Photo: Nicholas Eveleigh
$40 Software Defined Radio
A versatile TV tuner can display a wide spectrum
Author: Stephen Cass
The last time I ventured into the world of Software Defined Radio (SDR) was seven years ago when I reviewed Matt Ettus’s Universal Software Radio Peripheral. While it was an excellent product, the basic motherboard cost $550 at the time; daughterboards for different frequency ranges ranged from $75 to $275. I spent quite some time compiling the necessary software on my MacBook Pro. This time, I was able to get it done for just $40, with software installation and running taking about 2 minutes.
Antti Palosaari, a student of engineering in Finland and a Linux developer, made this little miracle possible. Last year, he discovered the unexpected features of the RTL2832U demodulator chip manufactured by Taiwan’s Realtek: The RTL2832U chip can also output raw digital streams describing amplitude and phase for decoding HDTV broadcasts in cheap USB dongle receivers. It calls I/Q data of signals over a wide frequency range.
Digital radio enthusiasts immediately began to adopt open-source tools to convert I/Q information into audio and data streams.The result is that low-cost SDR can adopt various modulation schemes to receive a wide variety of transmissions, including stereo FM from broadcasters, digital data packets from aircraft transponders, and single sideband (SSB) signals from amateur radio enthusiasts. Of course, the system is not as sensitive as dedicated SDRs and cannot transmit signals, but it is sufficient to see what is happening across a large spectrum.
Cheap Software Defined Radio: With some inexpensive hardware and free software, you can listen to digital and analog signals across various radio spectra.
Different receiver dongles pair the RTL2832U with different radio tuners, so the exact frequency range that can be received varies. I bought a Freeview P250 dongle from a Chinese supplier for $20, including shipping. The P250 combines the RTL2832U with the Elonics E4000 tuner, capable of receiving signals from about 52 MHz to 2.2 GHz, with gaps around 1.1 to 1.25 GHz.
The receiver came with a small antenna, but I replaced it with a $15 set of rabbit ears from RadioShack. Connecting the rabbit ears’ US coax cable to the dongle’s European socket adapter cost a few dollars.
To use the receiver on my MacBook Pro, I downloaded Elias Önal’s Gqrx software receiver port. Önal’s port is precompiled for OS X, so installation was as simple as downloading it to my hard drive. The application automatically detected my receiver, and I was up and running.
Gqrx is centered around a display similar to an oscilloscope, showing a slice of the radio spectrum (and a waterfall display tracking the last 30 seconds or so). Gqrx allows you to set the width of the slice, ranging from 1 to 2.4 MHz. You can select the frequency to be passed to the software demodulator by clicking on that frequency in the oscilloscope display. Demodulation modes include AM, narrowband FM, mono and stereo FM, SSB, and CW (for Morse code).
Viewing radio spectrum: This screenshot from Gqrx shows two FM stations. The central peak is an analog stereo broadcast, while the square signals on either side are digital radio transmissions. You can listen to and tune into the station by clicking on its center frequency. The gray band indicates the bandwidth of the software demodulator selected by the user.
Because the receiver can see so much spectrum simultaneously, you can use it to monitor activity on multiple channels at once. For example, in Boston, where I live, there are 460 narrowband FM channels ranging from 460.025 to 460.500 MHz, covering various regions, etc. The peaks on the display indicate when using any of these channels, clicking the mouse will play audio on my speakers.
This raises regulatory issues. In some countries, it is illegal to receive any frequency for which you do not have a license, except for public broadcast frequencies. In the US, you are free to receive almost any signal you can pick up. However, this general rule has some important exceptions, such as prohibiting listening to mobile phone frequencies or operating devices that can receive police signals in the car (the latter requires an amateur radio license).
I quickly found it cumbersome to connect the dongle and TV antenna to my laptop, and my home office doesn’t always get good reception. So I spent $35 on a Model B Raspberry Pi microcontroller.
The Raspberry Pi is an ARM-based microcontroller that supports Ethernet and has USB connectors, capable of running various versions of Linux. Following the instructions on the Ham Radio Science website, I was able to download and compile some supporting software to use the Pi and dongle (connected via a powered USB hub) in about 30 minutes. In turn, I connected the Pi to my home network hub in the front room via an Ethernet cable. Using the Pi allows me to place the receiver away from local radio sources (like the Wi-Fi transmitter of my hub) and also allows multiple machines to easily access the receiver; the Pi acts as a centralized SDR server through a command-line utility called rtl_tcp.
With the Pi running, I was able to call Gqrx on my Wi-Fi-connected MacBook, input the Pi’s network address, and control and decode signals as if the tuner were plugged directly into my laptop. (Admittedly, this represents a certain level of engineering overkill when it comes to listening to local FM stations.)
Now that I have the basic system up and running, I want to extend the bottom of the receiver’s range to longer bands, such as the popular amateur 20-meter band between 14.00 and 14.35 MHz. This would require modifying the dongle or purchasing or building a downconverter. However, unless I swap out my rabbit ears for a long-wave antenna, which is another kettle of fish for homebrewing, neither of these methods makes much sense.
https://spectrum.ieee.org/geek-life/hands-on/a-40-softwaredefined-radio
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