5
Technical Description
5.1
The Doppler Principle
The
Baby Dopplex
uses the Doppler principle for non-invasively monitoring
movement within the body.
The Doppler principle states that if a signal is transmitted at a fixed frequency and is
reflected by a moving body, the frequency of the received signal will be shifted. An
increase in frequency results if the reflector is moving towards the
transmitter/receiver, and a decrease results if moving away from the
transmitter/receiver. The amount of frequency shift is proportional to the velocity of
the reflector relative to the transmitter/receiver.
In the Dopplex range, a fixed frequency ultrasonic signal is transmitted from the
transducer into the body. This is reflected from, for example, the fetal heart. The
signal is reflected from the heart and is received by the transducer. Due to the
movement of the fetal heart, a frequency shift results, which is proportional to the
fetal heart velocity.
5.2
Doppler Audio Processing
The
Baby Dopplex
ultrasound transducer contains a transmitter and receiver. In use,
the transducer sends out a pulsed ultrasonic signal, generated by the piezo-ceramic
transmitter crystals, at 1.5 MHz.
This signal is scattered by blood cells or any other “interface” such as skin, muscle
layers, organs, walls of vessels etc. A small proportion of the scattered signal will be
reflected back and detected by the receiver.
By demodulating the received signal (removing the high frequency carrier) the
Doppler shifted component (i.e. the difference between the transmitted and received
signals) can be produced.
With typical target velocities found in the human body, this Doppler shift signal falls
within the audio frequency range. It can therefore simply be amplified and heard
through a loudspeaker.
It is important to remember that the sound you hear is an artificial sound, the
frequency (pitch) of which is proportional to the velocity of the moving target.
It is not the real sound made by the fetal heart.
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Issue: 2 Draft
Rev: a
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