Once you move away from the usual software defined radio (SDR) dongles, you have only a few choices unless you want to drop some serious cash. One common hobby-grade SDR is the KiwiSDR. This popular unit runs Linux and can receive up to 30 MHz. The platform uses a dedicated A/D converter, an FPGA, and BeagleBone computer. Success of course breeds imitators, and especially when you have an open source design like the Kiwi, you are going to find similar devices with possibly different end goals. That’s how the RaspberrySDR came to be. This is a very similar unit to the KiwiSDR but it uses a Raspberry Pi, along with a handful of other differences. What’s different? [KA7OEI] tells us in a recent blog post.
Other than the obvious difference of the computer and all that it entails, the RaspberrySDR has a higher speed A/D (125 MHz vs 66 MHz) and 16-bits of resolution instead of the Kiwi’s 14 bits. This combines to give the Raspberry a wider receive range (up to 60 MHz) and — in theory — better performance in terms of dynamic range and distortion.
[KA7OEI] measures a few key parameters on both devices and arrived at some surprising conclusions. The Kiwi appears to boost signals near its cutoff frequency to compensate for losses in the system. The Raspberry — using adapted software — looks as though it does the same trick, but does it around the Kiwi’s cutoff frequency, which is lower. Probably a software fix could take care of that, of course.
There are also tests of image rejection and front-end overloading. The tests revealed a few problems with signal strength measurement and some other problems with the RaspberrySDR. The biggest issue, though, was that the 16-bit A/D didn’t seem to have better performance. Without proper design, throwing more bits at a problem isn’t always helpful and this appears to be a good example of that.
In the end, the Raspberry looks like a cheap clone of the Kiwi with some benefits, but also some drawbacks. The blog post also covers some open source issues where Kiwi is now saying some parts of their code will only be binary in the future and there has been some difficulty finding all of the Raspberry’s files. If you are looking to buy one, you might not find the name “raspberrysdr” but [KA7OEI] suggests searching for “New 16bit 62M real-time bandwidth network shared SDR receiver” which does turn up some results.
Of course, you can always use a Pi with a more conventional dongle, and that works well enough. If you want to make a Pi just transmit, you can do that with little more than a wire, although the quality might not be perfect.