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 k
Ω
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
2
π
10 000
,
(
)
60
( )
-----------------------------------------
>
2
4
6
8
10
0
1
–1
Input Signal
Sampled Points
Reconstructed Signal
