Cheap, Full-Duplex Software Defined Radio With The LimeSDR

A few years ago, we saw the rise of software-defined radios with the HackRF One and the extraordinarily popular RTL-SDR USB TV tuner dongle. It’s been a few years, and technology is on a never-ending upwards crawl to smaller, cheaper, and more powerful widgets. Now, some of that innovation is making it to the world of software-defined radio. The LimeSDR Mini is out, and it’s the cheapest and most capable software defined radio yet. It’s available through a Crowd Supply campaign, with units shipping around the beginning of next year.

The specs for the LimeSDR mini are quite good, even when compared to kilobuck units from Ettus Research. The frequency range for the LimeSDR Mini is 10 MHz – 3.5 GHz, bandwidth is 30.72 MHz, with a 12-bit sample depth and 30.72 MSPS sample rate. The interface is USB 3.0 (the connector is male, and soldered to the board, but USB extension cables exist), and the LimeSDR is full duplex. That last bit is huge — the RTL-SDR can’t transmit at all, and even the HackRF is only half duplex. This enormous capability is thanks to the field programmable RF transceiver found in all of the LimeSDR boards. We first saw these a year or so ago, and now these boards are heading into the hands of hackers. Someone’s even building a femtocell out of a Lime board.

The major selling point for the LimeSDR is, of course, the price. The ‘early bird’ rewards for the Crowd Supply campaign disappeared quickly at $99, but there are still plenty available at $139. This is very inexpensive and very fun — on the Crowd Supply page, you can see a demo of a LimeSDR mini set up as an LTE base station, streaming video between two mobile phones. These are the golden days of hobbyist SDR.

41 thoughts on “Cheap, Full-Duplex Software Defined Radio With The LimeSDR

  1. I have only the vaguest notion of how far developments have come with these.
    Is it possible to hook one of these up to for instance an oscilloscope probe and have this perform as a crude vector network analyser?
    Cause I’m getting at a point where I’m looking into VNAs but then I’d get a less pricy old one that at best has GPIB and then I’d have to write devices drivers myself if I want to pull data off it.
    Of course, ‘real’ VNAs scan a larger bandwidth, so would be more ‘real time’ than an SDR based one, in which you have to step through the frequency range bit by bit.
    However, one made from a SDR would be cheaper, smaller, and have easier access to data.

    1. There is an open source demo of a LimeSDR VNA for the bigger brother to this unit. You need a good directional coupler also, but it looks to work ok. This uses the same chip and driver so I see no reason the code would not work on this alsob possibly with a couple of simple mods.

      Using a scope probe with a VNA is a bit flacky. Depends upon the frequency your looking at how bad it is.

      Whats your VNA budget?

      1. I know.. I’d probably have to get an active probe, but those are not that expensive either these days.

        £300-£400, which nowadays is roughly equivalent to $.
        I know, I know, hardly anything for a VNA. However, I have seen VNAs being sold for this price on Ebay.
        It’s not meant as a heavy duty VNA, just to start exploring effects of PCB editing and circuits on EMC, preferably in real time to get an actual “feel” for the situation. You know… adding caps and baluns and such and see what really happens.

        1. Its difficult when you dont have a connectorised 50ohm system to measure. Calibrating and demebedding a probe is not simple. You can often solder UF.L SMD connectors onto a board which makes things simpler(especially if you can add the footprint at design time).

          All that said an SDR like this and a small loop antenna probe works great for sniffing off emmisions in a spectrum analyser mode. I like the HackRF for this now as its got a new fast sweep mode that will do 0 to 6ghz in less than a second. The HackRF is not as clean as the LimeSDR though.

          1. Thanks, you’ve given me the scan time and you indicate that it shows up on a sweep plot.
            0 to 6 GHz is amazing!

            What’s demebedding? De-embedding? What do you mean with that?
            An active probes only have to be calibrated once, surely?

          2. Generally we calibrate a VNA before each measurement, or at least whenever we change things like cables, connectors etc. It both removes errors associated with these items and brings the measurement plane to the point of the connector. This is generally done with a set of open short and thru standards that you connect to the VNA before measuring anything.

            Active scope probes are more useful on spectrum analysers. They are useful for probing a circuit and looking at the general signal shape and harmonics. This is great for EMC work, but the way you connect them to the cct messes up the impedence match, so unless you can calibrate out the effects it really makes the Vector part of the VNA rather useless.

