Software Defined Radio (SDR) is a wireless communication technology that is based on software-defined radio protocols rather than through hardwired implementations. The bandwidth, air interface protocols, and functions can be upgraded via software downloads and updates without completely replacing the hardware. Today, we walk into a simplified SDR radio design with Uncle BG5WKP, which is currently in the experimental stage. Let’s take a look!
USX or uSDX QRP HF Transceiver
Written by: vk3yy
The architecture of this transceiver is based on an interesting design by Guido, PE1NNZ. This transceiver initially used the very successful QCX as its starting platform, but now has taken on a life of its own and has done some temporary builds using dedicated PCBs on forums.
In handling SSB IQ from the Tayloe IQ detector for SDR applications, Atmel ATMEGA 328 is pushed to near its limits. The speaker audio is provided directly from the 328 pin as PWM. Therefore, no audio amplifier is needed.
The transmitter uses Class D or E PA and implements envelope elimination and restoration (EER) functionality, providing an efficient and effective linear power amplifier.
However, there are indeed some trade-offs in the design of this implementation, as they run on a simple 8-bit processor with a 10-bit A/D converter.
Currently, this version uses two boards, namely the UI and processor board and the RF board, which are connected in a stacked form via plugs. For design experimentation, a test PCB has been placed on the RF side. The processor board is built using a prototype board.
The device measures approximately 7.5 x 8 x 3.5 cm, and it is hoped to be used during SOTA travels to reduce weight. It is painted bright orange to avoid losing it!
Completed prototype
RF test board
Processor UI prototype
PWM speaker audio sounds great on efficient speakers but not so well on a 50mm small speaker. An LM386 amplifier was added to boost the volume to useful outdoor SOTA levels.
The RF board ultimately has many links, and chips need to be replaced and experimented with. I might make the final PCB, but for now, others on the UCX forum are doing quite well.
To speed up the build, I used an Adafruit Si5351a adapter board, although this adapter board uses a 25MHz clock, so some changes in software and hardware need to be made.
On the right, the newer version weighs 150 grams
Newer RF board
Processor board
They are getting smaller, it is the uSDX top RHS
So how does it perform?
RX audio sounds pretty good, although there were additional filtering issues while keeping the PWM frequency outside of the extra audio range. At low volume settings, some residual hiss is quite noticeable. The Tayloe detector has a false response on odd harmonics, so it relies on the transmitter LPF to reduce these harmonics. A BPF or similar QCX tuned bifilar coil will improve selectivity.
Selecting 13dB attenuation in the menu can eliminate the regular “pop” sound on the receiver, but it reduces some sensitivity.
Others have encountered noise issues with the OLED display when entering the receiver, although this version doesn’t mention it. Good power rail bypassing and grounding is crucial.
Transmission audio sounds clear, although it sounds a bit avant-garde. The transmission spectrum looks clean enough for amateur use, with harmonics and spurious around 50dB. I have successfully contacted some SOTA chasers in New South Wales. The output power is about 7W, and due to the margins in this level of design, this power should be reduced to 5W.
Considering all of the above, it should be fine for most cases on SOTA, although you might have some feedback on the TX audio quality.
In summary, this is a remarkable simplified SDR design that compresses almost everything into the processor. I look forward to achieving higher A/D resolution, faster speeds, and higher bit rates on more powerful platforms.
A fascinating simplified design, stepping into today’s simple, small, low-power SDR transceiver, DIY reliably without complex tools. An external ADC on the branch board can enhance the MCU’s signal processing capabilities. A truly good combination will emerge soon.
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