FFT Analyzer Options

Spectrum Options

  • Frequency Resolution
    SignalScope simplifies FFT configuration by allowing you to choose a frequency resolution for narrowband spectral analysis. SignalScope then automatically calculates the FFT length required for that frequency resolution at the audio input hardware's current sample rate. It should be noted that in this case, frequency resolution refers to the distance between adjacent spectral lines (or frequency bins) in the spectrum. The smallest frequency difference over which two independent sinusoids can be resolved will also remain a function of the FFT analyzer's data window type.
  • Data Window
    SignalScope provides a selection of different data windows that may be applied to the time domain data, before the application of the FFT.
    See the Data Window Selection page for more information on available windows and user-defined parameters.
    When the Uniform data window is selected, the FFT Analyzer will display the overall level of the signal in the window represented by the current spectrum.
  • Overlap %
    It takes time to gather enough data to compute an FFT, so for longer FFT's the spectrum analyzer will update less frequently as it waits for data to be gathered for each FFT. Overlap processing allows the spectrum analyzer to be updated more frequently, which makes for a smoother response. The overlap percentage can be manually specified with one of the fixed values (50% overlap means the analyzer will be updated twice as often and 90% overlap will produce updates 10 times as often). When Overlap % is set to Max, SignalScope will automatically compute an overlap percentage that allows the spectrum analyzer to be updated as fast as reasonably possible for the selected sample rate and frequency resolution. Unless you need a specific amount overlap, it is generally recommended that Overlap % be set to either Off or Max. High overlap, combined with certain frequency resolutions, can consume too much available processing time with no apparent benefit.
  • Vibration
    If an accelerometer is in use (that is, if the input channel units are set to "g" or "m/s^2"), then the acceleration signal can be optionally integrated once or twice to produce velocity or displacement values, respectively. The Vibration option allows Acceleration, Velocity, or Displacement to be specified for vibration measurements in the FFT Analyzer.
    • If units are set to "g," velocity units will be "ips" (inches per second) and displacement units will be "mils."
    • If units are set to "m/s^2," velocity units will be "mm/s" and displacement units will be "mm."
  • Min Frequency (Fmin)
    A minimum frequency can be specified for the spectrum display. Only values that correspond to frequencies between Fmin and Fmax will be plotted. If Export Full Spectrum is disabled, only those same values will be included when exporting spectral data to a file. The Cursor Peak Track function will operate within the bounds of Fmin and Fmax.
  • Max Frequency (Fmax)
    A maximum frequency can be specified for the spectrum display. Only values that correspond to frequencies between Fmin and Fmax will be plotted. If Export Full Spectrum is disabled, only those same values will be included when exporting spectral data to a file. The maximum frequency can be specified independent of the hardware sample rate. The Cursor Peak Track function will operate within the bounds of Fmin and Fmax.
  • Auto Adjust Sample Rate
    When this option is enabled, the hardware sample rate will be automatically adjusted to an optimal value for the current Fmax. For example, if Fmax is set to a low frequency (e.g. less than 4 kHz), the sample rate will automatically be set to 8 kHz, if the hardware supports it (8 kHz is the standard minimum sample rate for iOS devices and many USB audio devices).
  • Export Full Spectrum
    When this option is enabled, spectral data exported to a file will include the entire spectrum (DC to Nyquist). If the spectrum is being displayed on a log frequency scale, the DC term may be omitted in the exported data.

Averaging

  • Averaging
    Successive spectra for each signal may be averaged according to the selection of this segmented control. Available average types include: Linear (Stable), Exponential, and Peak Hold. Linear averaging weights all spectra equally. Exponential averaging applies an exponential weighting such that more recent spectra contribute to the current average value more than older spectra. Peak Hold keeps the maximum value in each frequency bin over all the spectra included in the measurement (it is not a true average). When either linear or peak hold averaging is selected, the FFT analyzer will automatically stop after the specified number of averages has been reached.
  • Averages
    The number of spectra to include in the current average is specified in the Averages text box. When linear or peak hold averaging is the selected average type, the FFT Analyzer will automatically stop after the specified number of spectra have been included in the average. Exponential averaging allows the analyzer to continue running even after the specified number of spectra have been processed. The exponential average can be thought of as including only the most recent N (time-weighted) spectra, where N is the specified number of averages.

Triggering

  • Enable
    Use this switch to turn input triggering on or off.
  • Mode
    SignalScope offers three trigger modes: Normal, Auto, and Single Shot. In Normal mode, the displayed spectrum remains frozen until a trigger event occurs. In Auto mode, the display continues to be updated whether or not a trigger event occurs, so the signal can still be monitored even if it is not triggered. In Single Shot mode, the display will continue to be updated until a trigger event occurs. Once the trigger event occurs, the signal display will be frozen. Single Shot mode is convenient for capturing transient events with the oscilloscope.
  • Slope
    A trigger event occurs when a source signal exceeds some user-defined threshold value, with a user-defined slope (positive or negative). You may specify the slope by selecting Positive or Negative in the segment control.
  • Threshold
    A trigger event occurs when a source signal exceeds some user-defined threshold value, with a user-defined slope (positive or negative). You may specify the threshold value by tapping on the text box and entering a desired value.

Data Export

  • Current Location
    When this option is enabled, the latitude, longitude, and altitude at the time of the measurement will be included in exported data files. Horizontal and vertical accuracy (in meters) will also be included.

Loaded Measurements

  • Trace Opacity
    Use this slider control to adjust the opacity of static data traces (loaded from data files) in the plot.
  • Trace Thickness
    This segmented control can be used to select the thickness of the static frequency data traces in the plot.
  • Load From Data File
    Up to 7 static frequency data traces may be displayed in the plot by loading them from data files that were previously exported from the FFT Analyzer tool.
  • Add New Average Curve
    Static FFT data traces may also be created by loading data from 1 or more FFT data files and averaging the data to create a new spectrum. The loaded files must contain data with the same measurement units. When source data with differing frequency resolutions is loaded, the resolution of the new average curve will be defined by the largest (coarsest) resolution of all spectra included in the average.