原创 Designing an open source multiband RC transmitter

There is a conceptual project that I hope to make reality soon. I was able to present it at one of the Gadget Smackdown sessions at the EE Live! 2014 conference and exhibition. This project is an open-source radio transmitter for RC hobbyists. This transmitter will be reconfigurable, allowing it to operate over multiple frequency bands using multiple protocols. The plan is to create a modular, open-source, software-defined radio (SDF), but what brought about the need for something like this?

Typical hobbyist RC transmitter.

 

With the advent of WiFi and other chips operating in the 2.4 GHz band, there has been significant development in the miniaturization and integration of RFICs. Due to the integrated nature of these chips, and the many features they offer, the RC industry has moved from the traditional 72 MHz and 75 MHz bands to 2.4 GHz. This transition has been anything but clean. The old frequency bands operated under an (relatively) industry-standard protocol. This was a simple analog frequency-shift keying (FSK) protocol -- effectively pulse-width modulation imposed on an FSK signal. There were only two variations of this protocol, but both were well known and well- documented.

When the industry started its shift to 2.4 GHz, all the vendors used their own proprietary protocols. This means that transmitters and receivers from different vendors are incompatible with one another. This was not the first time the industry had tried this, and it quickly became apparent that the various vendors were up to the same old tricks. As the switch to 2.4 GHz continued, the number of proprietary protocols kept growing; there are now at least 10. The switch also reduced the number of vendors offering hardware in the old transmission bands. For those (like myself) operating RC submarines, this has been very detrimental, since 2.4 GHz signals don't penetrate the water.

What is one to do? This is where my solution comes into play. There have been great advancements in software-defined radios, which can transmit across multiple bands and be reconfigured to support multiple protocols. Lime Microsystems has a new RFIC that can transmit from 50 MHz all the way up to 3.8 GHz. This would allow for coverage of legacy bands as well as the new 2.4 GHz band. Since it is a software-defined radio, one could also configure the device for each proprietary protocol. As manufacturers develop new protocols, once they are reverse engineered, they could be uploaded to the transmitter. Then all the user would need to do is select the desired protocol at startup.

With the RF section conceptualized, the rest of the device would consist of a reconfigurable board that would allow the user to implement 1-20 channels. Each connection could be a simple on-off switch, an analog potentiometer for a standard set of gimbals found in current transmitters, or serial protocol devices. Devices using serial communications might include such things as motion and position sensors, magnetometers, or pressure transducers. Once connected to the inputs of the core, they would be configured either via a desktop application or on the screen of the transmitter itself. With the RF section, interface connections, and screen, this would form the core of a handheld transmitter.

The beauty of this approach to designing a RC handheld transmitter is that it allows users to create and customize their transmitters as required. Custom enclosure designs can be created and shared. If one needs to add another channel, this will be as easy as attaching the user input device to the core and configuring it. This transmitter customization approach will add a new element to the hobby. This element would be consistent with both the Maker/Hacker movements.

Personally, I want to design an enclosure similar to a device found from the TV series Leverage. In the "First Contact Job" in the fifth season, Harrison had a handheld device that he called the Marvin. This had a very unique shape that I think would lend itself to support the features I need for my RC submarines.

The Marvin, Leverage (Season 5, Episode 3, "The First Contact Job").

 

To pull this RC transmitter concept off, I will need to learn a lot. I have yet to really play with designing any sort of graphical interface on a microcontroller. I do not see this as a major hurdle, but it will require some learning. I will also need to learn a bit about general RF design and the nuances of ultra-high-frequency PCB design. Is this a lot to learn? Well, yes, but I find that having a project is the best way to start learning. If you happen to have any ideas, or if you would like to contribute your efforts to a project like this, please let me know in the comments below.

Adam Carlson is Senior Mechanical Design Engineer at Eagle Technologies.