Nuts and Volts - July 2015

Figure 3 is a photo of the DVB-T stick we are using and a short interface cable that was bought separately. Most DVB-T sticks have two main chips inside: a digitally controlled tuner and an ADC that samples the baseband signal and outputs the samples to a host computer through a USB port.

A very specific DVB-T stick is used in this project — one that has a Rafael Micro R820T tuner IC and a Realtek RTL2832U inside. These so-called RTL sticks are quite common and available from many sources. Beware, though, as some sticks have an E4000 tuner or some other kind of tuner IC inside. Those will not work in this project. Make sure your stick has an R820T inside. The R820T tuner is crucial because it has the lowest tuning range — tunable down to 24 MHz. Other tuners do not have that low-end range. As we will see later on, this capability makes our shortwave up-converter easy to build.

What makes the RTL stick so valuable is its inherent ability to demodulate FM signals and transfer the amplitude and phase information as raw in-phase and quadrature phase (I/Q) samples to the computer via USB. Annti Palosaari — a Finnish engineering student and Linux developer — discovered this special radio mode. Amazingly, this mode enables the tuner to output a stream of eight-bit I/Q samples at rates up to two million samples per second.

Once this discovery was grasped, enthusiasm grew for the development of a cheap SDR. The group from Open Source Mobile Communication (Osmocom) — particularly Steve Markgraf — developed a basic set of drivers and utilities to communicate with the RTL dongle. After that, other software developers began writing code to use these drivers and provide user interfaces. One of those programs, SDR# (pronounced SDR Sharp) is probably the most popular one. We’ll discuss it in more detail later on.

RF Converter and Assembly Details

The front end of our shortwave receiver is a frequency up-converter. An up-converter is a circuit that adds a constant frequency to the received frequency — the one received at the antenna. An RF mixer and local oscillator can be used to make a simple up-converter. If we use a local oscillator frequency of 24 MHz, then the output of the mixer will contain the received signal plus 24 MHz. Both the local oscillator and the mixer functions can be handled by a general-purpose SA612 device.