            De embedding is thr mathematical way which makes it possible to remove the effects of a non standard test fixture.

            Agilent has a good guide.

          3. Hmm I just looked up the HackRf and it’s a bit more pricey than I expected: £300
            And it’s an 8 bit device, so at best it will have a 50 dB range.
            Whereas a bench top spectrum analyser or VNA will typically have 120 dB range.

        2. I’m planning to build some directional couplers in the winter some time and use them with my LimeSDR. Having 50 ohm ufl sockets on the board is a really good tip so that the rf circuit, whatever it is, can be intercepted between the main components.

  2. As a cellphone repeater, can it work with all American carriers? Can it handle multiple phones at once or just one? Having a gizmo that can have one antenna inside and one outside of a building with concrete walls or metal roof and siding, and handle multiple phones, would be very useful for all kinds of businesses in such buildings, especially in places with spotty coverage.

    1. Yes it can work with all USA carriers but you have to have additional pcb s with duplex filter chips on them. It’s simpler than it sounds. The main issue is wether the LimeSDR can really handle the full Tx power of a cell phone or not without distortion or burning out the sdr.

      1. ‘available’? You mean ‘officially allowed’.

        I wonder if any effort is even done to track if someone transmit a normal power signal on some obscure frequency and he/she isn’t interfering with in-use frequencies.
        I can’t imagine that the FCC for instance has many teams available dedicated to such things.

        1. In my neck of the woods, you can get away with a lot (as in pirate radio stations) as long as no one is complaining. But when somebody does complain, our FCC will show up on your doorstep, confiscate all your gear and slap you with a fine.

    1. There isnt any info about the rx performance yet. This will come once the production hardware is locked down. However, its probably safe to assume that it will be similar to the bigger brother LimeSDR which has more published info.

  3. For US residents I suggest looking at the FCC part 15 frequency allocations. The permitted power levels are *very* low, typically 100uV/M @ 30 M, but there are allocations spanning almost the entire RF spectrum in which ANY modulation is permitted. For those who think that’s too low to be usable, I point out that by design a GPS signal at the receiver is well below the noise floor of the front end. The deficit is made up via processing gain. Processing gain is a function of the product of the bandwidth and time used to transmit a symbol, so this is inherently limited to low data rates. For higher data rates more RF output power which implies the need for a ham license.

    I have a LimeSDR waiting for an enclosure and antennae and a newly arrived Siglent SSA3021X-TG, so I hope to be able to offer more information about the LimeSDR soon.

    BTW A VNA would require synchronizing 2 LimeSDRs or 3-4 LimeSDR minis. So not especially cost effective vs commercial hobbyist units unless you’re also using the LimeSDRs for other purposes.

    1. For a single A/D converter – yes, it would.
      However, this uses IQ sampling, which uses 2 converters sampling 90° apart, not one. It’s one of those neat ways where you see algebra actually being useful ;-)

    1. Very low… also – dabling in paid bands (like the ones used for LTE) outside of a lab and without a license is a good way to get FCC called upon…you will not be happy if they show up…

  4. I’ve got a suggestion for all you hackers out there. You like to make complicated things like SDR radio’s work. Many of us would like to play with SDR radios, but don’t have the time or interest in hacking one out of components. For instance, we’d like a SDR radio that’d attach itself to our existing WiFi network and allow us to access it—perhaps high up in our attic or on a pole in our yard—via our computers, tablets or smartphones. And we’ll like to be able to buy it plug-and-play.

    I love following websites like Hackaday, but as I do, I keep wondering why connections are not established with those who, while they have not interest in hacking, would like to bring these products to market.

    1. Very good point Michael. Actually, VCs do connect with projects here but it’s often veiled in secrecy, especially near the beginning of the marketing stage, for obvious reasons. However, it would be pretty cool if hackaday had a bunch of VCs waiting in the metaphorical corridors to scoop up some of the more financial viable projects.

    2. those SDRs exist, just drop >$1K. Law enforcement has been buying Ettus stuff for ~10 years now, do you expect average FBI drone hacking away in the command line to make his cell site simulator work?

    1. Thanks for the link. I have a LimeSDR. Very nice! However, the PlutoSDR is what I have been looking for. The FPGA / Arm9 combo + Linux + AD936x for $99 is unbelievable ( I have experience with the AD9364). Wish I would have caught on before they sold out. On backorder, but on the way.

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