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I'm currently on an SDR kick and reverse engineering and then emulating all of the remote controls in my house. There are numerous solutions to transmitting signals over 315 or 433 MHz and you can get super cheap boards available on Amazon to do so.

One of my remotes is operating at 350 MHz (a Hunter ceiling fan). I have no clue how to cheaply replicate this signal. I can replicate using a HackRF One, etc, but using a cheap electronics board? I can't even find oscillators super cheap for that specific frequency, and am kinda at a loss.

I know that companies (Hunter, in this case) make these remotes super cheap, but I can't figure out how they are transmitting on that specific frequency (350, NOT 315/433). I haven't taken my remote apart, but research and FCC lookups suggest Hunter typically uses Holtek chips, but I haven't found anything directly relevant. The FCC lookup (internal photos) is a bit blurry and doesn't show the actual chip for my specific remote control.

How do I generate a 350 MHz signal with a microcontroller with cheap components? (i.e. NOT a $350 SDR)

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You usually don't buy an oscillator running at your RF – for example, there's no single "2.4 GHz oscillator" component in the wifi module of your phone. (for the low frequency of 350 MHz, you can actually build a relatively bad oscillator using a SAW filter and a transistor yourself, but don't.)

Instead, you buy some oscillator that generates a stable base oscillation, and then use a frequency synthesizer that generates a much higher frequency from that base oscillation. In the simplest case, you'd just buy a PLL chip, and a crystal oscillator running at an integer fraction of your target frequency (350 MHz).

A very much hacky way of getting a multiple of a base frequency is getting an output buffer that generates a square wave from the base oscillation, and then filter out the fundamental and all the harmonics you don't want, leaving you with just the harmonic you want. This works through the the discrete nature of the Fourier transform of the square wave. Look for the Fourier series representation of the square wave to understand where the harmonics appear!

Most RF SoCs include much more flexible designs for generation of clocks (see: fractional-N synthesis), where a much larger range of frequencies can be selectively synthesized from a reference oscillator.

The honest answer to how to build such a device is hence probably:

Look through the RF SoCs (which typically are a microcontroller, ADC/DAC and an RF frontend) of the major semiconductor producers. For example, Silabs has quite a few RF/microcontroller combos (example). That's the style of component most companies use when they need to add a sub-GHz RF interface to their devices. The example I picked can produce a whole range of frequencies, nearly continuous from 110 MHz to 965 MHz. It's not 100% an SDR, but chances are that the actual comms standard used by your remote control is indeed covered. You will, however, need to add impedance matching, antenna or connectors etc yourself. This is just a chip! Maybe there's cheap eval boards, I haven't looked into that.

Other than that: When paying for the HackRF (or other SDRs), you're paying the price for getting a device developed that works on a very large range of frequencies well, is tested, blocks out frequencies you're not interested in producing or receiving, comes with a whole firmware and driver infrastructure; when you don't need that level of sophistication, for example because you really only want to use a very limited set of frequencies and standards, things get cheaper, but you'll have to do them yourself, because they are now application-specific engineering of RF hardware.

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  • $\begingroup$ Thanks! I'll look into everything you suggested and hopefully come up with a solution. $\endgroup$ May 17, 2020 at 14:51
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I agree with everything Marcus wrote and have some further details on how to get 350 MHz easily and cheaply.

There are lots of overtone crystal oscillators supplying 350 MHz in a small footprint on the market for very low cost (ranging from \$3.94 to $\10.80 in single piece quantity from Digikey with manufacturers including Renesas, EPSON, ECS, Skyworks, and Abrocon. If a single frequency was needed at 350 MHz, this would be the way to go. I notice right now none are in stock and lead time has been a current problem in the component industry. Clicking on lead time indicates a 3 - 6 month wait.

For further flexibility in terms of being able to change to different frequencies, the Analog Devices ADF4360-7 is a good option: this is an Integer-N synthesizer with integrated VCO that provides an output frequency that goes down to 350 Mhz. This part is available from Digikey for \$10.44 in one piece quantities, to be used together with some external components including a low cost reference crystal. For a home project, it would be worth buying the eval kit for $160.65; the board is optimized for operation at 1200 MHz but can be easily modified for use at 350 MHz.

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