Options for RC model oscillators?

[Lightning]

Hello,

I am designing a transmitter circuit for an RC model aircraft.

The law dictates that I use the frequency range 34.95Mhz (ch.55) to 35.30Mhz (ch.90),
10kHz per channel!

Quite a narrow bandwidth, so I need a oscillator that I can easily set to somewhere within this range.

Any circuit suggestions/comments would be appreciated.

Thanks.

 

[KrisBlueNZ]

Have a look at http://www.digikey.com/product-search/en/crystals-and-oscillators/programmable-oscillators

Download a few data sheets to get an idea of what they do, how you use them, and whether they would do the job.

 

[Lightning]

Thanks for your comment.

Using crystals is a great idea but I think using interchangeable resonators with their discrete values would be less tedious when changing the frequency to a different channel.

Any ideas on how to construct an oscillator out of a crystal resonator?

 

[KrisBlueNZ]

Yes! Lots of ideas can be found here.

But ceramic resonators are typically only accurate to 0.5% so not suitable for this application. Also they are only available in commonly used frequencies.

If you had downloaded and viewed some data sheets, as I suggested, you would know that these devices can be programmed for different frequencies. As long as you can set them to the frequencies you want, and as long as they are accurate enough, they are obviously a much better solution than having discrete components that you need to change physically.

 

[Lightning]

Thanks,

There are resonators specifically designed for model aircraft and I have seen them used to select different channels. The key design point is definitely precision as you said.

I did have a look at the datasheets but I am reluctant to use this method as there is no way of knowing what the frequency is in the circuit, unless I carry around a very accurate multimeter.

You mentioned the tolerance of ceramic resonators but don't crystal resonators have a smaller tolerance?

 

[davenn]

individual crystals are available
for 30+ MHz they will likely to be overtone crystals rather than fundamental crystals
you would use a Colpitts or similar oscillator for the crystal

Dave

 

[KrisBlueNZ]

I did have a look at the datasheets but I am reluctant to use this method as there is no way of knowing what the frequency is in the circuit, unless I carry around a very accurate multimeter.

I would be comfortable trusting the manufacturer's specifications, after doing an initial test to make sure I was programming it correctly.

Of course any device needs an accurate frequency source; if the device doesn't include one internally, you'll need to provide one, and its accuracy must meet the specification.

You mentioned the tolerance of ceramic resonators but don't crystal resonators have a smaller tolerance?

If you mean quartz crystals, yes. Have a look at this data sheet: http://www.abracon.com/Resonators/ABLS3.pdf

I THINK (not certain) that it's pretty typical of crystals in this frequency range.

As you can see, the crystal is available in a wide range of frequencies, and in fundamental and third overtone (harmonic) versions. This series of crystals is available with an initial frequency tolerance at 25 degrees Celsius between ±5 ppm (which corresponds to about ±180 Hz) and ±30 ppm (±900 Hz). This is typical for crystals in this frequency range. As well as the initial frequency tolerance there is a temperature stability tolerance across the operating temperature range (typically 0~70 degrees Celsius) of between ±15 ppm and ±150 ppm.

You can get individual crystals in certain frequencies from Digikey, but if you want specific frequencies that aren't standard, you'll have to buy a reel of 1000 units! There is only one frequency in the range you gave that's available from Digikey: http://www.digikey.com/product-detail/en/ECS-352.5-18-1/X151-ND/226620

This is the main reason why I suggest a frequency synthesiser approach. Digikey will sell you single units with initial accuracy of ±10 ppm and temperature stability of ±10 ppm for around USD 1 each if you choose a standard frequency like 12, 18, 24 or 32 MHz. The frequency synthesiser chip can then do the work of producing the exact frequency you want.

 

[Lightning]

Thanks guys, I have coherent understanding now.

I will do a bit more research and see what I come up with.

If I have any more queries I will ask.