Chapter 6
Theory of Analog Operation
PCI-4451/4452/4453/4454 User Manual
6-12
www.ni.com
Anti-Image Filtering
A sampled signal repeats itself throughout the frequency spectrum. These
repetitions begin above one-half the sample rate (Fs) and, theoretically,
continue up through the spectrum to infinity, as shown in Figure 6-6a.
Because the sample data actually represents only the frequency
components below one-half the sample rate (the baseband), it is desirable
to filter out all these extra images of the signal. The PCI-4451/4453
accomplishes this filtering in two stages.
First, the
data is digitally resampled at eight times the original sample rate,
then a linear-phase digital filter removes almost all energy above one-half
the original sample rate and sends the data at the eight-times rate to the
DAC, as shown in Figure 6-6b. Some further (inherent) filtering occurs at
the DAC because the data is digitally sampled and held at eight times the
sample rate. This filtering has a sin x / x response, yielding nulls at multiples
of eight times the sample rate, as shown in Figure 6-6c. Still, images
remain, and they must be filtered out. Each output channel of the
PCI-4451/4453 has discrete-time (switched-capacitor) and
continuous-time analog filters that remove the high-frequency images,
as shown in Figure 6-6d.
