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Spectrum Analyzer Tab

The Spectrum Analyzer is one of the powerful frequency domain measurement tools as introduced in Unique Set of Analysis Tools and is available on all MFLI instruments.


  • Fast, high-resolution FFT spectrum analyzer
  • Signals: demodulated data (X+iY, R, Θ, f and dΘ/dt/(2π) ), PID, Boxcar, Auxiliary Inputs, and more
  • Variable center frequency, frequency resolution and frequency span
  • Auto bandwidth
  • Waterfall display
  • Choice of 4 different FFT window functions
  • Continuous and block-wise acquisition with different types of averaging
  • Detailed noise power analysis
  • Support for Input Scaling and Input Units
  • Mathematical toolbox for signal analysis


The Spectrum Analyzer provides frequency domain analysis of demodulator data. Whenever the tab is closed or an additional one of the same type is needed, clicking the following icon will open a new instance of the tab.

Table 1: App icon and short description
Control/Tool Option/Range Description
Spectrum Provides FFT functionality to all continuously streamed measurement data.

The Spectrum tab (see Figure 1) is divided into a display section on the left and a configuration section on the right. The configuration section is further divided into a number of sub-tabs.

Figure 1: LabOne UI: Spectrum analyzer tab

The Spectrum Analyzer allows for spectral analysis of all the demodulator data by performing the fast Fourier transform (FFT) on the complex demodulator data samples X+iY (with i as the imaginary unit). The result of this FFT is a spectrum centered around the demodulation frequency, whereas applying a FFT directly on the raw input data would produce a spectrum centered around zero frequency. The latter procedure corresponds to the Frequency Domain operation in the Scope Tab. The main difference between the two is that the Spectrum Analyzer tool can acquire data for a much longer periods of time and therefore can achieve very high frequency resolution around the demodulation frequency. By default, the spectrum is displayed centered around zero. Sometimes however it is convenient to shift the frequency axis by the demodulation frequency which allows one to identify the frequencies on the horizontal axis with the physical frequencies at the signal inputs. This can be done by activating Absolute Frequency on the Settings sub-tab.

By default, the display section contains a line plot of the spectrum together with a color waterfall plot of the last few acquired spectra. The waterfall plot makes it easier to see the evolution of the spectrum over time. The display layout as well as the number of rows in the color plot can be configured in the Settings sub-tab.

Data shown in the Spectrum tab have passed a low-pass filter with a well-defined order and bandwidth. This is most clearly noted by the shape of the noise floor. One has to take care that the selected frequency span, which equals the demodulator sampling rate, is 5 to 10 times higher than the filter bandwidth in order to prevent measurement errors due to aliasing. The Auto Bandwidth button adjusts the sampling rate so that it suits the filter settings. The Spectrum tab features FFT display of a selection of data available in the Signal Type drop-down list in addition to the complex demodulator samples X+iY. Looking at the FFT of polar demodulator values R and Theta allows one to discriminate between phase noise components and amplitude noise components in the signal. The FFT of the phase derivative dΘ/dt provides a quantitative view of the spectrum of demodulator frequencies. That is particularly useful in conjunction with the PLL or the ExtRef functionalities. The FFT of the frequency samples then provide a quantitative view of what frequency noise components are present in the reference signal and also helps to find the optimal PLL bandwidth to track the signal. Note that many of the signals in the Signal Type list are real-valued, rather than complex-valued. Their spectra are single-sided with minimum frequency of 0 Hz.

Functional Elements

Table 2: Spectrum tab: Settings sub-tab
Control/Tool Option/Range Description
Run/Stop Run the FFT spectrum analysis continuously
Single Run the FFT spectrum analysis once
Center Freq (Hz) numeric value Demodulation frequency of the selected demodulator used as input for the spectrum. For complex FFT(X+iY) the demodulation frequency defines the center frequency of the displayed FFT.
Frequency Span (Hz) numeric value Set the frequency span of interest for the complex FFT. A FFT based on real input data will display half of the frequency span up to the Nyquist frequency.
Auto Bandwidth Automatic adjustment of the demodulator bandwidths to obtain optimal alias rejection for the selected frequency span which is equivalent to the sampling rate. The functionality is only available if the spectrum is enabled.
Start Frequency (Hz) numeric value Indicates the start frequency of the FFT.
Stop Frequency (Hz) numeric value Indicates the end frequency of the FFT.
Refresh Rate (Hz) numeric value Set the maximum plot refresh rate. The actual refresh rate also depends on other parameters such as FFT length. In overlapped mode the refresh rate defines the amount of overlapping.
Overlapped FFT ON / OFF Enable overlapped FFTs. If disabled, FFTs are performed on distinct abutting data sets. If enabled, the data sets of successive FFTs overlap based on the defined refresh rate.
Power ON / OFF Calculate and show the power value. To extract power spectral density (PSD) this button should be enabled together with spectral density.
Spectral Density ON / OFF Calculate and show the spectral density. If power is enabled the power spectral density value is calculated. The spectral density is used to analyze noise.
Filter Compensation ON / OFF Spectrum is corrected by demodulator filter transfer function. Allows for quantitative comparison of amplitudes of different parts of the spectrum.
Absolute Frequency ON / OFF Shifts x-axis labeling to show the demodulation frequency in the center as opposed to 0 Hz, when turned off.
FFT length numeric value The number of samples used for the FFT. Values entered that are not a binary power are truncated to the nearest power of 2.
Sampling Progress 0% to 100% The percentage of the FFT buffer already acquired. The progress includes the number of rows and averages.
FFT Duration (s) numeric value Indicates the length in time of the samples used for a single FFT.
Window Cosine squared (ring-down) Several different FFT windows to choose from. Depending on the application it makes a huge difference which of the provided window function is used. Please check the literature to find out the best trade off for your needs.
Blackman Harris
Flat Top
Exponential (ring-down)
Cosine (ring-down)
Resolution (Hz) mHz to Hz Spectral resolution defined by the reciprocal acquisition time (sample rate, number of samples recorded).
Rows numeric value Number of rows
Averages numeric value Number of FFT averaged for each row. Setting the value to 1 will disable any averaging.
Waterfall ON / OFF Enable to show the 2D plot in waterfall mode. It will always update the lowest line.
Overwrite ON / OFF Enable to overwrite the grid in continuous mode. History will not be collected. A history element will only be created when the analysis is stopped.
Plot Type Select the plot type.
None No plot displayed.
2D Display defined number of grid rows as one 2D plot.
Row Display only the trace of index defined in the Active Row field.
2D + Row Display 2D and row plots.
Active Row integer value Set the row index to be displayed in the Row plot.
Track Active Row ON / OFF If enabled, the active row marker will track with the last recorded row. The active row control field is read-only if enabled.
Palette Solar Select the colormap for the current plot.
Colorscale ON / OFF Enable/disable the colorscale bar display in the 2D plot.
Mapping Mapping of colorscale.
Lin Enable linear mapping.
Log Enable logarithmic mapping.
dB Enable logarithmic mapping in dB.
Scaling Full Scale/Manual/Auto Scaling of colorscale.
Clamp To Color ON / OFF When enabled, grid values that are outside of defined Min or Max region are painted with Min or Max color equivalents. When disabled, Grid values that are outside of defined Min or Max values are left transparent.
Start numeric value Lower limit of colorscale.

Only visible for manual scaling.
Stop numeric value Upper limit of colorscale.

Only visible for manual scaling.

For the Math sub-tab please see the table "Plot math description" in the section called "Cursors and Math".