Among its many tricks, the Raspberry Pi is capable of putting clock signals signal out on its GPIO pins, and that turns out to be just the thing for synthesizing RF signals in the amateur radio bands. What [Zoltan] realized, though, is that the resulting signals are pretty dirty, so he came up with a clever Pi shield for RF signal conditioning that turns a Pi into a quality low-power transmitter.
[Zoltan] stuffed a bandpass filter for broadband noise, a low-pass filter for harmonics, and a power amplifier to beef up the signal a bit into a tiny shield that is cleverly engineered to fit any version of the Pi. Even with the power amplifier, the resulting transmitter is still squarely in the realm of QRP, and the shield is optimized for use as a WSPR beacon on the 20-meter band. But there’s plenty of Pi software available to let hams try other modes, including CW, FM, SSB, and even SSTV, and other signal conditioning hardware for different bands.
Yes, these are commercially available products, but even if you’re not in the market for a shield like this, or if you want to roll your own, there’s a lot to learn from [Zoltan]’s presentation at the 2015 TAPR Digital Communications Conference (long video below). He discusses the difficulties encountered getting a low-profile shield to be compatible with every version of the Pi, and the design constraints that led to the decision to use SMT components.