Once you move away from the usual software-defined radio (SDR) keys, you have only a few choices, unless you want to make serious money. One common hobby SPT is KiwiSDR. This popular module works with Linux and can accept up to 30 MHz. The platform uses a special A / D converter, FPGA and BeagleBone computer. Success, of course, gives rise to imitators, and especially when you have an open source design like the Kiwi, you’ll find similar devices with possibly different end goals. That’s how RaspberrySDR came about. This is a very similar unit to the KiwiSDR, but uses the Raspberry Pi, along with a few other differences. What is different? [KA7OEI] tells us in a recent blog post.
Aside from the obvious difference between the computer and everything it includes, the RaspberrySDR has a higher A / D speed (125 MHz vs. 66 MHz) and a 16-bit resolution instead of the 14-bit Kiwi. This combines to give Raspberry a wider reception range (up to 60 MHz) and – in theory – better performance in terms of dynamic range and distortion.
[KA7OEI] measured several key parameters of both devices and came to some surprising conclusions. The kiwi appears to amplify signals close to its cutoff frequency to compensate for system losses. Raspberry – using adapted software – seems to do the same trick, but does it around the Kiwi cutoff frequency, which is lower. Probably a software fix can take care of that, of course.
There are also tests for image rejection and front overload. Tests have revealed several problems with signal strength measurement and some other problems with RaspberrySDR. The biggest problem, however, was that 16-bit A / D didn’t look any better. Without proper design, throwing more bits in case of a problem is not always useful and this seems to be a good example of this.
In the end, Raspberry looks like a cheap kiwi clone with some advantages but also some disadvantages. The blog post also covers some open source issues, where Kiwi now says that some parts of their code will be binary in the future and there have been some difficulties in finding all the Raspberry files. If you want to buy one, you may not find the name “raspberrysdr”, but [KA7OEI] suggests looking for a “New 16-bit 62M real-time bandwidth network interface, shared SDR receiver”, which gives some results.
Of course, you can always use pi with a more conventional dongle, and it works well enough. If you want to make Pi just transmit, you can do it with a little more than wirealthough the quality may not be perfect.