# Measurement Functions

Measurement functions perform DFT-based and FFT-based analysis with signal acquisition for frequency measurement applications as seen in typical frequency measurement instruments such as dynamic signal analyzers.

Several measurement functions perform commonly used time domain-to-frequency domain transformations such as amplitude and phase spectrum calculation, signal power spectrum calculation, network transfer function calculation, and so on. Other supportive measurement functions perform scaled time-domain windowing, power and frequency estimation, and total harmonic distortion analysis.

You can use the measurement functions for the following applications:

- Spectrum analysis applications
  - Amplitude and phase spectrum
  - Power spectrum
  - Scaled time-domain window
  - Power and frequency estimate
- Network, or frequency response, and dual-channel analysis applications
  - Transfer function
  - Impulse response function
  - Network functions, including coherence
  - Cross power spectrum

The DFT, FFT, and power spectrum functions are useful for measuring the frequency content of stationary or transient signals. The FFT provides the average frequency content of the signal over the entire time that the signal was acquired. For this reason, you use the FFT mostly for stationary signal analysis, when the frequency content of the signal does not significantly change over the time that the signal is acquired, or when you want only the average energy at each frequency line. A large class of measurement problems falls in this category. For measuring frequency information that changes during the acquisition, use joint time-frequency analysis.

The measurement functions are built on top of the signal processing functions and have the following characteristics that model the behavior of traditional benchtop frequency analysis instruments.

- Assumed real-world, time-domain signal input
- Outputs in magnitude and phase, scaled in units where appropriate, ready for immediate graphing
- Single-sided spectrums from DC to , where fs is the sampling frequency
- Sampling period-to-frequency interval conversion for graphing with appropriate x-axis units, in hertz
- Corrections for the windows being used applied, where appropriate
- Scaled windows; each window gives the same peak spectrum amplitude result within its amplitude accuracy constraints
- Viewing of power or amplitude spectrum in various unit formats, including decibels and spectral density units, (V2/Hz, V/√Hz), and so on