Check: LimeSDR Mini Software program Outlined Radio Transceiver

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It is fair to say that software defined radio represents the most significant advance in affordable radios that we have seen in the past decade. The shift in signal processing from specially designed analog hardware to software has opened up so many exciting possibilities of what is possible with both more traditional and newer modes of radio communication that are only made possible by new technology.

It’s also fair to say that radio enthusiasts looking for a high-performance SDR would need to be prepared even with a high bank balance, as some of the components required to ship software-defined radios were quite expensive. The budget of the market was therefore for radios that use the limited baseband bandwidth of an existing analog interface such as a computer sound card, or in the case of lucky accidents during driver hacking such as the discovery of the cheap and now ubiquitous RTL2832 chipset, digital TV receivers could be used as SDRs -Receivers act. Sending was and is more expensive.

The bulky USB stick form factor of the LimeSDR Mini.

A new generation of budget SDRs, like today’s LimeSDR Mini, has lowered the broadcast price. This is the latest addition to the LimeSDR line of products, an SDR transceiver and FPGA development board in a USB stick format that essentially uses the same Lime Microsystems LMS7002M as the existing LimeSDR USB, but with a lower specification. One of the most important changes is that there is only one receiving and one sending channel to the two USB devices, the bandwidth of 30.72 MHz is halved and the frequency range at the lower end jumps from 100 kHz to 10 MHz. The most interesting lower number associated with the Mini, however, is its price, as early risers buy it for $ 99 – half the price of its predecessor. (It’s now available on Kickstarter for $ 139.)

We were fortunate to receive a pre-produced LimeSDR Mini for review by the MyriadRF staff – in fact, two of them were sent to us after it was discovered that the first had a hardware failure that was suspected of having a solder joint problem. We feel their pain, after all, those who did not have pre-production boards made mistakes at the worst times!

The laser cut case that will be available for your Mini.

The board itself is a printed circuit board approximately 33mm x 70mm (1.25 “x 2.75”) with a USB 3 male connector on one end and two SMA female connectors on the other end, one for receiving and the other for Send. The integrated circuits are all on top of the board, and although they contain footprint screening cans, they are not populated. There is a single multi-colored status LED between the SMA sockets. It’s worth noting that there will be a laser cut plastic shell for the board, which is likely worth it as it feels a bit vulnerable. Together with the board they supplied a pair of small rubber duck antennas for the 870 MHz band.

A couple of small 870 MHz antennas come with the board.

It is obvious that the LimeSDR Mini is an extremely capable board that could have the potential for great things in the hands of a true SDR and FPGA programming expert. It is also evident that, as your Hackaday writer, I am not an SDR extreme power user. Although I’ve had an amateur radio license for over three decades, I came into the world of SDRs relatively late and haven’t evolved beyond RTL SDRs or simple devices that use a PC sound card as a baseband. But it’s likely that many SDR programmer will actually buy this board, but the majority of its customers will be similarly new to the art. Hence, this review will be aimed at the SDR non-guru, the longtime radio enthusiast who viewed the LimeSDR Mini as the first ever transceiver.

The first task with an SDR is always to install the required software on the host computer. Here it means a copy of the latest Ubuntu distribution, but Windows and macOS computers are also supported. There is a handy page of instructions where in the case of Ubuntu you need to add a PPA repository for the drivers and then install the Lime Suite software and the SoapySDR abstraction layer. It is this final package that makes the LimeSDR an interesting prospect by shifting software compatibility onto the widely used abstraction layer that they hope will avoid some of the pain experienced with other products.

Test it out

A UK DAB digital radio multiplex seen through a LimeSDR.

Once the drivers are installed, it is time to decide which software to run first. The Lime Suite user interface that comes with the driver packages is the first place to go when testing the card. However, I was told that the version in the PPA at the time of writing was written with the unreleased mini with the LimeSDR USB port against it and so I should be using GQRX. In the case of Ubuntu, this can be installed through the graphical software installer, but luckily I already had it on my computer. If I selected “Other” as the SDR and inserted driver = lime, soap = 0, because my device string soon had the familiar interface in front of me and, with a suitable antenna, was able to hear my local BBC Radio 4 FM station in no time.

A simple FM transmitter derived from this example from [Gyaresu]Two things are immediately apparent to an owner of an RTL-SDR: the large number of glitches is gone and the background noise is much lower. Reading GQRX with different front ends is an inaccurate and even somewhat meaningless way of making measurements. With fully automatic AGC, the RTL has a noise floor of -60 dB and the LimeSDR a base value of -90 dB. If you only look at the FM band, there are stations that stand out from the noise and simply do not exist with RTL. Unsurprisingly, dedicated SDR hardware outperforms a $ 10 TV stick that hacked to turn it into an SDR. However, if you’re an RTL SDR user, you’ll be pleasantly surprised by the Mini when you see it in action.

Fortunately, there is nothing too sensitive controlled by this 433MHz waveform that I just revealed to the world.

Having a board like the Mini and just using it for GQRX means wasting so much of its potential. We are promised a library of bespoke applications via Snappy Ubuntu Core, but it will not be available until it is released. Your next stop would likely follow ours with GNU Radio, particularly its drag-and-drop GUI application, GNU Radio Companion. This is an absolute homebrew radio for the SDR age, just like analog radio amateur homebrewers solder their own radios while others bought shiny transceivers so the SDR homebrewer could use GNU radio to build their own sets. It’s a package beyond the scope of this test, but as an example playing with the Mini it was pretty easy to cobble together a little GNU radio receiver to pull in the signals from a 433Mhz remote control transmitter we have in the and extract house, and then vomit them through a 433MHz Baofeng antenna for the satisfactory sight of a table lamp at the other end of the bank that turns itself on. The Mini itself does not intervene in this process, only to do what it is told to do once its communication with GNU Radio is complete. Experimenting with the mechanics of decoding the bitstream itself became a matter of going through a series of tutorials and burning the midnight oil. The steep learning curve is amply offset by the satisfaction of playing with the instant gratification of radio chips without reworking the pain of a complicated soldering job.

In terms of transmission, the maximum output power of 100 mW is quite modest for anyone used to amateur radio. Given that many uses for this board involve sniffing and responding to more local devices rather than looking for contacts from other continents, this is likely just a problem for radio amateurs without the necessary requirements for a power amplifier. The dual antenna connectors are a bit annoying when you’re used to a single one on a simplex transceiver, unless you naturally transmit and receive on different frequencies.

A review of an SDR over a short period of time cannot hope to cover all of its many functions. So this was an impression of the Mini as a platform for experimenting and learning to use an SDR transceiver. But just in the time it was here in the bank, the Mini has opened up a significant new perspective on an RTL-SDR, and given a few months it can play radio on GNU, I’m sure to both get some useful radio uses provide and a seriously interesting learning process.

Earlier end-of-budget SDRs such as the HackRF were all quite expensive purchases that a typical radio amateur might find difficult to hide from their partner in their family accounts. With a price approaching impulse buy, the Mini has the potential to become an SDR transceiver for everyone. If you held back because of the price, maybe it is time you checked out.

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