The EELive! 2014 Conference and Exhibition earlier this year was a fantastic event. I had the opportunity to present three hobby projects that I currently have in the works. One of them is already completed, one is in process, and one is still in the planning stage.
The project I am currently working on is a 75MHz receiver for RC submarines. Now, you might be asking yourself: "RC submarines? Is this is some type of joke?" But it's true. All those times you were taunted for being out late at the submarine races, you may have actually been able to be so. This really is a fun hobby, but it does require some specialized electronics. In fact, my participation with RC submarines is how I got my start into electronics, but that is a story for another time.
My first RC submarine -- a converted 1:144 scale Seawolf class.
So why am I working on a VHF receiver for RC submarines? The answer, strangely enough, is WiFi. As WiFi became more prevalent, low-cost RFICs (radio frequency integrated circuits) came out for the 2.4GHz frequency band. In addition to offering more features, these chips were cheaper than the standard discrete components that were being used to build typical RC receivers and transmitters.
In the last decade, the industry as a whole made a mass migration to 2.4GHz, though this transition has not been a clean one. (See also Creating an Open-Source Multiband RC Transmitter for more on this issue.) Despite the benefits 2.4GHz can offer for many applications, one disadvantage is that it does not penetrate through water. With RC manufacturers stopping production of products for the old radio frequencies, this created an opportunity for me to try my hand at creating a 75MHz receiver for this small niche hobby.
I have actually had an interest in doing something like this for a while. Unfortunately, the combination of a lack of knowledge and the lack of parts suitable for what I was wanting to do slowed me down, but then I found a part that changed everything. This was when by Silicon Labs released the SI4704/5 -- a 3mm x 3mm QFN device that is intended for the reception of broadcast FM radio. The great thing about this part is that it covers all the way down to 64MHz and has an automatic antenna impedance matching feature.
The impedance matching feature is great for a number of reasons. First, it reduces the required area on the PCB. Second, I do not have the equipment to measure the impedance of an antenna, which would be required to be able to design a matching circuit. (Of course, if anyone wants to donate such equipment to "the cause," I would gratefully receive it.)
How then will this chip, which was originally intended for use with FM broadcast signals, be repurposed as an RC receiver? The signal that needs to be received at the receiver is an FM signal that is an FSK (frequency shift key) modulation scheme. What this means is that there is an underlying carrier frequency, and there is a shift frequency that gets added to the carrier frequency. In the case of RC products, that shift frequency is 5kHz.
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