A Software-Defined VLF (Very Low Frequency) Radio.
As a licenced radio amateur I have access to a range of bands and techniques, but my interests lie at the far ends of the radio spectrum. One challenge I found was the lack of equipment (either commercial or DIY) for the 2,200m band (137KHz). Since this is such a low frequency, it lends itself in many ways to building your own equipment. I figured I would have a try at this project.
Rather than build a conventional VFO (Variable Frequency Oscillator) I decided I would use the readily-available AD9850 DDS module. These are quite easy to use, stable and relatively inexpensive. This repo contains the code for driving one such module.
A simple test generator. Once running, you will see a 137.5KHz signal on the output pins of your module.
A better test generator. Uses an additional I/O line to change the freqency by 455KHz. Why? Ultimately this will be a transmitter and superhet receiver combo, so we need to adjust the receive frequency.
Now we are starting to look more like a complete system. This test adds a rotary encoder to act like the "dial" on a traditional radio. Existing functionality from the earlier tests is retained.
Watch this space!