2.1.2. Spectrum Analyzer

This application will turn your RedPitaya board into a 2-channel DFT Spectrum Analyzer. It is the perfect tool for educators, students, makers, hobbyists, and professionals seeking affordable, highly functional test and measurement equipment. The DFT Spectrum Analyzer application enables a quick and powerful spectrum analysis using a DFT algorithm.

The frequency span is from DC up to 62.5 MHz, where the frequency range can be arbitrarily selected. You can easily measure the quality of your signals, signal harmonics, spuriousness, and power. All Red Pitaya applications are web-based and do not require the installation of any native software. Users can access them via a web browser using their smartphone, tablet, or a PC running any popular operating system (MAC, Linux, Windows, Android, and iOS). The elements of the DFT Spectrum analyzer application are arranged logically and offer a familiar user interface.

The graphical interface is divided into four main areas:

1. Run/Stop and Export button: The “Run/Stop” button is used to start and stop measurements. With the “Export” button, you can select in which format you want to download the measured data (plotted spectrum). Two formats are available: .png and .csv.

2. Inputs / Cursors / Range / Axis control panel: This menu provides controls for inputs, cursors, and frequency range settings. Horizontal +/- buttons are used to select the span of the X (frequency) axis (zooming in/out). The vertical +/- buttons change the Y (amplitude)-axis range.

3. Graph area: Here, the currently calculated signal spectrum is plotted in the selected frequency range.

4. Waterfall plots: Waterfall plots are a different way of representing the signal spectrum where the colour of the plot defines the signal amplitude for a certain frequency. The waterfall plot is also useful to enable the representation of a signal spectrum in a time-dependent fashion.

2.1.2.1. Features

The main features of the DFT Spectrum analyser are described below:

2.1.2.1.1. INPUTS:

Input controls are shown in the picture below. With the “SHOW” select button, displaying the spectrum of the selected input can be enabled or disabled. The “FREEZE” button is used to stop the measurements of the selected input. The “MIN” and “MAX” select buttons are used to enable or disable the persist mode for the spectrum plot. The “MIN” signal spectrum plot will show the lowest values of the signal spectrum taken after enabling the “MIN” button. The same logic is used for the “MAX” signal, where the MAX values of the signal spectrum are shown. This feature is mostly used for detecting signal glitches and calculating the max/min spectrum amplitude values during the measurement.

For SIGNALlab 250-12, there are additional settings available where the user can select:

• Input attenuation

• Coupling

2.1.2.1.2. CURSORS:

The cursors are an additional vertical and horizontal pair of lines useful for extracting the values of the spectrum plots.

The cursors are interactive, and they can be set on any part of the graph while the frequency value is shown corresponding to the place where the X cursors are set and the amplitude value where the Y cursors are set. Cursor delta values are useful for measuring signal harmonics and relative ratios between amplitudes and frequencies.

2.1.2.1.3. RANGE:

The range settings are used to set a frequency span. This feature is useful when the frequency range of interest is smaller than the full frequency range of the Spectrum analyzer application.

2.1.2.1.4. PEAK DETECTION:

During the measurement, peak values of the signal spectrum are measured and shown in the “Peak Values” field. Peak values are the max values of the signal spectrum regardless of the selected frequency range. This peak finding prevents not seeing peak values that are outside the selected frequency span.

2.1.2.1.5. WATERFALL PLOTS:

Waterfall plots are a different way of representing the signal spectrum where the colour on the plot defines the signal amplitude for a certain frequency. The waterfall plot is also useful when enabling the representation of the signal spectrum in a time dependency.

2.1.2.1.6. AXIS CONTROLS:

Horizontal +/- buttons are used to select the span of the X (frequency) axis (zooming in/out). The vertical +/- buttons change the Y (amplitude)-axis range. When the Reset button is pressed, the frequency and amplitude span are reset to their default values.

2.1.2.1.7. OUTPUTS:

The Spectrum Analyzer applications also include a signal generator, so users can simultaneously generate a signal and observe the signal spectrum. For the signal generator settings and specifications, refer to outputs.

2.1.2.1.8. External reference clock (SIGNALlab 250-12 only):

The external reference clock input can be enabled through the settings menu. Once enabled, its status is displayed in the main interface. The “green” status indicates that the sampling clock is locked to the external reference clock.

2.1.2.2. Specifications

	STEMlab 125-10 (discontinued)	STEMlab 125-14	STEMlab 125-14 4-Input	SDRlab 122-16	SIGNALlab 250-12
Input channels	2	2	4	2	2
Bandwidth	0 - 50 MHz	0 - 60 MHz	0 - 60 MHz	0 - 60 MHz	0 - 60 MHz
Resolution	10 bit	14 bit	14 bit	16 bit	12 bit
DFT buffer	16384	16384	16384	16384	16384
Dynamic Range	60 dB	80 dB	80 dB	96 dB	74 dB
Input noise level	< -100 dBm/Hz	< -119 dBm/Hz	< -119 dBm/Hz
Input range	10 dBm	10 dBm	10 dBm	-2 dBm	10 dBm (when att. is disabled)
Input impedance	1 MΩ / 10 pF	1 MΩ / 10 pF	1 MΩ / 10 pF	50 Ω	1 MΩ / 10 pF
Input coupling	DC	DC	DC	AC	AC/DC
Spurious frequency components	< -70 dBFS Typically	< -90 dBFS Typically	< -90 dBFS Typically

2.1.2.3. Source code

The Spectrum analyzer source code is available on our GitHub.