TRX-Duo Is A Red Pitaya Clone For Software Defined Radio

If you’ve noticed the TRX-DUO software-defined radio transceiver, you might have wondered how it stacks up to other choices like Red Pitaya or HackRF.  [Tech Minds] obliges with a review of the Red device in the video below.

While this unit isn’t inexpensive, it also isn’t as expensive as some of its competitors. Sure, you can pick up an RTL-SDR dongle for a fraction of the price, but then you miss out on transmitting. The device is pretty powerful compared to a cheap software defined radio:

  • Frequency: 10 kHz to 60 MHz
  • CPU: Zynq SoC with a dual-core ARM Cortex A9
  • RAM: 512 MB
  • OS: Linux
  • Connectivity: Ethernet and USB connectivity (WiFi with a dongle)
  • ADC: 16-bit at 125 MS/s (2 channels)
  • DAC: 14-bit at 125 MS/s (2 channels)

The board boots off an SD card and there are several to choose from. The video shows two different images. One has a number of applications that run on the device and will also run on a Red Pitaya. The device shows a browser menu with various options and the result is quite impressive. Using the box as a WSPR beacon, it was heard fairly well given the low output power. It was, however, able to hear the world easily.

You can get a less capable Red Pitaya model for about $100 less than the going price. However, for something comparable, you will pay more for the Red Pitaya and — depending on capabilities — perhaps a lot more, although you do get more capability for the increased price.

You can do a lot more with a transmitting SDR — having both transmit and receive opens up many new projects. Of course, canned applications are great, but if you get one of these, you are going to want to try GNU Radio.

33 thoughts on “TRX-Duo Is A Red Pitaya Clone For Software Defined Radio

      1. QRPp. I was told 25 watts is not QRP.

        I have seen it defined as under 100 watts.

        100 watts is the level of most multiband HF transceivers without an external amp.

        I identify my hr2600 as QRP, a hundred watts output would help make more qsos.

          1. “QRP” is one of the q codes, originating in the days of hams using Morse code. It means “lower your power”. There’s also “QRO”, raise your power.

            In the fifties, there was an escalation, lots wanting the 1KW power limit. This was the late days of AM, which isn’t so efficient. As ssb was phased in, it was easier to get 1KW.

            So some time in the sixties, there was a sort of countermovement, “let’s see what we can do with low power”. I’m not sure the level was well defined, but it was helped by the changeover to solid state. It took some time for hams to get more power out of transistors.

            100W is QRP compared to a kilowatt. 30 watts is higher than 5, but not that.much. Under a watt is definitely QRPp.

    1. 4 dBm for the “TRX duo” and 9 dBm for the Red Pitaya. And approximately the same power level for all SDR at the DAC output (including all OpenHPSDR motherboards)

      Building a linear amp is easy for all licenced ham radio, and several projects could be adopted by people able to use a soldering iron. For example :
      – the openHPSDR pennywhisle . Old but efficient.
      – the “good old 20W G6ALU”
      – and a new and modern approach by XQ6FOD specifically designed for this kind of baseband SDR and using a 50 W LDMOS dual transistor

      2 to 10 dBm is the perfect “sweet spot” for a modern high-end exciter

      VY 73′

      1. “The output stage is a single stage amplifier that uses a pair of TO-220 “16 Watt” Mitsubishi RD16HFF1 parts in push pull and has approximately 19 dB gain, depending where you bias it, so it will deliver 16 to 20 watts output with 1/4 watt drive. ”

        Hm. That’s 250mw input. Not 2,5mw.

        73, Joshua

        1. your just need to change the “rare to find” input transistor with a MMIC. (I think this mod is described in my blog)

          a fourth option (with a 5 W output stage) woud be in a small “OPA2677/AFT05MS003 ” amp like the one used in the HermesLite V2

          VY 73′

    2. That is a slightly ignorant comment. What matters is the quality of the signal. If it is good then you can always amplify to kW if you like. But if it is poor, with noisy sidebands, spurii etc you musty stick to vert low power.

      1. You’re not wrong my friend, but there’s a catch.

        The output power is so low (TTL oscillator level, but sine), that it’s useless for driving a linear.

        It requires at least an one-transistor-amp, say BC548/2N2222, in order to reach 100-500mw even.

        From there on, another amp is required to reach a few watts.

        Now, commercial PAs for shortwave, like the Yaesu FL-2100b, need 130W on its input to fully produce a KW.

        Sure, this works. But the problem is noise. Each amplifier stage will introduce more noise because of the transistor (it has self-noise).

        And last but not least, there are harmonics. Each transistor stage would need some filtering, if we want to do things properly. This can either be a band-pass or a low-pass.

