In her latest video Sarah from the SignalsEverywhere YouTube channel describes how it is possible to use an RTL-SDR to measure RF filters when combined with a noise source and the Spektrum SDR software. In the video Sarah comprehensively explains how to set all the various parameters in Spektrum, before demonstrating a filter measurement with a noise source. Sarah explains how the power measurements may not be entirely accurate, however it is enough to get some idea about the shape of a filter.
Spektrum is a popular spectrum analyzer program that is used with RTL-SDR dongles. It is based on the command line rtl_power software and is compatible with both Windows and Linux. Thanks to it's easy to use GUI it is an excellent piece of software for scanning and determining where active signals exist, or for measuring filters and antenna SWR with a noise source.
Recently SV8ARJ (George) and SV1SGK (Nick) have been working on extending the original open source Spektrum code. Their improvements focus around the UI and making it more functional and easier to use. Currently the updated branch is in alpha, and they are hoping that any testers could help report bugs, issues and wishes to them. The code is available on their GitHub and the latest Windows test build can be downloaded from their DropBox.
The changelog reads:
- 2 Cursors for Frequency axis.
- 2 Cursors for Amplitude axis.
- Absolute and differential measurements with cursors.
- Zoom functionality of the cursors's defined area (gain + frequency).
- Mouse Wheel Gain adjustment on graph (Top area for upper, low area for lower).
- Mouse Wheel Frequency adjustment on graph (left area for lower frequency, right for upper).
- Mouse Wheel in the centrer of the graph performs symetric zoom in/out.
- View/settings store/recall (elementary "back" operation, nice for quick zoomed in graph inspection).
- Right click positions primary cursors.
- Right Double Click positions primary cursors and moves secondary out of the way.
- Left Double Click zooms area defined by cursors (Amplitude + frequency).
- Left Mouse Click and Drag on a cursor moves the cursor.
- Middle (mouse wheel) Double Click resets full scale for Amplitude and Frequency.
- Middle (mouse wheel) Click and Drag, moves the graph recalculating limits accordingly.
- Reset buttons to Min/Max range next to Start and Stop frequency text boxes.
- Cursor on/off checkbox now operate on all 4 cursors.
- ZOOM and BACK buttons.
- Filled-in graph option (line or area).
- Display of frequency, Amplitude and differences for all cursors.
- Modified: Button layout.
- Fixed: Save/Reload settings on exit/start. IMPORTANT : delete the "data" folder from the installation location if you have it.
- Filling in graph option (line or area).
Recently a reader of RTL-SDR.com, Pavel wrote in to let us know about a new program called “Spektrum” which he has written. Spektrum runs on Windows and Linux and turns an RTL-SDR dongle into a spectrum analyzer in a similar way to rtl_power GUI front ends and RTLSDR Scanner. However one key improvement is that it is based on a version of rtl_power that has been modified by Pavel in order to make it more responsive and remove the need to wait until a full sweep is completed before you can see any results. The modified version of rtl_power can be found at https://github.com/pavels/rtl-sdr.
Spektrum also has an additional “relative mode” feature. This allows Spektrum to be easily used together with a wideband noise source to measure things like filter characteristics and the VSWR of antennas. See our previous tutorial on this here, but note that in our tutorial we used Excel instead of Spektrum to do relative measurements.
The Processing language was used to create Spektrum and Pavel has also released his processing library for accessing rtl_power functionality over at https://github.com/
Ready to use releases of Spektrum for Windows and Linux 64-Bit OSes can be downloaded from https://github.com/pavels/spektrum/releases. Note that there may be a bug with the current release which causes only a gray window to show, but we’ve contacted the author about it and it may be fixed soon.