ESP-SDR Firmware Turns Bare ESP32 Chips into 2.4/5GHz SDR Receivers

ESP-SDR, a new open-source firmware, bypasses the ESP32's fixed-function Wi-Fi and Bluetooth modems to tap directly into the chip's internal ADC signal chain, capturing raw I/Q baseband samples. The developers found this capability by using LLMs to reverse-engineer the adctrig function in Espressif's librftest library, uncovering a debug sampling path apparently left in silicon for factory wafer testing. This lets supported chips—including ESP32, ESP32-C3/C5/C6/C61/S2/S3, and the preview ESP32-S31—act as SDR receivers with no external RF hardware required.

Depending on the chip, the firmware captures up to 80 MSa/s across 13-54 MHz of bandwidth, tuning from 2.2-2.7 GHz and, on the ESP32-C5, up to 4.8-6.0 GHz; the C61 can even pick up LTE Band 7 and 5G NR Band n7 signals. Samples stream over USB or UART to a browser-based tool called ESP-WebSDR for real-time spectrum and waterfall visualization, though USB/UART bandwidth limits force short snapshot captures rather than continuous streaming. A companion project, SoapyESPSDR, aims to enable continuous I/Q streaming over Gigabit Ethernet from the ESP32-S31 to tools like GNU Radio and Gqrx.

The discovery came from Florian Euchner and Zoltan Doczi of the ESPARGOS project while building an eight-antenna Wi-Fi sensing array. Released under GPL-3.0, ESP-SDR turns sub-$5 ESP32 boards into basic spectrum analyzers, offering engineers a low-cost entry point into RF signal analysis without dedicated SDR dongles.