        But that’s complicated to implement, if the whole linear is supposed work on a wide scale (all shortwave bands). For a monoband PA (say 80m, 40m, 20m), it’s possible with moderate efforts, though, I think.

        If this TRX just had ~100mw out, at least, things would be much easier. A single homebrew PA made with a simple transistor or an old 12BY7A driver tube would produce a few watts easily. With merely little extra noise.

        That being said, I’m not a fan of QRO (100W up), either.
        I also have an aversion against those CBers who don’t understand that radiation patterns matter or that one S level increase requires another 6dB or 4x the power.
        (4, 16, 64, 256, 1000, 4000 etc)..

        On the other hand, it’s also some sort of illusion, not to say propaganda, to believe that 0.5W is enough to work the world.
        Because, QRP requires effective antennae and there are so many factors that come into play.

        A long wire antenna with a matchbox in a garden may or may not do. Ground/earth is an issue, often. With a sandy ground, the antenna won’t work properly, for example.

        Sure, a little antenna works great on a boat or by the sea.
        Water reflects, provides excellent earth/ground.. Amateur radio on a tropical island must be fun, even with QRP! 🙂

        And an half-wave dipole doesn’t need extra ground. But for 80m band, that’s two wires of 20m each. Who has that space these days? Sure, if you’ve got a mansion..

        That’s something that always angers me.
        The QRP advocates don’t mention that low-power operation requires an effective antenna system.

        And a location with little man-made noise. In other worlds, a typical field-day location.
        This will frustrate all these younglings who believe the stories about QRP.

        Which in turn end up being heartbroken and totally disappointed. Which in turn makes them leave amateur radio.

        Or make them limiting themselves to VHF/UHF bands.
        There, they will find abandoned repeaters, where in worst case no one is willing to answer a CQ call coming from a stranger. Very sad.
        Gratefully, APRS and amateur satellites are still there, at least. And Echolink, as a last resort.

        In other words, QRP advocates raise false hopes among those beginners that dream about making QRP contacts from home.
        But in a city, with a 5m wire, a few mW just won’t cut it. 5, 10W (or 25W) are more reasonable here.

        If those QRP fans only were honest about the expected results.
        If they would only mention, as well, in their blogs and videos about their failures and what’s really necessary to work QRP successfully. Then, it could really be fun. But currently, they raise false hopes. Present themselves in best light, don’t care about the beginners.

        Let’s remember, about 10W is the maximum we hams in some EU countries can radiate without the need for registering our aerials.

        Together with a magnetic loop antenna, the noise/QRM (mainly electric fields) can be blended out on the recever side.

        But once we’re using a mag. loop for transmission, as well, we need those 10W+ input again, because of the size limitations.

        A magnetic loop is space efficient in comparison to half-wave dipol, but a properly sized model for 80/40/20m would still be large. So we must use a smaller model, and for compensation, feed it with more power.

        Best regards, 73s,

          1. A warm antenna, outside in the garden. On a snowy winter’s days, this prevents the antenna from freezing. 😁

            Seriously, though. A warm shack is no good sign. This can happen, if the SWR is high.

            But not necessarily, if the antenna is resonant, but ineffective (like a very short antenna with a coil).

            Another cause for a warm shack is another bad habit: Using an antenna tuner (aka matchbox) in the shack. That’s nonsense. Seriously.

            Not only is by comparison the efficiency lower than that of a proper antenna, but the tuner wastes a lot of energy also (becomes heat).

            It’s as if matching a light bulb. It works, but it’s not an efficient antenna.

            An antenna tuner must be connected directly to the antenna feed point – not via a long coaxial cable, also.

            If a coaxial cable is connected to the ANT port of a tuner, it gets matched, too. The whole coaxial cable becomes a radiating part of the antenna system.

            Lots of CBers and HAMs do that mistake: They do have the antenna tuner standing directly besides the radio. That’s silly. For a field-day, it’s okay at best.

            So all in all, it’s better to use an antenna that “looks” electrically fine by itself. It can have a loading coil, too. Or a matchbox at its feed point. Depending on the specs of the antenna itself, a little “help” by these things won’t necessarily prevent a reasonable efficiency.

            Best wishes, vy73,

    1. Sadly the LoRa modem and waveform are patented, so there’s no good TX/RX SDR stacks for it.
      As far as I know there’s only one receive only implementation based on reverse engineering.

      As for LoRa on that pitaya clone, one would have to use one of the upper Nyquist zones from the DAC and a band pass filter.
      Conveniently there’s absolutely no filtering on it’s output, so one can select their desired harmonic or Nyquist zone as desired.

