Raspberry Pi as QRP (very QRP) CW transceiver?

[Toontje]

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?

Ton.

[unclejed613c]

a pi3 has just enough horsepower to do sdr, and using a HackRF would be the best way to do it. i've installed linrad on a pi3, and used it as a sdr receiver, but haven't tried out the transmit functions yet. the HackRF has 8 bit converters, so some filtering is required, but is capable of more than just cw.

[Ronaldlees]

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?

Ton.

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.

[Toontje]

You know, i have been receiving so many posts on why not to do this and i agree with all of them. I ran WSPR on the Pi and it screwed up all the other bands, so that was a very short but indeed interesting experiment.
The idea is not to build a 10mW transceiver for production use. I was just wondering, given that all the components are available on a standard Pi, why nobody ever built it. For the sake of proving that you can, not for production use. There are much better and cheaper alternatives out there to do CW without the problems the Pi has.

[Ronaldlees]

We have SDR/RX covered pretty well, with the RTL dongles as well as some other very inexpensive but more dedicated alternatives (all <=$20). The holy grail right now is cheap SDR/TX. This is somewhat ironic, as SDR incorporates the idea of putting half the radio into software (software=free, usually) - and still the price remains high! The least expensive transceiver kits are around $170, and most homebrew BOM lists tally up to $100 or more.
Go figure ...

[Toontje]

Pixies are $7. Limited, but still, as an experiment it should be a nice board to hook up to the Pi. Hook up the iambic keyer to the GPIO of the Pi, let the Pi generate the dits and dahs, hand those over to the Pixie and on the air it goes.

[Ronaldlees]

Pixies are $7. Limited, but still, as an experiment it should be a nice board to hook up to the Pi. Hook up the iambic keyer to the GPIO of the Pi, let the Pi generate the dits and dahs, hand those over to the Pixie and on the air it goes.

I've seen those advertised for a while, and even had a number of QSOs with hams who were using them. They're a little too minimalist for me though. I'm considering mating my Pi2 powered SDR receiver tablet with a simple (cheap) ~$35 range 5 watt VXO transmitter. The station would have only the RX half of an SDR radio, but would be completely Pi2 controlled (The Pi could still control frequency and power on TX, and on RX the Pi could control frequency and gain. It's just that the TX signal would not be processed in software. A half step.

For around another $35-40 it could be a DDS controlled transmitter.

[MartinGrrrr]

Well, the RXTX ensemble from fivedash http://fivedash.com/index.php?main_page ... 2b8dd23ae1 costs only 90$ and is capable to use the Pi3 as a full blown transceiver, not just CW but also other digital modes and SSB as well...

So, get yours and wire it up, even CW-keying (the RTXTX ensemble has a separate CW keyer port!) through a GPIO-pin is doable...

[ab1jx]

It seems like a good old fashioned tank circuit (resonant coil and capacitor), or maybe 2 stages might clean up square wave RF enough to be usable especially at QRP. You could even come up with a bandswitching arrangement like the front end of a superhet receiver in reverse to get on different bands. A tank is a little like a wheel, it has its own resonant frequency and it rejects inputs much above or below that. Junkbox parts, and nothing very fancy, could work. You'd need a dip meter to build/tune it the first time though.