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Chapter 5 Adding Components for Special Functions
© National Instruments Corporation 5-13 SCB-68 Shielded Connector Block User Manual
Lowpass Filtering Applications
Noise filtering and antialiasing are two applications that use lowpass filters.
Noise Filtering
You can use a lowpass filter to highly attenuate the noise frequency on a
measured signal. For example, power lines commonly add a noise
frequency of 60 Hz. Adding a filter with f
c
< 60 Hz at the input of the
measurement system causes the noise frequency to fall into the stopband.
Referring to Equation 5-4, fix the resistor value at 10 kΩ to calculate the
capacitor value and choose a commercial capacitor value that satisfies the
following relationship:
(5-6)
Antialiasing Filtering
Aliasing causes high-frequency signal components to appear as a
low-frequency signal, as Figure 5-14 shows.
Figure 5-14. Aliasing of a High-Frequency Signal
The solid line depicts a high-frequency signal being sampled at the
indicated points. When these points are connected to reconstruct the
waveform, as shown by the dotted line, the signal appears to have a lower
frequency. Any signal with a frequency greater than one-half of its sample
rate is aliased and incorrectly analyzed as having a frequency below
one-half the sample rate. This limiting frequency of one-half the sample
rate is known as the Nyquist frequency.
C
1
2π 10 000,()60()
-----------------------------------------
>
2468100
1
–1
Input Signal
Sampled Points
Reconstructed Signal