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Playing with Software Defined Radio

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Introduction

Most Ham Radios these days, receive signals through an antenna, convert the signal to digital, process the signal with a built-in computer, and then output the result converting back to analog for the speaker. This trend to doing all the radio signal processing in software instead of using electronic components is called Software Defined Radio (SDR). The ICOM 7300 is built around SDR as are all the expensive Flex Radios.

Inexpensive SDR

Some clever hackers figured out that an inexpensive chip used in boards to receive TV into a computer, could actually tune to any frequency. From this discovery, many inexpensive USB dongles have been produced that utilize this “TV Tuner” chip, but to tune radio instead of TV. This is possible because all this chip does is receive a signal from an antenna and then convert it to digital for the computer to process. I purchased the RTL-SDR dongle for around $30 which included a small VHF/UHF antenna.

I run Linux, both on my laptop and on a Raspberry Pi. I looked around for software to use with this device and found several candidates. I chose CubicSDR because it easily installed from the Ubuntu App store on both my laptop and on my Raspberry Pi.

I tried it first on the Pi, but it just didn’t work well. It would keep hanging and the sound was never good. I then tried it on my laptop and it worked great. This led me to believe that the Raspberry Pi just doesn’t have the horsepower to run this sort of system. Either due to lack of memory (only having 1Gig) or that the ARM processor isn’t quite powerful enough. Doing some reading online, the consensus seemed to be that you couldn’t run both the radio software and a GUI on the same Pi. You needed to either have two Pi’s or use a command line version of the software. I was disappointed the Pi wasn’t up to the challenge, but got along just fine using my laptop.

Enter the NVidia Jetson Nano

I recently acquired an NVidia Jetson Nano Developers Kit. This is similar to a Raspberry Pi, but with a more powerful quad-core ARM processor, 4Gig or RAM and 120 Tegra NVidia GPU processors (it also costs $99 rather than $35).

I installed CubicSDR on this, and it worked right away like a charm. I was impressed, getting software for the Nano can sometime be difficult since it runs true 64-Bit Ubuntu Linux on ARM, so you need to have that built. But CubicSDR was in the App Store and installed with no problem. I fired it up and it recognized the RTL-SDR and it recognized the NVidia Tegra GPUs. It took over 10 of them for doing its signal processing and worked really well.

Below is a screenshot of CubicSDR playing an FM radio station.

CubicSDR

CubicSDR is open source and free, it uses GNURadio under the covers (low level open source radio processing). CubicSDR has quite an impressive display. Like fancy high end radios you can see what is happening on the frequencies around where you are tuned in. The interface can be a bit cryptic and you need to refer to the documentation to do some things. For instance the volume, doesn’t honor the system setting and you have to use the green slider in the upper right. Knowing what the various sliders do is quite helpful. Tuning frequencies is a bit tricky at first, but once you check the manual and play with it, it becomes easy. Using CubicSDR really is like using a high end radio, just for a fraction of the cost.

It is certainly helpful to know ham terminology and to know what radio protocol is used where. For instance most VHF communications use narrow band FM. Most longer wavelength ham communications are either upper or lower sideband. Aeronautical uses AM. Commercial FM stations use wide band FM.

Antennas

Although the RTL-SDR supports pretty much any frequency, you need the correct antenna for what you are doing. The ham bands that bounce off the stratosphere to allow you to talk to people halfway around the world use quite long wavelengths. The longer the wavelength, the larger the antenna you need to receive them. Don’t expect to receive anything from the 20 meter band without a good sized antenna. That doesn’t mean it has to be expensive, you can get good results using a dipole or end-fed antenna, both of these are just made out of wires, but you do have to string them high up and facing the right direction.

What About Transmitting?

This RTL-SDR only receives signals. If you want to transmit as well, then you need a more expensive model. These sort of SDR transmitters are very low power, so if you want to be heard, you will need a good linear amplifier, rated for the frequencies you want to use. You will also need a better antenna.

If you transmit you also require a ham radio license and call sign. You are responsible for not causing interference and that you signal doesn’t bleed through to adjacent channels. Since you are assembling this all yourself, an advanced license is required.

Summary

SDR is great fun to play with and there are lots of great projects you can create with this and an inexpensive single board computer. It’s too bad the Raspberry Pi isn’t quite up to the task. However, more powerful Pi competitors like the Jetson Nano run SDR just fine.

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Written by smist08

April 16, 2019 at 2:08 am