© PCE Instruments
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should select a higher low-pass frequency for differential probes
than for absolute probes. Thus the minimum low-pass frequency to
be selected for a particular application depends on the test speed
and the coil system of the probe you use.
The easiest way to determine the minimum low-pass frequency is
probably by trial and error.
High-Pass Filter
Switching on of a high-pass filter activates the field for setting up the
frequency HF from 1 to 5000 Hz with a step of 1, 10, 100, 1000 Hz.
The frequencies from the set value of Hz to F
sampling
/2 Hz (where
F
sampling
– sampling frequency) are allowed.
If you use a high-pass filter, the signal dot always returns to the
selected compensation point, if the probe is not moved relative to the
surface of the
test specimen. This can be likened to an „automatic
reset". Thus, if you move the probe across materials with varying
electric conductivity, the signal dot briefly moves to a position on the
screen that varies depending on the material; the dot then returns to
the selected compensation point.
The filters are intended to improve the signal/noise ratio and/or
supress undesired signals so that the signal evaluation and test
sensitivity are optimized. The noise can be
“electronic” (caused by
the instrument or the probe), or it can be caused by other factors
such as scanning rough surfaces. You can easily identify electronic
noise by the so-
called “fuzzy” signal image instead of a steady dot.
This happens frequently, if you carry out inspections with low
frequencies or high gain. The filter frequency does not depend on
the alternating frequency of the current that flows through the coils,
but on how frequently the test signals change. If for example you
move a probe across a crack, the frequency depends on how fast
you do so. The high-pass filter is designed so that only those signals
whose frequency is greater than that of the high-pass filter are
depicted by deflecting the signal dot. If the high-pass fil filter is active
the signal dot on the screen remains on the compensation point.
Bandpass
Switching on of a bandpass filter activates its settings:
Carrier frequency F is set from 1 to 5000 Hz with a step of 1; 10;
100; 1000 Hz;
