NI 9225E Operating Instructions and Specifications
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Understanding NI 9225E Filtering
The NI 9225E uses a combination of analog and digital filtering
to provide an accurate representation of in-band signals while
rejecting out-of-band signals. The filters discriminate between
signals based on the frequency range, or bandwidth, of the signal.
The three important bandwidths to consider are the passband, the
stopband, and the alias-free bandwidth.
The NI 9225E represents signals within the passband, as quantified
primarily by passband flatness and phase nonlinearity. All signals
that appear in the alias-free bandwidth are either unaliased signals
or signals that have been filtered by at least the amount of the
stopband rejection.
Passband
The signals within the passband have frequency-dependent gain or
attenuation. The small amount of variation in gain with respect to
frequency is called the passband flatness. The digital filters of the
NI 9225E adjust the frequency range of the passband to match the
data rate. Therefore, the amount of gain or attenuation at a given
frequency depends on the data rate. Figure 6 shows typical
passband flatness for the NI 9225E.
