System Design

The system design will be described in order starting from the antenna right through to the results display.

Antenna

See '3 m TVRO Dish Antenna': Configuration #1 - HI for details.

RTLSDR Dongle

The ubiquitous RTLSDR dongle is actually a DVBT USB-based receiver re-purposed by some clever people. They have made available a range of software which allows access to the 8-bit IQ RF data stream. This system uses a modified (by the author) version of the provided 'rtl_sdr.exe' console application.

The dongle itself has been modified to accept an external TCXO 28.8 MHz input - improving the stability of the sampling rate...


...and a tinplate (from the lid of a salmon tin) fin has been soldered to the pad below the RTL2832U chip onto which a small 5V fan blows air. The provision of this heatsink has been shown to improve the stability of the data stream - especially at the higher sampling rates.

An unmodified unit shown in the foreground.

Installation instructions can be found here. This is left as an exercise for the reader to achieve. That is, please do not ask the author for help with this - a non-response may offend... Remember - Google is your friend...

If you can get your dongle to work with SDRSharp then it should work with the console applications which can be used to capture data from the dongle and save to a file.

Details about RTLSDR and available applications can be viewed here.

Data Stream Interface

The IQ data is acquired via a USB interface connected to the RTLSDR dongle. General wisdom says that the dongle should not be plugged in directly into the PC USB port, but connected by a good-quality cable of a few metres in length. This is to reduce the level of RFI induced from the PC. The dongle is a USB 2.0 device, so any port capable of supporting that speed should suffice. If an external USB hub is used it is strongly recommended that it be a self-powered hub. Ideally a dedicated internal USB card would be used, with only the dongle plugged in to ensure unfettered access to bandwidth and power.

Data Acquisition Software

The data acquisition consists of a special version (produced by the author) of the standard console application 'rtl_sdr.exe' and a Windows C# GUI wrapper. The author has modified the source code and recompiled to produce a version of 'rtl_sdr.exe' which uses 64-bit variables - necessary to handle files > 4 Gb in size. At a nominal sampling rate of 2.4 Msps the data size produced is about 17 Gbytes/hour.

The modified 'rtl_sdr.exe' console application is called from within the Windows GUI application with parameters set via the GUI. The IQ data is saved in a binary unsigned 8-bit file identified by a number of parameters in the filename. These filename parameters are used by the data analysis software to identify the conditions under which the data was acquired - e.g., centre frequency, sampling rate, doppler correction, time of acquisition, hardware configuration, target object...

The GUI calculates the time when the target passes through the antenna beam and initiates a scheduled data run of a duration corresponding to the passage of the target through the beam.

Note: the GUIs shown here are for illustration purposes only. Please don't request copies - they are bespoke tools for the particular system setup here and I have neither the time nor the inclination to provide support. Sorry.

Data Analysis Software

The data analysis software is accessed through a separate GUI window. Here various analysis parameters can be set and the result displayed...

The data analysis GUI allows various graphical result display options including display limits, tilt correction, offset, running average. A typical result display is shown below...

Results can be exported to other programs via the clipboard or to a graphic file in various formats.