Minimalist Amateur Xcvr: 3-Band

Receiver

• Bandpass filter for each band (double tuned circuit), switched in with relays. QRP Labs bandpass modules are the starting point for receive filter designs. Principle of operation covered here.

• Single conversion superheterodyne architecture with 10MHz IF

• IF crystal filter bandwidth is ~3kHz for SSB reception

• ~500Hz audio filter can be switched in for CW reception. Filter bypassed for SSB reception

• ??? dB unwanted sideband rejection (CW filter)

• Bypassable LNA. ~-140 dBm minimum discernable signal (LNA on, CW bandwidth)

• Potentiometer sets max audio level. Variable gain for headphone volume control

• Audio output for normal earbuds or 4ohm speaker

Transmitter

• Class E amplifier with 8x 2N7002 surface mount MOSFETs

• 8W transmit power @ 15V input, 5W transmit power @ 12V input

• Key shaping circuit

• Low pass filter for each band, switched with relays

• Semi break-in CW

• SMA connector for RF output. Relay selects between 50ohm output or integrated EFHW transformer

• TX harmonics at least 43dB below fundamental (per Section 97.303: https://www.ecfr.gov/current/title-47/chapter-I/subchapter-D/part-97/subpart-D/section-97.307)

Power

• USB-C connector

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

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

• 2 pin header (works with RC battery JST connector) for 6-15V DC input

• TX: 530-660mA @ 12V

• RX: 100mA @ 12V

Digital & Connectivity

• ESP8266 System on Chip (SoC) can connect to an existing WiFI network for operation at home

• ESP8266 creates its own WiFi network when home network not available

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

• Over-the-Air (OTA) firmware updates

• LED for status indication

• Radio implements REST API for remote control. Currently only a browser UI is implemented, but the same interface could work with a phone app, python scripts, etc.

• Expansion header w/ I2C, power, and RF output. Daughterboard can support 2m VHF and microphone

The following features are not implemented, but should be possible with existing hardware:

• Direct frequency entry via CW key

• CW beacon

• RTTY

• CAT interface over USB port

• CW decoder

• RIT

• Split operation

• Contact logging and QSO transcript logging

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 radio work?

The receiver is based on the SA612 mixer, which has relatively poor large signal performance. (See: https://studyres.com/doc/7790754/pa1dsp---why-not-to-use-the-ne602). Robust band-pass filters are included to avoid interaction with out-of-band signals, and otherwise performance is generally acceptable in remote operating locations (such as a SOTA peak). This is a similar receiver architecture to the LNR Mountain Topper series, which is widely regarded as suitably high performance for SOTA. Subjective RX quality was similar to a MTR-2B in a field test.

The IF crystal filter is wider than most CW only radios in order to support SSB reception and minimize ringing from the crystal filter. An additional CW analog filter provides ~500Hz bandwidth when CW reception is selected.

FCC Part 15

This project should not be considered a "product." The board is a useful subcomponent of a larger radio system that a builder can design, and once fully integrated can be one of the builder's home-built devices allowed under section 15.23: https://www.ecfr.gov/current/title-47/chapter-I/subchapter-A/part-15/subpart-A/section-15.23. 

The WiFi section of the board is not implemented with a FCC-certified module, so operation of this device should be in accordance with ham radio best practices and the ARRL band plan. No encryption should be used, identification should be done via a callsign in the SSID, and operation should not cause interference for other users of the spectrum.

Sparkfun electronics has a brief intro to topics like this: https://www.sparkfun.com/tutorials/398

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-3B 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 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.