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Latest Lime SDR board builds on Raspberry Pi CM3

Oct 22, 2018 — by Eric Brown — 2333 views

The open spec, 125 x 65mm LimeNET Micro is Lime’s first fully embedded SDR board, featuring the Raspberry Pi Compute Module 3, MAX 10 FPGA, u-blox GNSS, RF transceiver, Ethernet with PoE, and optional enclosures.

UK-based Lime Microsystems has returned to Crowd Supply to launch its first fully autonomous, embedded software defined radio (SDR) platform, and the first to include integrated PoE and GNSS. The successfully funded LimeNET Micro is available through Dec. 6, starting at $269, including the integrated Raspberry Pi Compute Module 3, with shipments due Feb. 25, 2019. Other packages add enclosures and omni-directional antennas.



LimeNET Micro, front and back
(click images to enlarge)

With its previous LimeSDR Mini, Lime began moving toward a more autonomous, embedded approach to SDR. Like the original LimeSDR-USB, the smaller LimeSDR Mini requires a tethered computer running Ubuntu Core. Yet, earlier this year, Lime launched a Grove Starter Kit bundle that combines the LimeSDR Mini mainboard with a Grove board controlled by the Lime Suite SDR stack running atop Ubuntu Core on a user-supplied Raspberry Pi.


LimeSDR Mini

At 125 x 65mm, the new LimeNET Micro mainboard is slightly larger than the 100 x 60mm LimeSDR Mini, and the two enclosure options appear to be about the same size as the Mini enclosure. However, the Micro enables more compact systems by integrating the 67.6 x 31m Raspberry Pi CM3 module rather than requiring an attached desktop or SBC computer.

The LimeNET Micro is once again equipped with an Intel/Altera MAX 10 FPGA with 16k programmable logic gates. It also integrates Lime’s SDR-optimized LMS7002M RF transceiver. Other specs are similar, including the 10MHz to 3.5GHz range, 12-bit sample depth, full-duplex operation, single TX and RX channels, and precision oscillator.

As seen in the comparison table below, however, the RF bandwidth and sample rate are much lower than the Mini, at 0.27MHz and 1.0 MSPS, respectively. It’s unclear if this is due to the lower processing power of the RPi CM3 compared to other attached computers, or if additional onboard components play a role. Like the original RPi 3, the CM3 module offers a quad-core -A53 Broadcom BCM2837 clocked to 1.2GHz. It ships with 1GB LPDDR2 and 4GB eMMC.



LimeNET Micro angle views
(click images to enlarge)

The LimeNET Micro offers a $30 option for an adapter and ribbon cable that lets you attach a full-sized Raspberry Pi. It’s unclear if the attached SBC plays any role in LimeNET Micro processing or if it simply offers you easier access to I/O and features such as WiFi/BT, which are unavailable on the CM3.

The Micro lacks the full-sized USB port of the earlier Lime boards. However, it supplies a micro-USB for booting, as well as a 10/100Mbps, RJ45 Ethernet port with 12V, passive Power-over-Ethernet (PoE). There’s also a microSD slot and a 12V DC input jack.



LimeNET Micro block diagram (left) and comparison with earlier models
(click images to enlarge)

Dual coaxial RF (SMA) connectors are provided for the LMS7002M chip, which can be switched between high- and low-frequencies. A $45 pair of omni-directional antennas is available, optimized for 800-960 MHz, 1710-2170 MHz, and 2400-2700 MHz.

You also get SMA connectors for external clock source in and out, and for the integrated u-blox NEO-M8 (PDF) GNSS module — a first for a Lime board. The -167 dBm sensitive GNSS module supports BeiDou, Galileo, GLONASS, and GPS / QZSS, and can concurrently receive up to three GNSS signals. A GPS antenna with a 28 dB gain and 3-meter cable is available or $49.

The LimeNET Micro is further equipped with FPGA-connected GPIO and JTAG connectors, FPGA switches, 6x LEDs, and a buzzer. There’s also a 30.72 MHz VCTCXO clock, and the RF module includes dual 128KB EEPROMs.



Acrylic (left) and aluminum case options
(click images to enlarge)

You can buy the LimeNET Micro board on its own or house it one of two enclosures. The $99 acrylic case integrates a fan and a heatsink. There’s also a $459 aluminum case option that unlike the acrylic option, integrates a RPi CM3 module with dual antennas. The aluminum case offers much better noise interference compared to the acrylic case.

The aluminum case is also the only option that preloads the CM3 with the open source PantaHub container software, which “lets you connect your devices and transform them into bare-metal infrastructure.” PantaHub integrates a Pantavisor device agent and uses container technology to develop, deploy, and manage firmware on Linux devices.

Like the other Lime systems, the Micro is an open source product. It will be supported with schematics, layout and project files, and open Lime Suite firmware code.

The LimeNET Micro supports most of the SDR applications enabled by other Lime products, although with lower RF bandwidth. The Lime systems can send and receive using UMTS, LTE, GSM, WiFi, Bluetooth, Zigbee, LoRa, RFID, Digital Broadcasting, Sigfox, NB-IoT, LTE-M, Weightless, and any other wireless technology that can be programmed with SDR. They can drive low-cost, multi-lingual cellular base stations and wireless IoT gateways, and are used for various academic, industrial, hobbyist, and scientific SDR applications, such as radio astronomy. Earlier this year, the European Space Agency purchased 200 LimeSDR Mini boards to prototype SDR-enabled 5G satellite networks.

 
Further information

The LimeNET Micro is available on Crowd Supply through Dec. 6, starting at $269. Shipping charges apply outside the U.S., and shipments are due Feb. 25, 2019. More information may be found on the LimeNET Micro Crowd Supply page, and more details should eventually appear on the Lime Microsystems website.
 

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