X90 mobile modules
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X90 mobile system User's manual V 1.20 - Translation of the original manual
Depending on the resolution selected, the lines manifest themselves in different ways on the spectrum.
Buffer storage
The values that have been sampled are stored for 300 ms in the module's internal buffer and must be transferred
during this time.
Write selected value
to buffer
The size of the buffer is constant and can store 8192 measured values. This results in the following relationship
between the sampling rate and the duration of measurement.
Measurement duration = Buffer size / Samples per second
Since the number of values stored depends on the configured sampling rate and not the hardware-based sampling
rate, not all values that are measured are stored. At a measurement duration of 318 ms, every second value is
stored; at a duration of 15.9 ms, every hundredth value is stored.
The number of stored values can be set indirectly through registers
"MaxFrequencyEnvelope" on page 185
and
"MaxFrequencyRaw" on page 186
Aliasing effect
The input signal is sampled at regular intervals. If the sampling rate used is too low, the input signal will be sampled
incorrectly and a flawed image of the oscillation will occur. This undesirable phenomenon is called the aliasing
effect.
To avoid such false results, the requirements of the theorem known as Nyquist's sampling theorem must be fulfilled
in the sampling. This sampling theorem describes the necessary frequency ratio between the sampling and the
signal and states that the sampling rate must be more than double the maximum frequency of the measured signal.
Example of incorrect sampling
Sine wave with 4 kHz sampled at 6kHz. The red wave shows the sine wave measuring 2 kHz, which is a result
of the sampling being too low.
4 kHz
2 kHz
Time
Sampling 6 kHz
Amplitude
Figure 77: Incorrect sampling with 6 kHz and 4 kHz
Information:
The module ensures that Nyquist's sampling theorem is always fulfilled.
With a wanted signal of 10 kHz, a reduced sampling frequency of 25.7812 kHz is used!
