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 HF2 Series instruments.
Features

Fast, highresolution 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 blockwise 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 subtabs.
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 subtab.
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 subtab.
Data shown in the Spectrum tab have passed a lowpass filter with a welldefined 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 dropdown 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 realvalued, rather than complexvalued. Their spectra are singlesided 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. 
Absolute Frequency 
ON / OFF 
Shifts xaxis 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 
Rectangular 
Seven 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. 
Hann 

Hamming 

Blackman Harris 

Exponential (ringdown) 

Cosine (ringdown) 

Cosine squared (ringdown) 

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 readonly if enabled. 
Palette 
Viridis 
Select the colormap for the current plot. 
Inferno 

Balance 

Turbo 

Grey 

Solar 

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 subtab please see the table "Plot math description" in the section called "Cursors and Math".