MicroZed Chronicles: A Look at the Artix-7-Based xSDR

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I think, for the good of my bank account and my accountant’s sanity, I am going to have to block Crowd Supply on my company network. Last week, another item I had ordered, and might have forgotten about arrived, the xSDR.

The xSDR is based around an Artix-7 FPGA coupled with a Lime Microsystems RFIC and is implemented in a compact M.2 format. For ease of use, I purchased the USB breakout board, which provides only one RF input and output, but for my purposes this is sufficient.

Its arrival was well timed, as we have been looking at DSP and signal processing across several blogs. In these DSP and signal-processing blogs, we have implemented FFTs and examined butterfly structures, designed IIR and CIC filters, and built waterfall displays that let us see how a spectrum evolves over time.

All of that work was performed using simulations, or signal generators and ADCs on the bench, to provide the stimulus.

In this blog, we are going to look at an application where these techniques are very useful: software-defined radio (SDR), which is exactly what the xSDR from Wavelet Lab is designed for.

What is the xSDR

The xSDR is a software-defined radio implemented on a single-sided M.2 2230 module. This means it measures only 30 mm by 22 mm and, in this format, can be installed in a range of embedded processing devices in place of Wi-Fi modules, for example.

Its footprint might be tiny; however, the xSDR provides us with a 2×2 MIMO architecture with two receive and two transmit channels.

This architecture provides an operating frequency range from 30 MHz to 3.8 GHz, sample rates of up to 100 Msps, and channel bandwidths of up to 90 MHz.

Host connectivity is typically provided by either USB 2.0 or a PCIe 2.0 ×2 interface, while typical power consumption is around 1.9 W. For this example, I have the USB carrier card for the module. The solution comes with a heatsink, thermal pads, and tiny RF cables to connect the xSDR ports to the SMA connectors on the USB carrier card.

Of course, what makes the xSDR interesting from our perspective is its architecture.

At its heart are two devices: a Lime Microsystems LMS7002M RF transceiver and an Artix-7 XC7A50T FPGA.

This pairing of a field-programmable RF transceiver and an FPGA is a classic SDR architecture, especially for more cost-optimised solutions. As such, it is useful to understand how the system is split between the two devices.

The LMS7002M

Regular readers will be familiar with FPGAs and may even have come across FPAAs. The LMS7002M is a field-programmable RF transceiver.

It is a full-duplex, direct-conversion, zero-IF transceiver that provides two complete transmit and two complete receive chains.

Each receive chain contains low-noise amplifiers, quadrature mixers, programmable analogue low-pass filters, and 12-bit ADCs, while the transmit chains mirror this with DACs, filters, mixers, and output drivers. These structures are similar to those you would see in an RFSoC commonly used in higher-end SDR applications.

The connection to the FPGA uses digital I/O implementing a 12-bit data transfer, which can operate in SDR or DDR mode, while configuration is handled over an SPI link.

The FPGA

As the LMS7002M handles the RF transmissions, the role of the FPGA in the system is to manage the system, communicate with the host, and, of course, generate and process the data sent to and from the host.

This is where the FPGA provides a very flexible implementation. Its logic, DSP48s, and BRAM enable it to implement the buffering and processing features needed for the design, such as FFTs and filters, along with the USB and PCIe interfaces.

One very nice feature of the xSDR is that the design is available as open source for inspection. This means we can see not only the hardware design but also the inner workings of the FPGA. This is particularly useful because it allows us to modify the design if we have a custom application that we want to implement in the SDR.

Applications

To use the xSDR, there is a web front end that allows you to develop custom SDR applications or use one of the default applications provided. The xSDR has some surprising capabilities: it is able to implement simple cellular communications, AM and FM signal generation and reception, along with waveform generation, signal monitoring, and analysis.

When you connect the xSDR to your system, you should be prompted to connect to the WSDR.io application, which is the web-based environment for developing applications. You will need to set up an account to do this, but it is pretty straightforward.

Now that I have mine fully assembled, I am going to spend some time exploring WSDR.io and work out how to create my first application, which I will report back on soon.

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