PSK-31 Beacon Mark 2

I've added PWM support for generating the baseband and envelope signals to the code base. This generates much cleaner looking signals, but I've run into a timing jitter problem.

Experiments with my new DSO and some pin-toggling code under cli(); has shown the internal clock of the ATtiny13V is hopelessly unstable and has pretty extreme phase noise - typical of an HF RC oscillator. The ATtiny13 also lacks a second timer, so I am busy-waiting to generate the baudrate and using the only timer to drive the two PWM channels. This means the code is a bit of a kludge and can't use interrupts to generate the baudrate without a major rewrite and probably dropping to assembler and cycle counting (adding NOPs) to balance out the delay in various execution forks. It would be much easier to produce well-timed PSK-31 from a xtal locked MCU that has at least two independent timers, the ATtiny85 comes to mind, and it has a lot more space for messages or even a varicode table.

In an attempt to xtal lock the MCU I changed its low fuse bits to run off an external clock. At the time I knew this would break my simplistic programmer, but to work around this I wired up a simple pierce oscillator on a solderless breadboard and simply held the output wire against pin 2 while programming. This worked fine - ONCE. Upon the second attempt I must have slipped while it was programming and glitched the heck out of the clock - programming the RSTDISBL fuse - BUGGER! (Although at the time I likely used more colourful language.) This naturally rendered the device unprogrammable via SPI even with a clock source!

Time to build a HV programmer...

I did get a lot further with the RF hardware side of things. I built a buffered xtal oscillator and a cascaded feedback amplifier pair to bring the output of the DBM up into the 10-100 mW region. The xtal oscillator is buffered and then padded very severely giving a very stable signal, but at less than -10 dBm. The modulation happens at this rather low level - probably a problem if I add more power gain later, so I may put an amplifier between the LO source and the modulator. The "power amplifier" currently implemented still has plenty of headroom, but I may change the output device (currently a BD139) to something with more gain at 20 metres.

Beacon RF Modules

Note that while I am generating the phase and amplitude signals, I am currently only using the baseband one with the DBM modulator.

Experiments will continue once I unbrick the microcontroller.


Here is a video of the MCU blinking some LEDs at about 1 baud. I slowed it down so the phase relationships between the three signals could be clearly seen. What you are seeing here is a few of the "0" transitions in the preamble/idle before the actual message is sent.

Signals Displayed with LEDs
Signals Displayed with LEDs
(4.822 Mbytes)

Note that the "phase" line transitions as the "amplitude" signals hits its minimum, and the "baseband" signal crosses through half-rail at the same point. Graphically it looks like this:

PSK-31 Modulation Diagram

The Code

I wouldn't recommend using this code for anything but a rough starting point, but I've attached it for your amusement.



title type size
PSK-31 Modulation Diagram Source application/postscript 9.481 kbytes

Parent article: PSK-31 Beacon Mark 1.