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
The front end of our
shortwave receiver is a
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
If you search the
Internet, you will see
some designs that use
42 July 2015
FIGURE 3. RTL tuner (with R820T and Realtek
RTL2832U inside) and BNC adapter cable.
FIGURE 4. Detailed schematic — shortwave radio
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