CaribouLite is an affordable, educational, open-source SDR platform that is also a HAT for the Raspberry-Pi family of boards (40-pin versions only). It is built for makers, hackers, and researchers and was designed to complement the SDR current eco-systems offering with a scalable, standalone dual-channel software-defined radio.
Unlike many other HAT projects, CaribouLite utilizes the <B>SMI</B> (Secondary Memory Interface) present on all the 40-pin RPI versions. This interface is not thoroughly documented by both Raspberry-Pi documentation and Broadcomm's reference manuals. An amazing work done by [Lean2](https://iosoft.blog/2020/07/16/raspberry-pi-smi/) (code in [git repo](https://github.com/jbentham/rpi)) in hacking this interface contributes to CaribouLite's technical feasibility. A deeper overview of the interface is provided by G.J. Van Loo, 2017 [Secondary_Memory_Interface.pdf](docs/smi/Secondary%20Memory%20Interface.pdf). The SMI interface allows exchanging up to ~500Mbit/s between the RPI and the HAT, and yet, the results vary between the different versions of RPI. The results further depend on the specific RPI version's DMA speeds.
In our application, each ADC sample contains 13 bit (I) and 13 bit (Q), that are streamed with a maximal sample rate of 4 MSPS from the AT86RF215 IC. This channel requires 4 bytes (samples padded to 32-bit) per sample (and I/Q pair) => 16 MBytes/sec which are 128 MBits/sec. In addition to the 13 bit for each of I/Q, the Tx/Rx streams of data contain flow control and configuration bits. The modem (AT86RF215) IC by Microchip contains two RX I/Q outputs from its ADCs (one for each physical channel - sub-1GHz and 2.4GHz), and a single TX I/Q intput directed to the DACs.
4. A single system level 3.3V power (while the FPGA still receives 2.5V and 1.2V for its core). A linear regulator (rather than a switching DC-DC) was used to reduce conducted (power and ground) noise levels.
5. More detailed changes in the [schematics](hardware/rev2/schematics/cariboulite_r2_sch.PDF).
In CaribouLite-R2 the PCB design has been thoroughly re-thought to meet its educational needs. The RF path has been annotated with icons to ease the orientation in the schematics sheets, friendly silk writing was added describing system's components by their functionality rather than logical descriptors, and more.
The gaps are defined by the design constraints of the system and may not exist in real-life hardware. Actual modem synthesizer outputs test show wider margins at room temperature than those written in the datatsheet, but, as noted by Microchip, performance may suffer.
CaribouLite is a test equipment for RF systems. You are responsible for using your CaribouLite legally.
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