MediaTek MT7921AU Wi-Fi adapters picked up two SDR drivers

An ordinary USB Wi-Fi adapter can now double as a wideband software-defined receiver. Two independent projects have released SDR drivers for the MediaTek MT7921AU chipset, Hackaday reports. The tuning range takes in the 13 cm amateur allocations around 2.4 GHz.

This follows the discovery about two weeks ago of an undocumented I/Q sampling feature in ESP32 chips, which W5PRO covered earlier. Since then, similar hidden capabilities have turned up in LoRa and Wi-Fi chipsets. The MT7921AU is the second MediaTek part repurposed this way, after the MT7996.

The first project, BirriaSDR, uses the raw I/Q samples the chip already exposes for debugging and calibration. It streams them to the host computer at 40 Msps with 14-bit resolution and tunes from 2156 to 2610 MHz. It talks to the adapter's registers directly through libusb and makes no permanent firmware changes, so the adapter returns to normal Wi-Fi duty afterward. Its programming interface is modeled on the RTL-SDR's, which should make it familiar to anyone who has used one of those popular dongles.

The second project, Mtk79ICAP, presents the chip through the standard SoapySDR interface that many SDR applications already support. It does modify the chip's firmware and needs a modified driver. In exchange, it reaches up to 80 MHz of bandwidth and can tune the 5 GHz bands. The adapter can also run in Wi-Fi monitor mode while streaming SDR data, but only with automatic gain control enabled.

For hams, this is early-stage and receive-only. The coverage overlaps the 13 cm band, and the 5 GHz capability reaches toward the 5.7 GHz amateur allocation. Many of these adapters have external antenna connectors, so adding a mixer to reach other frequencies should be straightforward, though Hackaday notes the Wi-Fi adapters are not quite at that point yet.