Chapter 2
NI 5450 Overview
©
National Instruments Corporation
2-33
NI PXIe-5450 User Manual
Baseband Interpolation Considerations
The NI 5450 implements baseband interpolation in order to reduce the
power of aliased images. Interpolation is a process that effectively turns a
lower sample rate into a higher sample rate. Because the signal is now at a
higher sample rate, images are moved to higher frequencies, where they fall
in the rejection band of the image rejection filter and are suppressed.
NI-FGEN automatically selects the largest interpolation factor to achieve
the maximum possible DAC sample rate. The interpolated sample rate can
then be read with the Sample Rate property or the
NIFGEN_ATTR_ARB_SAMPLE_RATE
attribute.
Note
For optimum performance, National Instruments recommends maintaining the
sample rate between 270 MS/s and 400 MS/s due to the fixed frequency characteristics of
the image rejection filter. For more information about the image rejection filter, refer to the
NI 5450 specifications.
Interpolation Settings
To determine the total interpolation and interpolated sample rate, divide
400 MS/s by your desired sample rate and round down to the nearest step,
as noted in the Interpolation table for your total interpolation value. Then,
consult the following table for settings that will allow you to achieve your
desired interpolated sample rate.
Note
With OSP enabled, Desired Sample Rate (Data Rate) = Symbol Rate ×
Samples/Symbol.
Example
: At a desired sample rate of 4.5 MS/s, the total interpolation will
be determined by the following calculations:
400 MS/s ÷ 4.45 MS/s = 89.88× total interpolation
A total interpolation rate of 89.88× falls in the 24-8192 Total Interpolation
range in the Interpolation table. As this range is measured in steps of 8, you
must round down to the nearest multiple of 8, yielding a total interpolation
rate of 88
×
. This value corresponds to a sample rate of 391.6 MS/s and a
worst case image at or below –100 dB.
