(2023)
Very nice. It's not a "crystal radio", though. It's a direct sampling receiver. That is, the processing takes place at the RF frequency directly. Most receivers use heterodyne conversion to down-convert a desired chunk of spectrum to a convenient working frequency. This doesn't. It just runs the raw RF through an A/D.
As A/D devices and processing have improved, the frequency upper limit for direct conversion receivers has moved up. There are low-cost direct conversion receivers going up to 50MHz, but they have to use an FPGA rather than doing all the work in software.
Very cool write up! I'm amazed that it's running on AAA batteries.
The introduction to SDR (software defined radio) is much appreciated.
Edit: defined, not designed
The staggered quadrature sampling trick also comes up in the Crank-Nicholson method for solving e.g. the Schrodinger equation, optics etc.
The LTC6227, used in this design, is "last time buy." The replacements aren't as good: either higher noise or lower GBW, and they too will vanish in a few years. That is, of course, deliberate, forcing designers to climb the curve of ever more expensive, exclusive components. Another component in this design, the FST3253, is also a figment of history: no longer produced. Electronics is a nasty rat race, with AD, for example, having bought literally all of their competitors except TI, we have an perfect ADC/DAC/op-amp oligopoly, with everything getting worse with each passing day. So the notion of a cheap, easily reproduced "crystal radio for the digital age" isn't actually workable: nothing you can dream up will be buildable more than 2-3 years after you design it as the components you've chosen cease to exist.
Not sure I would have gotten very far with this as a 7 year old, but as a 62 year old it looks quite interesting :-)
That's really cool, but as long as AM exists, crystal radio will be crystal radio
Indeed very simple and within reach of a beginner.
Another illustration how RPi 2040 PIO is an exceedingly helpful thing. Without it, the RF-related circuits would have to use either bit-banging, or a bunch of small discrete components.
When I look at that crystal radio kit, my fingers hurt. I had a kit like that as a kid in the 90s - not sure if it was a radio or more generic electronics - with the springs to hold wires together. At some point, made a short circuit and burned my fingers. Immediately upon seeing that, the memory comes back. I guess it's burned into my memory.
I was looking at digital fm chips a few years back that specifically pick up digital fm and more importantly to my project at the time digital text that stations broadcast.
Deviation aside this is a cool project and impressive as someone said that it runs on alkaline batteries.
Was looking into Meshtastic recently, as a means to have basic communication in disaster situations (hurricanes, war, etc.) but haven't picked a device yet. Any recommendations?
Neat project I do wonder what more could be done with the RP2350.
Note that the RP2350, being M33 based has square root, and can add 16MB of PSRAM behind the XIP that will help with the flash writing problem, The solder party RP2350 Stamp XL has a SOP8 pad to add PSRAM and is actually shipping for anyone who is looking to play with them, no connection with them, just a user.
The extra PIO block would also allow for S2C output which may be nice rather than a TPA2012D2, which will require most people to buy a breakout board anyway.
The rp2040/rp2350 series is incredible. The DMA/SIO/PIO features are just so powerful and even the esp32/stm32 seem restrictive to me to go back to. It is amazing what people are doing with them.