Toontje wrote:The Pi has GPIO ports to connect the CW keyer to.
The Pi can transmit 10mW into an antenna (yes, license and pass-band filter required, i know).
Pi can run SDR (RTL-SDR?) hard and software.
Why has nobody created a very low powered CW transceiver yet?
If you're talking about the built-in programmable frequency generator - and using it to generate RF - it will be much too noisy to use directly as part of a QRP rig. No bandpass filter will be sufficient. AFAIK it's meant to be a wired clock generator, it's square wave, and it's incremented over the entire spectrum. You're definitely looking to have bad and/or lots of spurious output if using it to generate RF.
If you're talking about using software on the Pi to generate appropriate signals, and send them via a good audio subsystem (like the Wolfson/Cirrus boards) - to an outboard SDR transmitter (the TX hardware part of the SDR), then yes, the Pi is very good for that. I have used a Pi2 in combination with the Cirrus audio board, python based SDR software, and an inexpensive SDR receiver (the RX hardware part of the SDR) to receive shortwave, ham, and weather fax (also shortwave) signals. Works well, and I have pics on my pages (look for the "Using a Pi with Shortwave" post).
Note: the 10 mW would be a very significant challenge anyway. Most QRP >= 1 watt. I'm assuming that you're a ham, given the mention of CW. Some people use the RTL dongles for SDR, but for any frequency where CW is used (significantly), you'd also need an HF down-converter.