Multi-mode Beacon on the RPi

Inspired by some comments from Jim on Bill Meara's Soldersmoke Blog, I dusted off the m0xpd PayPal account (again!!!) and splashed out on one of the AD9850 DDS Modules from eBay...

Of course, you also need some kind of controller to make any use of these little synth modules. Fortunately, inspired by their beer-budget price, the modules are generating quite a buzz at the moment, so there's a lot of supporting software out there.

Andy Talbot, g4jnt, has written about the eBay modules in the current number of RadCom (but the dear old RSGB hasn't got around to updating their website at the time of blogging, so the link points to the previous edition - funny how the paper copy can be more up -to-date than electronic news!).

You can find Andy's PIC-based controller on-line - start with his description here .

Rather than follow the rest of the flock with a PIC controller (having already played with a PIC controller for DDS devices), I thought I'd like to use my new Raspberry Pi, so I had a further sniff about the 'net and found Ron, nr8o's blog, where he presents an Arduino sketch for setting up the AD9850.

I hacked that Arduino sketch into a piece of Python code for the Pi, which might be useful for others... 



Having got the synth to run, I wanted something to do with it - so I built up a "beacon" transmitter...

You can see (from the left) my GPIO breakout (here in the "High Rollers' edition"), then a quad bi-directional level converter module I made to couple the 3V3 signals from the RPi into the 5V world. Next there's the eBay module.

Its signal goes to a little QRP PA I knocked together (drawing on the PA stage in my "Funster Plus" rig) and a low-pass filter for 40m. Currently, I'm measuring 5.8V RMS at the output of the filter which (if we were to allow ourselves to be comforted by the delusion that the load thereafter is 50 Ohms) implies that I'm throwing a rather healthy 600mW at my half-g5rv. I'll throttle back to more sociable power levels when I get a chance.

Here's the amplifier...

and here's the filter...

See how my obsession with Plugin Modules is getting worse!

With the work I'd already done on sending Morse Code from the RPi, it was easy to get the system to send my call (this time at QRSS speeds, with a 3 second dit length).

Here's the first spot (in the red box) of my transmissions at pa9qv's "Double Dutch" grabber...

With all the processing resource of the RPi, it was also easy to transmit Hellschreiber messages making this a multi-mode beacon (in the spirit of my original).

Here's my multi-mode signal arriving in the Netherlands...

I've added the annotation in red to help you see it! I only transmitted the rather flamboyant "Raspberry Pi" element of the message for fun. After the buzz subsides, I'll go back to sending just my call!

I guess the next step is to get the RPi beacon to send WSPR as well - but first, I must get onto 30m - there are more grabbers on that band - so more chance of seeing my Raspberry Pi proclaim itself to a waiting world!.

 ...-.- de m0xpd

Update :-

The beacon is now even more "multi-mode" than before ... in addition to FSK-Morse and "Hellschreiber", it now also transmits the digital "WSPR" mode.