Chapter 5
Signal Generation Fundamentals
©
National Instruments Corporation
5-15
NI PXIe-5450 User Manual
The third filter, Analog Filter 3, has a much higher 3 dB point than the first
two analog filters. Because of the higher 3 dB point, the filter is very nearly
flat in the passband (0 to 0.43f
s
). Analog Filter 3 does not filter the images
produced at f
s
and 2f
s
at all, but this shortcoming can be alleviated with a
digital interpolation filter.
To ease the requirements of the analog filter and to get more output
bandwidth, NI signal generators use a halfband digital filter to interpolate
one, three, or seven samples between every two waveform samples at two
times, four times, and eight times the sample frequency, f
s
. Also, the DAC
operates at an effective sampling rate that is two times (2f
s
), four times (4f
s
)
and eight times (8f
s
) the sample frequency—specifically, the rate at which
the data is clocked from the memory into the DAC.
In the following figure, the two times interpolating filter is used and the
effective sample rate of the DAC is 2f
s
. The images at f
s
± f
o
are no longer
an issue, and the images are now at |2f
s
± f
o
|.
Analog Filter 2
Analog Filter 1
f
s
0
2f
s
3
f
s
0.5f
s
Images
f
0
Signal
Power
4f
s
–
3
dB
Analog Filter
3
