3 Pressure Detecting Principle
The pressure detecting applies the Wheatstone bridge circuit. Refer to figure 8-1, the pressure
signal has been transformed into voltage signal for measuring. Two pressure stain gauges R1 and
R8 are installed on both sides of the spring, the spring’s being forced causes the resistance of R1
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