0
$\begingroup$

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)

$\endgroup$
0
$\begingroup$

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.

| improve this answer | |
$\endgroup$
  • $\begingroup$ Thanks! I'll look into everything you suggested and hopefully come up with a solution. $\endgroup$ – slipvelocity May 17 at 14:51
0
$\begingroup$

To generate higher frequencies, the technique I've seen the most with hobbyists is the use of harmonics and mixers. Whether it is using a stable oscillator (transistors or xtal) and doubling the frequency a few times (mix it with itself -- a bad diode can do just that, or you could use a SA612), or using digital synthesis (DDS).

Look into the Si5351 chip, it's been used a lot by hobbyists, and even professionals. It is a clock generator (square wave) and you can use the filtered harmonics as your LO signal or even for your carrier. There are a lot of projects out there that have used it up to 1000 MHz using 3rd and 5th harmonics. Its maximum frequency is around 200-250 MHz, but given it's a square wave, you could set it to 116.667 MHz and use 3rd harmonics to generate a signal at 350 MHz, with proper filters in place.

I've also seen techniques using 3rd and 5th harmonic crystals to get to a frequency near the target frequency, and using an adjustable oscillator through a mixer to tune the rest of the frequency gap. For example, say you want a frequency range of 300-375 MHz, you can take the 5th harmonic of a 50 MHz crystal, which will get you a stable 250 MHz, and you can mix it with a variable frequency source of 50-125 MHz, to get your intended range.

There are also VCO chips that can reach pretty high frequencies, but they're usually not cheap and you would need a PLL to stabilize the output frequency. Using a PLL you can have a relatively low frequency reference (xtal generally) and increase it to much higher frequencies, as long as the VCO supports those frequencies. This is maybe the technique I've seen the most in professional products.

| improve this answer | |
$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.