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 spectra have been averaged.
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.
Source
A trigger source can be chosen for all selected channels in the FFT Analyzer tool. If the
selected source is Self, then each channel will trigger itself. This is the standard
behavior in earlier versions of SignalScope. If any other source is selected, then
all channels will be triggered from that same source.
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.