Experiments on Large Structures - Dynamic Behavior of Concrete and Seismic Engineering - page 204

determined in order to make the necessary corrections when processing the signal. A
frequency response curve for a filter with a 20 Hz cut-off frequency is shown in
Figure 6.1.

1

0.8

0.6

0.4

0.2

0

-0.2

-0.4

-0.6

-0.8

-1

0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40

Time, s

Figure 6.2. Frequency aliasing

Hydrophones are high-accuracy pressure sensors used to measure hydrodynamic
pressures in dam reservoirs. These sensors behave as high-pass filters, as Figure 6.1
shows. The strain and displacement sensors do not require any measuring
corrections, but the signal from such instruments has to be amplified before
acquisition.

The different instruments are connected to the data acquisition device via
several-conductor electric wires, which can reach several hundred meters in length
due to the large dimensions of typical structures. These wires often cause noise with
or without any load amplifier. Noise originates mainly from three sources:
electromagnetic noise, ground loops and tribo - electric effects.

Electrical wires conveying large amounts of alternating current cause
electromagnetic effects. Such cables are surrounded by powerful electromagnetic
fields that vary over time, and these induce static voltages in parallel cables. We
recommend the use of sheathed wires with a conducting surface, which protects the
inner part from external electrostatic and electromagnetic fields. The wires carrying
signals should be placed as far as possible from electrical wires, and the use of
wound cables liable to turn a transformer should be avoided.

A ground loop arises when a common connection inside a system is grounded to
more than one point. This happens, for example, when the shielding of the wire