NI-9253 Getting Started

The NI-9253 uses programmable hardware filtering to provide an accurate representation of in-band signals and reject out-of-band signals. The filters discriminate between signals based on the frequency range, or bandwidth, of the signal. How the filter discriminates signals based on their frequency is known as frequency response. In general, the frequency response of a filter is described by a signal attenuation (magnitude response) and a input delay (phase response) for every input frequency.

• Magnitude Response—The three important frequency ranges, or bandwidths, to consider for magnitude response are passband, transition band, and stopband:
  • Passband—The range of frequencies at which the filter attempts to pass a signal without modifying it. The small amount of variation in magnitude at these frequencies is called passband flatness. This is the frequency range of signals that you want to measure.
  • Transition band—The range of frequencies in which the filter magnitude response has started to roll-off such that it attenuates signals by some amount, but has not reached the full attenuation amount. The shape of the transition band has an impact on the alias rejection and how signals are represented in the time domain (for example, step response).
  • Stopband—The range of frequencies at which the filter attenuates input signals to its maximum attenuation level. Ideally, you want to choose a filter with a stopband that covers frequencies of noise sources that you do not want in your measurements.
• Input Delay—Filters delay the input signal by some amount when processing data. In some cases, the delay is a function of the input signal frequency; when this is the case, the input delay plot is useful for knowing the exact delay at different input frequencies and the maximum variation between signals of different frequencies within the passband.

  
  

  

  
  

Each NI-9253 filter has a different frequency response to serve different applications:

• Butterworth—Has a passband independent of the sampling rate (as opposed to the comb filter), which offers more flexibility when filtering out noise that is below one-half of the sample rate. However, depending on your settings, you may see alias components of higher frequency signals in your measurement that extend beyond one-half of the sample rate due to the larger transition band.
• Comb—Has a smaller passband because its transition band starts early in the frequency range. The comb filter has shorter group delay than other filters and better representation of signals in the time domain (step response). The comb filter's transition band features equally-spaced notches at different frequencies. It is common to use the comb filter with a specific sample rate to align the notches of the transition band thereby removing a specific noise-source frequency from measurements.

The NI-9253 filter delay across signals in the passband varies between filters:

• Butterworth—Delays signals by a variable amount depending on their frequency.
• Comb—All input frequencies have the same amount of delay when going through the filter. Choose this filter for applications where linear phase, short delay, or data correlation of different devices and configurations is required.

Refer to the specifications for details on the amount of variation in the passband gain and input delay you can expect for different input frequency ranges.

The shape of the magnitude and phase responses of a filter impacts how signals look in the time domain. The step response of a filter is typically used to identify the behavior of a filter in the time domain.

Three important factors of the filter step response are group delay, rise time, and overshoot/undershoot. The three filters differ in step response across signals in the transition band:

• Butterworth—Has a short group delay and the longest rise time. The output signal shows overshoot.
• Comb—Has the shortest group delay and the shortest rise time. The output signal does not show overshoot or undershoot.