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Appendix B Waveform Sampling and Interpolation
DAQArb 5411 User Manual B-2
©
National Instruments Corporation
To ease the requirements of the analog filter and to get more output
bandwidth, the DAQArb 5411 uses a half-band digital filter to
interpolate a sample between every two samples at twice the update
frequency, 2f
c
. Also, the DAC operates at twice the sample frequency.
This increase pushes the images from f
c
to 2f
c
and the analog filter
roll-off easily rejects any images from the output spectrum. This
behavior can be seen in the frequency domain representation from
Figure B-2 and in the time domain representation from Figure B-3.
Figure B-2. Digital Filter, Analog Filter, and Signal Images with Digital Filtering
Figure B-3. Waveform Updates
Note: The digital filter will be operational only for sample rates of 40 MHz and
20 MHz. For other sample rates, the digital filter will not be of any use.
f
c
0
2f
c
0.5f
c
Images
f
0
Analog Filter
Digital Filter
Signal
Power
(Interpolation Frequency)
4f
c
Without Interpolation
With Interpolation After Analog Filtering