Homemade 457KHz Avalance Beacon?

[casper.bang]

While researching for my cat location project, I stumbled upon commercial avalanche beacons and trackers, you know, those devices which can be used to locate avalanche victims under snow on the slopes. Apparently there are older analog and newer digital versions, using the 457kHz band. To my surprise, these devices, even old analog ones, are quite expensive and I also have not been able to come up with a simple diagram for how to do just a transmitting beacon (I'd be ok with buying a commercial tracker unit, but the transmitter would have to be as simple and as small as possible).

The beacon appears to transmit for about 200ms once a second but does anyone have an idea how the signal looks like, what's modulated or whether a carrier frequency is all that's needed? A 457kHz xtal seems impossible to find, so would a Colpitts oscillator (or similar) be drift-stable enough to stay within the +/- 100Hz?

[dannyf]

You can generate that easily with a mcu.

[fcb]

No.  It's unlikely you'll be able to make a colpitts *that* stable.  Especially in that application.

I would use a PLL (like the 4046) to build a frequency synthesized transmitter.  You could even do the divide by 457 in a micro-controller - although i'd wrap the whole thing up in a 4046 & small CPLD or a 4059.

It is very close to the ubiquitous 455KHz I.F. frequency - so you might be able to pull a SAW filter- based oscillator that far off...

[SArepairman]

how about using a DDS chip

[DavidGoncalv]

How about a programmable XO? SiLabs make them, and they are in stock at DigiKey for a little over $3.

http://www.silabs.com/Support%20Documents/TechnicalDocs/Si501-2-3.pdf

[dannyf]

for a little over $3.

Why?

Give me a minute and I turn an Attiny to a 457Khz oscillator,

[DavidGoncalv]

If it can meet the +/- 80 Hz spec over -20 to 45 C, then yah that would be better since you can put the keyer logic in there too.

[calexanian]

RF induction heaters work in that freq range. they are also several thousand watts.. How far might your cat go??  O0

[casper.bang]

You can generate that easily with a mcu.

Give me a minute and I turn an Attiny to a 457Khz oscillator

Love that idea (I have some ultra small ATtiny10's) but it's my understanding, that the internal RC oscillator of the ATtiny are very unstable. So how would you achieve this, clocking with a 10MHz xtal and dividing down 22 times by software gets you 454.5KHz which is still 1.5KHz off?

There IS a multiple-of-457KHz xtal in the form of 1.828MHz (divide by 4) but they appear hard to source.

[Maxlor]

While a beacon should transmit at 457kHz +-80Hz, the receivers actually handle +-180Hz. You can get 457.143kHz by dividing a 16MHz oscillator by 35. Easy enough for an MCU

[dannyf]

I was thinking about using the pwm mode to generate the signal (OCRx to set the top). After that, everything is done in hardware and the mcu is free to do other things, like modulating the carrier signal.

Yeah, the 1.8Mhz crystal would work. There is another that is off by just 0.002Mhz. Potentially more options in the oscillator land.

[fcb]

for a little over $3.

Why?

Give me a minute and I turn an Attiny to a 457Khz oscillator,

And?