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Yes—a mass-produced MediaTek MT7921-family Wi‑Fi dongle can be used as a receive-only software-defined radio. Luigi Cruz’s experimental BirriaSDR project configures an internal I/Q capture engine that was intended for chip calibration and debugging, then streams the samples to a computer over USB 3.0.
RTL-SDR.com reported the discovery on October 14, 2026, describing a roughly US$30 dongle that can deliver 40 million complex samples per second at 14-bit resolution. The current implementation is useful for experimentation, but it is not a calibrated, full-band laboratory SDR and it cannot transmit.
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What was hidden inside the Wi‑Fi chipset?
Modern Wi‑Fi silicon already contains a wideband receiver, digital signal-processing blocks and high-speed memory. Those parts are normally controlled by vendor firmware and exposed only through ordinary Wi‑Fi interfaces. BirriaSDR shows that at least one MT7921-family design also contains an internal capture path called ICAP (I/Q capture), intended for calibration and engineering diagnostics.
The project configures ICAP to place raw in-phase (I) and quadrature (Q) samples into circular SRAM. A DMA engine then moves that data to the USB interface. A small original 1,120-byte RAM program is loaded through the chip’s boot ROM to set up the process. According to the project documentation, BirriaSDR does not download vendor firmware and does not write flash memory or eFuses.
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That distinction matters: the software is exploiting a diagnostic function already present in the silicon rather than replacing the dongle’s operating firmware with a new Wi‑Fi stack.
What the current BirriaSDR build can do
| Specification | Documented result | Qualification |
|---|---|---|
| Tested hardware | COMFAST MT7961/MT7921AU USB dongle | The board combines an MT7961 digital die with an MT7972 analog die. |
| USB identification | 3574:6211 |
Use the USB ID to distinguish the tested device from superficially similar adapters. |
| Tuning range | 2,156–2,610 MHz | Live retuning is supported; sensitivity has not been verified across the entire range. |
| Sample format | 40 MSps complex | The nominal instantaneous sampled span is 40 MHz. |
| Resolution | Signed 14-bit I and Q | Both components of each complex sample use signed 14-bit values. |
| Receive channels | One, chain 0 | There is no documented multi-channel capture mode. |
| Transmit | None | BirriaSDR is receive-only and cannot replace a Wi‑Fi transmitter. |
| Host link | About 161 MB/s over USB 3.0 bulk transfers | This figure comes from the tested setup, not a universal guarantee for every computer or hub. |
| Tested host platform | Linux on Raspberry Pi 5 | macOS is listed as untested with hardware. |
Which frequencies are actually covered?
The documented 2,156–2,610 MHz span covers the 2.4 GHz Wi‑Fi region and adjacent spectrum, with the ability to retune while running. The current BirriaSDR README does not document 5 GHz operation. News descriptions that mention a broader 2.4/5 GHz Wi‑Fi chipset should therefore not be read as proof that the present software can capture 5 GHz signals.
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Raw I/Q access also does not guarantee equal performance everywhere in the range. The project says gain and frequency accuracy are uncalibrated, and sensitivity has not been measured across all tuning points. A strong signal outside the selected 40 MHz span can alias into the captured data, so apparent signals should not automatically be treated as being at their displayed frequency.
How to try BirriaSDR
The repository supplies a host library named libbirriasdr, a command-line recorder, firmware source for the small RAM program and a CyberEther proof-of-concept flowgraph. The documented setup is Linux-oriented.
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- Use the right adapter. Start with the tested COMFAST MT7961/MT7921AU variant and verify that the operating system reports USB ID
3574:6211. A different MT7921-family product may use another board design, firmware state or USB identifier and is not automatically compatible. - Provide a USB 3.0 path. Connect the dongle directly to a USB 3.0 port where possible. The reported approximately 161 MB/s transfer rate was measured on a USB 3.0 setup; a USB 2.0 connection cannot be assumed to sustain the stream.
- Prepare the Linux build environment. Install a C compiler, CMake and the libusb development files, then build the supplied host library and recorder from the BirriaSDR source.
- Keep the kernel Wi‑Fi driver away from the device. The capture program must claim the USB interface. If the normal wireless driver has already attached to the dongle, detach or blacklist that driver for the session before starting BirriaSDR.
- Check the device’s boot state. The README warns that a vendor ROM patch already loaded in the device can prevent operation. If the adapter arrives in that state, BirriaSDR may not be able to configure the capture engine.
- Load the capture program and start recording. BirriaSDR sends its small RAM program through the chip’s boot ROM, configures ICAP, and begins DMA transfers into the host recorder. This process does not flash the dongle or install vendor firmware.
- Process the samples. Use the command-line recorder for raw files or the supplied CyberEther proof-of-concept flowgraph to inspect the stream. Begin with known 2.4 GHz sources and conservative gain settings, then test retuning and watch for aliasing.
What this is—and what it is not
A practical low-cost receiver
The discovery makes high-rate I/Q capture accessible with hardware that RTL-SDR.com described as costing roughly US$30 at the time of its October 14, 2026 report. The 40 MSps complex stream and 14-bit samples provide substantially more raw data than the ordinary Wi‑Fi interface exposes, while USB 3.0 provides enough host bandwidth for continuous transfers on the tested setup.
Not a general-purpose Wi‑Fi replacement
BirriaSDR does not provide packet transmission, a second receive chain or a documented 5 GHz mode. It is a receive instrument for a defined slice of spectrum, not a way to turn the adapter into an arbitrary software-configurable transceiver.
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Not a calibrated measurement instrument
Because gain and frequency accuracy are uncalibrated and sensitivity is unverified across the tuning range, frequency and amplitude readings should be treated as experimental observations. External calibration, filtering and a known reference signal are needed for measurements that require defensible absolute accuracy.
Why the discovery matters for SDR experimentation
Previous Wi‑Fi-to-SDR projects have often been limited by low host bandwidth, undocumented hardware behavior or narrow proof-of-concept demonstrations. BirriaSDR combines a commodity USB form factor with a documented 40 MSps complex stream and a reproducible host interface. That makes it an interesting platform for signal visualization, protocol research and educational experiments in the 2.4 GHz region.
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The important lesson is broader than this one adapter: wireless chips may contain diagnostic receivers that are invisible to normal driver APIs. Finding and safely controlling those paths requires hardware-specific reverse engineering, and compatibility depends on the exact digital/analog die pairing, USB ID and boot state—not merely on a product name that includes “MT7921.”
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