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.
Features¶
- 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
Description¶
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.
| 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.
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¶
| 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. |
| Power Correction | ON / OFF | When activated, applies power correction to the spectrum to compensate for the shift that the window function causes. Power correction is useful for noise measurements to correct the noise floor. When deactivated, amplitude compensation is applied which corrects the peak amplitudes of coherent tones. |
| 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. |
| Rectangular | ||
| Hann | ||
| Hamming | ||
| 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. |
| Viridis | ||
| Inferno | ||
| Balance | ||
| Turbo | ||
| Grey | ||
| 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".