Sunday, January 22, 2012

Propeller takes to the air

Today I have been playing around with MEPT beacon code on my Gadget Gangster Parallax Propeller board. First I wandered over to Jeff, KO7M's blog and borrowed his WSPR beacon code.

Gadget Gangster in use as a 25mW WSPR/Opera beacon

WSPR was the thing I originally wanted to try when I decided to get the Gadget Gangster: I hadn't even heard of OPERA at that time. But I was unsure whether I would be able to generate the FSK frequencies WSPR uses: four tones separated by about 1.48Hz. Jeff decided to shoot as near as he could and programmed for a 2Hz separation of tones, and found that the signal was decoded by K1JT's WSPR program. So no problem!

I measured the RF output from the Propeller board and it was somewhere in the region of 25 to 40mW, depending on which measurement method you believe. I also looked at the output using my oscilloscope.

Output waveform from Parallax Propeller on 80m
It wasn't a pure sine wave, there's obviously some harmonic content there, but it was not as bad as I feared it might be. As I would be using my MFJ magnetic loop on 30m, which has a very narrow bandwidth, I decided not to bother with a low pass filter for the sake of these initial trials. I watched the seconds tick over on my shack radio-controlled clock, started the beacon and was soon rewarded by several WSPR spots!

WSPR spots for 25mW Propeller beacon
Because WSPR is a time-synchronous mode I had to start the beacon when the seconds ticked over to 00. This brought to light a problem Jeff had already observed: the Propeller drifts. The drift seems to be worst during the first few minutes of operation, so leaving the beacon running so it can reach a stable temperature would appear to be the solution. However that is not so easy when you have to power it on at an exact time. I will need to look in to implementing a real-time clock for WSPR, unless I want to interface a GPS receiver to the Propeller - which is certainly possible and something else I hope to try as I'd like to have a go at making an APRS tracker.

An advantage of the OPERA mode is that it is not time synchronous so I can leave the beacon running in that mode with an arbitrary delay between transmissions. My first OPERA transmission also produced several spots, including reports from fellow bloggers PC4T and G4NKX.

OPERA spots for 25mW Propeller beacon
I can also generate Morse and QRSS beacons using the Propeller chip. There is still a lot to do to reach my goal of a multimode, frequency-agile beacon, including adding a PA and some switchable bandpass filters. But so far this project has turned out to be easier than I thought it would.
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