Thank you to Joshua R. for writing in and sharing some links on how an EFR32 microcontroller can be used as a software-defined radio. The EFR32 is a microcontroller designed for IoT applications such as Zigbee, Z-Wave, Thread, Bluetooth. Of interest to us is that the EFR32 has a special mode that allows for the demodulator/modulators to be switched off and instead allowing for raw IQ data to be transferred from the chip.
An example of the EFR32 being used as an SDR has been provided by OH2EAT/tejeez and can be found in this write-up and in his 'geckokapula' GitHub repo. This MIT-licenced repo provides hardware schematics and software for creating a full handheld FM, AM, USB, LSB, and CW receiver with FM, CW, USB, and LSB transmit capabilities. It also supports a 12 kHz waterfall display. The tuning range is nearly 13.2 MHz to 2.9 GHz, but there are some gaps.
This Knowledge Article by Silicon Labs also explains the SDR mode of the EFR32.
This is an interesting low-cost chip, but the limiting factor appears to be the small bandwidth.
