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4 Measurement of Biosignals and Analog Signal Processing

only used in a few transmission techniques anymore. However, amplitude modula-

tion can also occur by chance. Let us continue to assume the measuring cable as an

antenna. If the position of the measuring cable to the transmitter changes, for ex-

ample because the patient moves, then the amplitude of the radiation captured by the

antenna also changes, which corresponds to amplitude modulation. The electronics

consists of components needed for demodulating the high-frequency radiation. For

example, a simple envelope demodulator consists of only a diode, a capacitor, and

a resistor. Here, the pn junction of a bipolar transistor or the metal-semiconductor

junction of a field-effect transistor can act as a diode. It is therefore possible that the

amplitude-modulated high-frequency radiation is coupled into the electronics and de-

modulated. The result is the superposition of the modulation signal with the biosignal.

Therefore, the greatest possible distance from transmitters must be ensured.

4.3 Transducer for Non-Electrical Biosignals

In section 4.1 the measurement of electrical biosignals was discussed in detail, where

the biosignal is already present as an electrical voltage and can be fed directly to the

electronics via electrodes. In addition, there are a number of non-electrical biosignals,

as can be seen from the overview in Figure 3.34. Their electronic detection requires the

conversion of the physical quantity in which the biosignal occurs into a voltage or a

current in order to be able to process it further. In this section, selected transducers

for the conversion of non-electrical biosignals are presented.

4.3.1 Sound Transducer

Listening to the body, called auscultation, is one of the oldest diagnostic procedures

in medicine. Especially the sounds of the heart and lungs provide information about

their function and possible diseases. The standard instrument used for auscultation

is the stethoscope. It consists of a transducer with a membrane for the conversion

of solid-borne sound⁵ in airborne sound and a branched sound tube, at the ends of

which are two ear olives, which provide the hermetic seal with the auricles of the ex-

aminer. Sound recording found its way into clinical practice in the 1950s in the form

of phonocardiography, i.e. the recording of heart murmurs. At that time, the devices

consisted of a solid-borne sound measuring head, analog measuring electronics and a

paper recorder for recording. Since the phonocardiogram is evaluated both in its tem-

poral course and in its spectral composition, the signal was fed to five different analog

5 Solid-borne sound is a term from acoustics. Rather, the term is used to describe the propagation of

sound in solids and, in some cases, liquids, which differs from propagation in gases (e.g., airborne

sound).