National Instruments SC Express PXIe-4331, Руководство пользователя

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Chapter 2 Using the Module
NI PXIe-4330/4331 User Manual 2-28 ni.com
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 PXIe-4330/4331 adjust the frequency range of the passband to match
the sample rate. Therefore, the amount of gain or attenuation at a given frequency depends on
the sample rate.
Stopband
The filter significantly attenuates all signals above the stopband frequency. The primary goal
of the filter is to prevent aliasing. Therefore, the stopband frequency scales precisely with the
sample rate. The stopband rejection is the minimum amount of attenuation applied by the
filter to all signals with frequencies within the stopband.
Alias-Free Bandwidth
Any signal that appears in the alias-free bandwidth of the NI PXIe-4330/4331 is not an aliased
artifact of signals at a higher frequency. The alias-free bandwidth is defined by the ability of
the filter to reject frequencies above the stopband frequency, and it is equal to the sample rate
minus the stopband frequency.
Filter Group Delay
The digital filtering performed by the NI PXIe-4330/4331 produces a delay of many samples
worth of time between when an event occurs on the input signal going into the
NI PXIe-4330/4331 and when the data associated with that event is available at the output of
the acquisition and filtering process. This delay is called the group delay.
In order to simplify the process of acquiring data from the NI PXIe-4330/4331 modules and
correlating that data with data from other modules, the NI PXIe-4330/4331 compensates for
this group delay in the following ways:
• The Sample Clock output from the NI PXIe-4330/4331 is generated at the point in time
when the input signal is valid at the ADC input pins. When acquiring data, the
NI PXIe-4330/4331 generates a Sample Clock, then waits for the data associated with
that Sample Clock to be acquired, then returns that data. As a result, any other
acquisitions timed with this Sample Clock line up with the data returned by the
NI PXIe-4330/4331.
• Any triggers generated or received by the NI PXIe-4330/4331 are interpreted based on
their relationship to the Sample Clock being generated. For example, a Start Trigger that
starts an acquisition results in data from the next Sample Clock being returned as the first
point
in the acquisition. Refer to the Triggering and Filter Delay section for more details
about
how this affects analog trigger events.