Four Band Digital HF SDR Transceiver Offers High Performance For Only $60

Amateur radio is a hobby that is often thought of as being exclusive to those with a healthy expendable income. In recent years however, the tides have turned. Cheap microcontrollers and signal generators have helped turned things around, and the $60 USD QDX from QRP Labs goes even further by sending the performance/price ratio through the roof. You can see more details in the video below the break.

The QDX is the creation of [Hans Summers] who is well known for producing affordable high performance amateur radio kits that are focused on low power transmission, called “QRP” in ham radio parlance. What is it? It’s a pocket sized four band (80, 40, 30, 20 Meters) software defined radio (SDR) that is designed to be used with some of the most popular digital radio modes: FT8 and JS8Call, as well as any other FSK based mode such as RTTY. It’s also been tested to work well (and within spec) on 60 Meters.

While classic radios have to be connected to a computer through a special hardware interface, the QDX is designed to connect directly to a computer through a standard USB A>B cable. CAT control, PTT, and Audio are all handled directly by the QDX, and no special interface is needed. While the radio is essentially plug and play, configuration, testing, and troubleshooting can be done by connecting to the QDX’s unique serial console, which among other things contains a text based waterfall. For those who want to run their own SDR receiver, I/Q output can be sent directly through the sound card.

Now for the bad news: due to global chip shortages, the QDX is out of stock at the moment, and there’s no telling when they might start shipping again. QRP Labs is looking to source parts wherever they can to get more of the units made, but of course, so is everyone else right now.

31 thoughts on “Four Band Digital HF SDR Transceiver Offers High Performance For Only $60

  1. I have purchased three of his radio kits. They are absolutely rad. I haven’t dabbled in digital yet and am still holding out for the all-modes all-band transceiver . Because of the incredible price point and ease of use the radios perfectly hit the sweet spot. I can credibly claim a homebuilt station but I frankly do not have time to do a full homebrew build nor funds for a commercial ready to use QRP CW transmitter- those are several hundred dollars. No thanks.

    Great job. Great radios.

  2. Interesting hardware. I’d love to see a RX comparison with something like a Perseus SDR (~€839/~$950) or an Airspy HF+ Discovery (~€150/~$169.00). Because it is using a 24-bit delta-sigma ADC (AK5386), it should not be a totally unfair comparison. The one thing that may limit it in performance is the choice of 48kHz for the ADC, which I’m not quite clear why. Maybe the MCU (STM32F401RBT6) is being under clocked (for less EMI) and that is what is limiting the USB 2.0 FS (12 Mbps) to less than the maximum I2S allowed of 192 kHz.

    But for TX, to me, it looks like it is using the Si5351A Digital PLL signal generator chip to output a single sine wave that can be adjusted in frequency. So technically RX is SDR, but the TX is a little bit unusual.

    1. That’s how FSK was originally sent, switch a small value capacitor in and out of an oscillator circuit, be it a crystal or LC oscillator.

      Audio tones into an SSB transmitter came later, in part to avoid modifying the commercial rig.

      If the software generated quadrature tones, it would be easy to feed into an IQ modulator.

      Instead, I gather this decodes the tones, and then the controller has the synthesizer shift to the needed frequencies.

    2. The receiver is only designed to receive a 3 kHz slice of the band, as that is all that is needed for the popular digital modes such as FT8. Using a higher sample rate rather than 48 kHz would just waste power, which is not desirable in a radio that may be powered using batteries. The ADC will use more power at a higher sample rate, as will the CPU because of additional processing demands.

      1. That’s not strictly true:

        > Using a higher sample rate rather than 48 kHz would just waste power

        High oversampling ratios still lead to larger dynamic ranges after digital filtering & decimation.

        The question is whether that matters, or whether you’re in a regime where noise, interference, local oscillator phase stability… are dominating and having more effective bits in your samples helps at all.

        1. The dynamic range of a 24 bit ADC already substantially exceeds the performance of the rest of the front end. Trying to enhance it further with decimation would be gilding the lily. That technique is important on a DDC receiver, where the front end ADC has an output of 16 or fewer bits, but not so much here.

    1. Nope, no voice. The transmitter works by amplifying the signal from one of the outputs of the Si5351A. The firmware monitors the audio frequency at the input and changes the output frequency of the synthesizer over I2C appropriately. There is no modulator in the conventional sense, and it can only send one tone at a time. It works for RTTY, FT8, FT4, and any other digital mode that sends a single tone. It is not designed for on-off keying or for multi-tone digital modes.

      The receiver receives SSB and sends the audio to the computer over USB. It can receive CW, voice, and other modes.

  3. I was hoping to just study the schematic, build my own, and then flash the QDX firmware but the way they do firmware is interesting. When placed in DFU mode, the microcontroller presents itself as a storage device where you place an AES-256 encrypted binary and I’m guessing the microcontroller decrypts the binary and saves it to the EEPROM. Maybe you can glitch the microcontroller into reading back the firmware, grab the key, and then decrypt a firmware binary to install yourself but that would require you to own a legit QDX in the first place. There’s no way to bootstrap a clone. That’s a great way to market this thing, show everyone how it all works but keep the secret sauce secret.

    1. Are you sure about AES-256? I just looked briefly at the latest firmware file, way to much 0x00 and a repeating string for a file entirely encrypted. Either it’s some chunks of data that are individually encrypted or whatever.

      (I don’t want to copy this firmware or to break this guy business, i am just curious…)

      And BTW, you mean FLASH, not EEPROM probably.

    2. I don’t fully understand your comment, but I did accidentally create magic purple smoke with one of my radios, and much to my happiness I could just order the main chip directly from him, firmware and all, for $6.

  4. Nice, I got him interested in DDS tuning years ago discussing mods to one of his SSB QRP kits, he asked if I wanted a FT-817 or a kit, and I answered I wanted both!
    Cool that he made it and sorry I missed the first crack at it.

  5. I managed to get one about a minute after the launch started. I’m looking at the box sitting on the table next to me. I’ve got a bunch of Hans’ radios, two original QCXs, one for 40m, the other for 30m, three QCX-Minis, two I purchased pre-assembled, and one as a kit, one 40m, two 20m. I’d bought the 20m kit, then he started offering pre-assembly. The only single-band QRP rig that competes with the QCX-Mini for size and performance (that I’ve run across so far) is the VN-XX02 by Haru JL1VNQ. I have the 20m VN-2002 hanging fire to be built. I’ve had some trouble with chronic pain keeping me from building, or frequently, from even operating, so I have a number of kits sitting waiting.

    Another very nice, even though rockbound kit, is the CRK-10a from CRKits. It’s very small and packed with features. If you’ve got a bunch of crystals sitting around, this may be a nice little quick operating rig. But Adam Rong has followed suit with his “13 Transistor Transceiver for Digital” that costs $59, including slow air shipping and the enclosure. It’s only available for 40m currently. I have the QRP-Guys DBD digital mode kit, but I’m not really impressed with it, and I think I’ve already borrowed a part from it to finish another rig rather than wait to get one from Mouser.

  6. Actually five bands. Although the 60 meter band isn’t officially included, hams have tried it out on that band using the 40 meter setting; it works just fine and complies with FCC spectral purity requirements. The legality of FT8 operation on 60 meters in the US (and other countries such as Canada where the rules on the band are based on the US ones) is debatable, but that’s a discussion for another day.

        1. Yes! WSJT-X communicates with and controls QDX over the Virtual COM interface using common CAT protocols. WSJT-X can put it into transmit and receive. That depends on the mode you use in WSJT-X, some are fully automatic (WSPR), others are not.

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