SBITX: Hackable HF SDR For The Raspberry Pi

Cheap, easy to use SDR dongles are an immensely powerful tool for learning about radio technology. However, building your own SDR is not something too many hackers are confident to tackle. [Ashhar Farhan, VU2ESE] hopes to change this with the sBITX, a hackable HF SDR transceiver designed around the Raspberry Pi.

[Ashhar] introduced the project in talk at the virtual “Four Days In May” annual conference of the QRP Amateur Radio Club International. Watch the full talk in the video after the break. He first goes over the available open source SDR radios, and then delves into his design decisions for the sBITX. One of the primary goals of the project was to lower the barrier of entry. To do this, he chose the Raspberry Pi as base, and wrote C code that that anyone who has done a bit of Arduino programming should be able to understand and modify. The hardware is designed to be as simple as possible. On the receive side, a simple superheterodyne architecture is used to feed a 25 kHz wide slice of RF spectrum to an audio codec, which send the digitized audio to the Raspberry Pi. The signal is then demodulated in software using FFT. For transmit, the signal is generated in software, and then upconverted to the desired RF frequency. [Ashhar] also created a GUI for the 7″ Raspberry Pi screen.

At the moment the sBITX is still in the development stage, information is spread between the video after the break, it’s accompanying PDF, the GitHub repo, and a thread on the BITX20 group.

[Ashar Farhan] is well known in the ham radio community for low cost radio designs like the BITX, and it’s successor, the μBITX. He also created the Antuino, an Arduino based antenna tester.

21 thoughts on “SBITX: Hackable HF SDR For The Raspberry Pi

  1. Interesting starting out on the premise that “not something too many hackers are confident to tackle”..
    Have to be pretty out of touch these days to not notice SDR’s are something covered on almost every Youtube white/grey hat’s channel.

    1. Maybe I misunderstood but I believe the author Danie meant specifically building your own SDR from scratch. Not plugging in an RTL-SDR dongle into a Pi. But I haven’t watched the entire talk, yet, so maybe he’s Ashar is just doing that. But I doubt it :)

      1. More importantly this is a TX/RX device, not a simple reciever. I haven’t gone through the github but if its 10+ watts that could be a great starter HF radio or portable unit. The antenna placement and size is usually the biggest part of a HF install anyway (thus his antenna analyzer project!)

    2. This is far from the consumer using the receive only dongle ($30). This is much closer to the HackRF device ($300) but much easier to modify and customize the internals if desired. They serve different purposes.

    1. The problem with that design is it has a pathetic dynamic range in the range of 50 db and is very low performance …. This design here uses 24 bit audio codec with a dynamic range of over 90 db which makes it perform substantially better and still can be had a a very reasonable price. RTL dongles are great for starting out and getting your feet wet but this is a design that can compete with traditional Ham radios in excess of $1000 and in many ways outperform them

      1. Well, the samplerate of the RTL is with 2.4MSPS about 50x more than the audio codec, basically adding another 17dB processing gain on top of the 50dB 8-bit ADC range. So actually this make the audio codec solution (only) roughly 20dB better in terms of dynamic range.

        Looking into what SBITX does, it’s seems that opposite sideband rejection is sticking at 70dB due to the crystal IF, not to speak what this heterodyne setup negatively does to the overall IP3 performance.

        SBITX seems to me a complex solution without evidence for a real performance improvement…

  2. If you tell people “this is hard”, it self-perpetuates.

    SDRs are treated as black boxes because they are treated as such.

    I remember an early arricle that explained in general terms, no schematic because at that time it wasn’t feasible for the hobbyist (because of the lack of fast A/D converters, and I think computing power). But somehow they never took off. I think it’s the 2001 ARRL Handbook that shows an SDR that uses a soundcard. By then, the “very technical” articles were shuffled off to specialized venues, so the average ham didn’t see them, unlike earlier times when articles about parametric amplifiers and under the noise reception were next to one tube transmitters and how to solder coax connectors. So many didn’t seesuch articles, while evenfor a whike there were still direct conversion and simple superhet receivers in view.

    Just within the last few years, some guy who wants to be a leader denounced SDRs, too complicated, when a computer is doing most of the work.

    An SDR is a direct conversion receiver times fwo. The fact that the hard part that used to be done with audio phasing networks and filters is done with a computer makes it simpler, for better results.

