Minimalist Amateur Xcvr: Software Defined Radio

Receiver

• Diplexer routes VHF signal to module on main board, HF signal to HF board.

• Upconversion superhet architecture to allow continuous 0-30MHz reception. After a 0-30MHz lowpass filter, the entire input spectrum is upconverted. Similar strategy to uBITX v5

• ADE-1 first mixer terminated in diplexer and amplifier.

• 45MHz / 30kHz wide monolithic crystal filter acts as roofing filter. Followed by three  hybrid cascode IF gain stages with AGC input.

• IF section of radio is reversible for use in both RX / TX

• ES8388 codec interfaces with Quadrature Sampling Detector / Encoder

• PCM5102 audio DAC drives headphones

Transmitter

• Push-pull power amplifier with flexible biasing. CW mode uses fixed VGS bias, SSB mode uses onboard current sensor to control to bias to precise current per transistor

• Variable transmit power. Adjustable with I/Q output amplitude, AGC voltage, and power device biasing

• RD16HHF1 power transistors soldered to aluminum PCB for heatsinking. Aluminum base plate exposed through enclosure to allow optional external heatsink

• Temperature sensing and protection

• Three lowpass filters to span portions of 0-30MHz operating range. Less sensitive to toroid winding variability than Class E inductors on MAX-3B

• Semi break-in CW

• Key shaping through DSP (not yet implemented)

Power

• USB-C connector

  • USB-PD negotiation for 9-15V, >=1.5A

  • If USB-PD negotiation fails, 5V supports reprogramming

• Barrel jack for DC input

• Internal 3S battery for IFR14500 LiFePO4 cells (AA size)

• Switching regulators synced to processor-controlled clock source to avoid interference

• TX: 1.6A @ 12V, 10W CW out on 20m

• RX: 205mA @ 12V

Digital & Connectivity

• ESP32-S3 System on Chip (SoC) creates its own WiFi network or joins an existing WiFi network

• Dual core 160MHz processor

• Optional QSPI flash and PSRAM

• Multi-color LEDs for status indication

• Microphone input w/ PTT

• Onboard USB-serial converter presents COM port. Used for programming, and can support CAT interface (future work)

• Radio implements REST API for remote control

• TRRS headphone pushbutton as backup straight key (future work)

• Over-the-Air (OTA) firmware updates (future work)

Is this actually a SDR? There's no waterfall and the bandwidth is only 8kHz.

Yes, it's a SDR in the sense that I/Q data are passed to the processor, and any modulation/demodulation scheme is possible with DSP. Demodulation of USB/LSB, narrow CW audio filters, and volume controls are all implemented in firmware.

Does this only work with iPhones?

Any web browser, including a computer, can act as the front panel for the MAX-3B. Multiple devices can connect to radio.local simultaneously. The radio's API could work with a mobile app.

How well does it work?

Good enough that I've completed a SOTA activation with it! This is a hobby project - is not a commercial product and I have not measured performance.

There are a few shortcomings I have identified:

• AM breakthrough occurs on my home antenna. A small inline high-pass filter solves the issue and will be added to the next board revision

• Sideband suppression is not as good as the MAX-3B.

• Strong signals within the passband of the 15kHz wide roofing filter change audio quality

• Power amplifier linearity needs improvement. 

Can I modify this project? Can we collaborate?

Collaboration is appreciated on this project. I'm not a software engineer or web developer, so the project could use some expertise adding features and improving the quality of the code. Please contact me if you would like to work together on this!

This radio is meant to be a high quality platform for amateurs to develop solutions that serve their needs and preferences. The software is licensed under GPLv3 to encourage collaboration and tinkering on derivative hardware. The hardware is licensed under the TAPR Open Hardware License to encourage disclosure of design materials in any derivative work.

I found a bug. What can I do?

Please file a bug on GitHub or send an email to ki6syd@gmail.com

How can I get one?

Currently, the MAX-SDR is not sold anywhere as a finished product, and I am not able to commit to a schedule for production runs of new hardware. If there's sufficient interest, I will coordinate assembly runs and kits of parts so builders can make their own. I may have a small number of initial boards set aside for people who commit significant time to field testing or software development. Please send an email to ki6syd@gmail.com if you are interested in a board. Please note that this is a project for the advanced builder, and I may not be able to help debug issues.