I just built what might be the coolest gadget of the year: a Software Defined Radio (SDR) powered by none other than the legendary MOS Technology 6502 CPU. And the timing couldn’t be better – this chip turns 50 years old on September 8th!
If you already know what SDRs and 6502s are, your jaw may have just dropped. But if not, don’t worry. I’ll catch you up.
The source code for everything is on Github: https://github.com/AndersBNielsen/PhaseLoom
You can also jump straight over to my store and buy one: https://www.imania.dk/index.php?currency=EUR&cPath=204&sort=5a&language=en
What’s an SDR Anyway?
Think of an SDR as the Swiss Army knife of radios. Instead of filling a workbench with dedicated devices, like a garage door opener, a satellite receiver, and a shortwave set, you just plug in an SDR and let software do the heavy lifting. Tuning, filtering, demodulating is all handled by code.
With an SDR, you can:
- Pull down data from satellites
- Decode airplane transponders
- Listen to ham radio or shortwave broadcasts
- Experiment with frequencies you didn’t even know existed
They come in all shapes and sizes, from cheap USB dongles to professional-grade equipment worth thousands. Some just receive, others transmit. But they all share the same principle: turn analog radio waves into digital data, then let software handle the magic.
And the 6502?
Ah, the 6502. This little 8-bit CPU powered the Apple I & II, Commodore 64, Atari consoles, and even the NES. For many, it was the chip that introduced the world to affordable personal computing. And now, half a century later, it’s back—this time running the front end of my homemade SDR.
Meet the PhaseLoom
I call my creation the PhaseLoom: a Quadrature Sampling Detector Phase-Locked Loop SDR frontend. Don’t worry about the jargon—what matters is that it’s a custom circuit that converts radio signals into the digital I/Q streams needed for software processing.
In practice, it works like this:
RF Frontend – A simple low-pass filter to clean up incoming signals.
Quadrature Sampling Detector – Uses clever switching to split the signal into in-phase (I) and quadrature (Q) components.
Local Oscillator – Generated by an SI5351 clock chip, controlled by 6502 assembly code.
Output – The I/Q signals can be piped into a sound card, oscilloscope, or eventually a higher-speed ADC for real SDR processing.
When I first powered it up, I wasn’t sure what to expect. But sure enough, with an antenna hooked up, I could tune into the 40-meter ham band. It’s rough, noisy, and very much a prototype—but it works. A 6502-powered SDR is alive.
Why This Matters
Most SDRs today rely on powerful modern processors or FPGAs. Running one from a CPU designed in 1975 is absolutely ridiculous—and that’s exactly why it’s so fun. The 6502 may not be doing the heavy DSP (yet), but it’s orchestrating the whole show. And with more development, I plan to push it further—maybe even squeezing in some real signal processing routines on that ancient silicon.
The Road Ahead
This is just the beginning. Better filters, wider bandwidth ADCs, and smarter software are all on the horizon. And yes, the PhaseLoom will be open hardware—schematics, code, and boards are already available.
The 6502 started a revolution 50 years ago. Today, it’s tuning in the airwaves. And I can’t wait to see how far this project goes.