    A simple SDR has limitations, but because of this perception, few are working on the part they can handle, building a better receiver before it converts to digital.

    Fifty years ago, frequency counters were starting to become viable as a project. Lots of tediius wiring, but little hard. But you get a better dial, without lots of fussy measurement. Synthesizers were starting to be a thing in ham magazines that year, greater stability and direct readout. But people are still building the simple direct conversion and superhet receivers of fifty years ago.I

    Maybe it was all an illusion, most hams weren’t builders?, but at least we saw good articles presenting that technical world.

    1. What we need is really good open source SDR software (including a proper gui and frontend) and then a standard for interfacing with the hardware. The biggest problem with most hacked SDR setups i played with a few years back was trying to figure out and locate each different software package for each function and then getting everything playing nice.

  3. The Tayloe detector receivers work very well–I gather what has put a damper on them is the lack of stereo inputs on laptops. The in-phase and quadrature channels need to be separated and the stereo microphone or line-in jack was the way to do it. Those aren’t so common any more and USB soundcards with stereo inputs are on the expensive side. DE AD8Y

  4. There is a long way to go from prototype to product. Ashhar is still working on ironing out these, so sBITx is still work in progress. A very interesting radio for sure, and out of the box thinking by the designer, as usual

  5. I have to admit that I am not impressed by this. I watched the entire hour long video. I admire Farhan for the BITX20/40 work that he did, and it was my own induction in radio homebrew. So for that, thank you Farhan.

    But this isn’t an SDR. It’s a superhet with DSP. Farhan posits it is difficult to get sufficient sideband rejection because of I/Q signal imbalance. I am sorry- this just isn’t true. But instead of learning how to solve this problem, Farhan has sidestepped it by leaning on old knowledge- the superhet design, something he’s already familiar with. So he slaps a DSP on a superhet and calls it SDR, but that’s *not* an SDR. Sorry Farhan. I’m out on this one.

      1. Real SDR’s do not have a crystal filter and feed quadrature signals into a two separate ADC’s. Sideband cancellation, demodulation, audio amplification, and really any other item you can imagine installing in a radio to make it perform (AGC, compression, IF shift, etc) is done in software. A radio demodulates a signal and then feeds a non-quadrature signal into a DSP is just another radio.

        1. So RTL-SDR is not an SDR :D
          As the sideband cancelation is done in the r820t2 tuner’s image reject mixer and “I/Q” is generated from the sampled IF inside the rtl2832u chip with it’s ADC.
          Naturally the sticks with E4000 tuners then are SDR¨s, as they do I/Q sampling with Zero-IF.

          Current hamrigs like Icom IC-7300 do direct sampling, so there’s only a single fast ADC and everything after that is done in the FPGA and DSP.

          And yeah this is quite a bit nitpickery.

          If the demodulator is software, the rig is SDR. In my opinnion anyway.

        2. The fact that it is direct sampled IQ waveform is not what makes it SDR, the fact that the waveform processing is done by software is what makes it SDR. Direct Sampled SDR is just better SDR, and analog tuners and filters in front of an ADC still produces digitally sampled waveforms for software processing hence SDR.

      2. To be fair, you can add an input that skips the mixer stage and goes right to the A/D converter, and alparently goid for the shortwave range.

        So it’s sort of a hybrid, surely because getting a faster A/D converter for tv frequencies is still expensive.

        I can’t decide if the comment is nitpicking or right. I can remember when these things weren’t imagined (barely) so I don’t see “just DSP” as something lesser. I’m not in the camp where “SDR” means a black box, off the shelf easy solution to reception. I’m also not in the camp that dismisses SDRs because they use computers or “too complicated”.

        So I can see middle ground. There was a top end Collins receiver in the early eighties that was hybrid. Some of the commercial ham transceivers are hybrid. I don’t know enough to know how much current hardware is a tradeoff, so maybe those hybrids get you a better receiver for the money.

        But hams are still building the same basic receivers that they did in 1985 or 1971. They want “simple” even if a bit of change or complication gives you better.

        In that context, this hybrid is a good thing. Not too different,but a fusion of the familiar with the “new”. It’s a start to building better SDRs.

  6. I am very disappointed that he didn’t go for a wider digitization bandwidth for a semi-custom hardware design. There are many PC sound card designs which are all limited by audio frequency bandwidth.

    I wish him success, but I want more.

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