Today’s single-chip radio tuner chips make it possible for hobbyists to construct decent quality broadcast receivers. One chip that has been around for 15 years or so is the Skyworks (formerly known as Silicon Labs) Si4735. This chip contains a full LW/MW/SW/FM radio tuner from antenna to audio. It requires a microcontroller to control it, for example the 8-bit AVR processor used by Arduino. There are many hobbyist and open-source designs based on this chip. The Si4735 is an SDR radio internally, it does all filtering and demodulation on its internal DSP. There is a custom firmware blob for it that can demodulate SSB. However, this firmware blob is not open source. The AVR sends the Si475 its commands to tune to a specified frequency and set its other operating parameters, The AVR controls the display and the user controls of the radio. There are many ready-made Chines radios designed around this chip (or the Si4732) and an Arduino/AVR. This chip gives nearly continuous LW/MW/SW coverage up to 30 MHz.It is also the main part (or the final IF filter/demodulator) of nearly all world band radios currently on the market.
Another single-chip radio tuner is the TEF-6686, designed by NXP. This chip contains a very capable FM tuner, along with LW, MW and SW. SW coverage is limited to 27 MHz (not 30) and there is no way to do SSB. As microcontrollers are now predominantly 32-bit, the TEF-6686 radios (and some newer Si473x designs too) uses an ESP32 microcontroller. One such open-source design was provided by PE6PVB https://www.pe5pvb.nl. This design has found its way to numerous ready-made Chinese radios too.
There is also the Skyworks Si4684, a complete FM/DAB+ radio on a chip. PE5PVB designed a very capable DAB+ radio around it, using this chip, an ESP32 and a colour display. As the ESP32 does slide show decoding and display, the software uses nearly all the flash of the ESP32, so the FM side of the chip cannot be accessed using the current software. Unfortunately, there are no ready-made Chinese versions of this radio yet. DAB+ is not a thing in China and many markets these Chinese radios get exported to.,
As interesting as these projects are, they leave the nitty-gritty radio stuff to a closed-source DSP and the open-source part is only about user control and sometimes RDS decoding or decoding of other digital data that comes with the radio signal. The Pico Rx project is different though. See https://github.com/dawsonjon/PicoRX There a Raspberry Pi Pico does all the hard work of an SDR in software. It uses minimalistic front-end circuitry (a set of analogue multiplexers to act as an IQ mixer and some frontend filters) and uses the on-board circuitry of the Pico itself for the rest, including A/D converters and the I/Q oscillator signal generation. This circuit gives you a capable all-mode HF receiver (0-30 MHz), optionally with a spectrum display. All the DSP stuff is done on the Pico, which is a fairly powerful dual-core 32-bit microcontroller in its own right. The radio can run on batteries and draws little current.
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