# 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, 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¶

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