  1. This device doesn’t seem to have an RF frontend. So I’m not sure how it’s transmitting / receiving. Is the mixing (with a carrier wave) happening in software? The Pluto+ SDR goes for about the same price on as the TRX-Duo on Aliexpress, but has an actual frontend. It has modded hardware to enable two transceivers off of the AD9363 (make it operate like a AD9361), a Zynq7010 running Linux, and a high quality oscillator. Anyone know how the TRX-Duo stacks up to the Pluto+ SDR ?

    1. the TRX duo is a 16 bits 125MSPS fully openhpsdr compliant. You can add any kind of compatible frontend you want : simple bandpass if you don’t want to use all the SDR features, or evolved frontend like Alexiares with variable bandwidth filter, configurable attenuators, high band LNA a.s.o . With Pavel Denim’s gateware, it also run on an embedded linux (alpine) and the board also uses Xilinx Zynq SoC family devices. As a solid baseband platform, it (TRX duo or Red Pitaya 16) could be used as a high dynamic HF (100 kHz/65 MHz) rig, or as a low phase noise “I.F.” in conjuction with low noise transverters.

      Pluto is an entry level 12 bits SDR (I don’t remember exactly, but probably 40 MSPS) with a bunch of mixers (and birdies) and very weak frontend preselector.
      It’s a nice and cheap piece of hardware, particularly efficient when dealing with UHF frequencies.

      you can hardly compare Pluto and TRX Duo/red pitaya. Both have their own advandages, the Duo/Pitaya beeing more dedicated to ham radio and experimenters interested by medium to high perfs equipments in the HF field, the Pluto targeting people looking for an “off the shelve” UHF device with medium performances.

      (and yes, I’m using both ;- ) )

  2. I see responses about the super low output. That’s nice. Personally, I think those interested enough in purchasing the unit and working with the software will either be satisfied with just listening or will be experimenters and Hams. The latter will have an interest in modifying and advancing it to a useful output. If it doesn’t have enough output, and you want SDR, go with a commercial unit already built and ready to go. I’ve had fun with the inexpensive dongles for listening. I have a Xiegu G90 that is 25 watts, and for higher power, an Alinco DX-SR8. I’ve also built tube based 5 watt transmitters, and receivers for them. If you want to learn and experiment with today’s technology, this may be the ticket. Sounds like a great stepping stone like so many of the magazine articles of my youth…

  3. The improvement of TRX-DUO compared to redpitaya is that its front end is designed according to 50ohm normalized impedance, unlike the bad high impedance of redpitaya, you need to use a balun for conversion, otherwise you cannot get good reception performance, two independent channels RX can achieve diversity reception, which cannot be achieved by other receivers

    1. Hi Justin. I’m afraid you’re confusing the Red Pitaya 14 and 16 bit boards. The 14-bit did have a high impedance input (easy enough to modify by the way), but the Red Pitaya 16 -of which the TRX-Duo is a clone- has a native 50 Ohms input.

      You are quite right to insist on the reception capacities in diversity mode. We could add that the double ADC can also be used as an input port for pre-distortion (compensation of linearity errors in the TX amplification chain) and that Pavel Denim’s gateware offers the possibility of experimenting with very high efficiency linear amplifiers (85% and more) with the envelope elimination /restitution -EER- mode. All these improvements are impossible to achieve with a Pluto board or with any other commercial device (except for OpenHPSDR platforms)


      1. Yes, I totally agree with you, high-level functions such as diversity reception and digital pre-distortion used to require expensive equipment to get, I think now TRX-DUO makes burgers taste the experience of foie gras

  4. And everybody forgets to look at the used FPGA. In the STEMlab it is a 7010 like in this piece of equipment which is enough for 8 parallel receivers for instance for FT8 or WSPR. In the SDRlab 16 bit it is the 7020, enough for 16 parallel receivers. This piece of equipment is half the price because it has half the power in the FPGA (even less when you compare available IP/DDS etc). So it is a kind of 50 Ohm SDRlab front-end with a “standard” Red Pitaya back-end. Not a clone, but a new idea.

    1. No, both are using a couple of 125 Mega sample per second analog to digital converter. In other words, they are able to cover 125/2=62.5 MHz (more exactly 122/2 for technical reasons). And no more.

      Depending on the input antialiasing filter, you can listen to the first nyquist zone, in other words from 0 to 61 MHz on both systems.

      As Erwin wrote, the main difference between these two 16 bits dual ADC boards is in the choice of the FPGA.

      For US export regulation reasons, it is almost impossible to build a 16 bits baseband SDR able to offer more than 125 MSPS… thus explaining this 61 MHz bw limit.

Leave a Reply

Please be kind and respectful to help make the comments section excellent. (Comment Policy)

This site uses Akismet to reduce spam. Learn how your comment data is processed.