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Yes—a mass-produced MT7921-family Wi‑Fi dongle can work as a receive-only software-defined radio. Luigi Cruz’s experimental BirriaSDR project exposes an internal I/Q capture engine that can stream 40 MSps of 14-bit complex samples from part of the 2.4 GHz band. RTL-SDR.com reported the discovery on October 14, 2026.
What BirriaSDR revealed
Wi‑Fi chipsets already contain wideband receivers, but their internal sample paths are normally reserved for calibration and debugging. BirriaSDR configures that hidden path—called ICAP in the project documentation—and turns it into a host-visible raw I/Q stream.
The implementation uses the chipset’s circular SRAM as a sample buffer, moves data with DMA, and streams it over USB 3.0. A small original 1,120-byte RAM program is loaded through the chip’s boot ROM. The project says it does not download vendor firmware and does not write flash or eFuse.
RTL-SDR.com described the hardware as a roughly US$30 USB 3.0 Wi‑Fi dongle. The result is not a general-purpose laboratory SDR, but it is a striking example of functionality already present in inexpensive wireless silicon.
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Documented BirriaSDR specifications
| Capability | Current documented result |
|---|---|
| Tested hardware | COMFAST MT7961/MT7921AU USB dongle, USB ID 3574:6211 |
| RF tuning | 2156–2610 MHz, including live retuning |
| Instantaneous sample span | Nominally 40 MHz |
| Sample rate | Fixed 40 MSps complex |
| Resolution | Signed 14-bit I and Q |
| Receive channels | One channel, chain 0 |
| Transmit capability | None |
| Host link | USB 3.0 bulk transfers at about 161 MB/s on the tested setup |
| Tested host | Linux on a Raspberry Pi 5 |
| macOS status | Listed as untested with hardware |
The USB ID matters more than the product name on a shop listing. Different dongles can use similar marketing names while containing different chip revisions or firmware states.
What frequencies it can receive
The repository documents tuning from 2156 to 2610 MHz. That covers a substantial portion of the 2.4 GHz ISM region, but it is not evidence that every signal anywhere in the band will work equally well. BirriaSDR has not verified sensitivity across the entire range.
The current README documents no 5 GHz support. News coverage may describe the underlying Wi‑Fi chipset in broader 2.4/5 GHz terms, but the tested BirriaSDR capability is the narrower 2156–2610 MHz receive range.
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The 40 MSps complex stream represents a nominal 40 MHz sampled span. Strong signals outside that span can alias into the capture, so a clean-looking center frequency does not guarantee that unwanted out-of-band energy is absent.
Why this is an SDR—and why it is still experimental
An SDR exposes digitized radio samples so software can perform filtering, demodulation, decoding or spectrum analysis. BirriaSDR meets that basic definition: it delivers signed in-phase (I) and quadrature (Q) samples instead of only Wi‑Fi packets.
It is nevertheless experimental. Gain and frequency accuracy are uncalibrated, and sensitivity has not been established throughout the tuning range. Measurements should therefore be treated as exploratory rather than equivalent to a calibrated commercial SDR. The project provides one receive chain and no transmitter, so it cannot replace a Wi‑Fi adapter for normal network operation or transmit arbitrary signals.
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How to reproduce the capture
- Obtain the matching hardware. Start with the documented COMFAST MT7961/MT7921AU variant and verify USB ID
3574:6211. A similar-looking dongle is not sufficient proof of compatibility. - Use a supported host setup. The project has been tested on Linux with a Raspberry Pi 5. Connect the dongle through a USB 3.0 port; the documented transfer rate depends on the tested USB setup.
- Prepare the build environment. Install a C compiler, CMake and the development files for libusb. The repository supplies the
libbirriasdrlibrary, a command-line recorder, firmware source and a CyberEther proof-of-concept flowgraph. - Prevent normal Wi‑Fi driver binding. A kernel Wi‑Fi driver must not claim the device while BirriaSDR is using it. Detach or blacklist the applicable driver according to your Linux distribution before starting the recorder.
- Check the device’s boot state. The README warns that a vendor ROM patch already loaded in the dongle can prevent operation. If the device remains unavailable after driver detachment, this is a documented possibility rather than evidence that the RF path is absent.
- Build the supplied components. Compile the library and recorder with the repository’s CMake-based instructions, then load the project’s small RAM program through the chipset boot ROM. BirriaSDR’s documented method does not write flash or eFuse.
- Start a capture and tune within the tested range. Record the one available receive chain at 40 MSps complex, then use the supplied processing flow or your own I/Q tooling for inspection and demodulation.
What the hardware design means in practice
The board pairs an MT7961 digital die with an MT7972 analog die. BirriaSDR is exploiting an existing receive and memory architecture rather than adding an RF front end to a microcontroller. That is why the approach can deliver a relatively high-rate stream over USB from inexpensive hardware.
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Common failure points
The operating system keeps reconnecting the adapter
Normal wireless networking software may immediately bind the dongle. Confirm that the kernel driver is detached before launching BirriaSDR, and use a separate adapter for network access if necessary.
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The recorder cannot initialize the device
Verify the USB ID and inspect whether a vendor ROM patch is already present. The project specifically warns that such a patch can block operation.
The spectrum contains unexpected signals
Remember that the gain and frequency response are uncalibrated, sensitivity varies by frequency, and strong signals outside the nominal sampled span can alias into the capture. External filtering may be needed for crowded RF environments.
You need 5 GHz, transmission or multiple channels
Those requirements are outside the current documented BirriaSDR capability. The README lists no 5 GHz support, provides one receive chain and implements no transmitter.
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Who should try it
BirriaSDR is suited to Linux users who want to investigate raw 2.4 GHz signals, experiment with I/Q processing or study how commercial wireless silicon works internally. It is a poor choice when you need calibrated frequency and amplitude measurements, guaranteed sensitivity, 5 GHz coverage, transmit capability or a polished cross-platform workflow.
Bottom line
BirriaSDR turns a specific MT7921-family USB Wi‑Fi dongle into a practical, receive-only SDR experiment: 40 MSps complex, 14-bit I/Q, one channel and a documented 2156–2610 MHz tuning range. Its low cost and USB 3.0 export are compelling, but exact hardware identity, driver control and the project’s uncalibrated experimental status are essential constraints.
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