National Instruments NI 9234 Understanding NI 9234 Filtering

Understanding NI 9234 Filtering

The NI 9234 uses a combination of analog and digital filtering

toprovide 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 9234 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.

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 9234 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. Figure6 shows typical

passband flatness for the NI9234.