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How to make ez430-Chronos-868 working on 915mhz?

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Hi,

 

Is it possible to change RF frequency of the Chronos from 868mhz to 915mhz?

If it is possible, could you, please, say how?

 

Thanks a lot, 

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I don't know of any easy way to do it, but I would think that you would have to pretty much re-design the board to change the frequency. 

 

This is a really good app note on moving from frequency to frequency on the same antenna. This brings back a bit of nightmare flashbacks to my e-mag courses, but if you're into RF (or think you want to be) this may help : http://www.ti.com/lit/an/swra161b/swra161b.pdf 

Where I found the above App Note

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It is possible to make an antenna that works on both 868 and 915 MHZ, but it wouldn't be a passive trace. You can use "traps" (RC in parallel) to basically chop your antenna short at a given frequency. So by putting these traps at several point in your antenna you can effectively change the length of the antenna depending on the frequency you're working on.

This is only concerning the antenna, so you'd need to be able to switch between tuning networks to actually use the antenna, since your radio tranceiver required the observed impedance to be 50 ohms. Switching can be done either by having a tranceiver with multiple outputs between which you can switch from software, or by using a RF multiplexer.

If you only want to use a given (preferably higher) frequency, you can replace the tuning circuit components with dimensionally identical components to match your new antenna. Since a higher frequency means your antenna will be shorter you could use a knife or mill to shorten your PCB trace antenna. If you have a chip antenna you might be able to replace that as well.

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You can do it in software only, and leave the hardware alone.  The range will suffer.

 

The Chronos is quite cleverly designed... except for the antenna which is an afterthought.  The antenna performance is bad in any band, so the range out-of-band is going to be really bad.  Even so, it might be good enough for your purposes.  You should use the low channels (902-904, etc) to get the least attenuation.  You are somewhat lucky because, the way filter work, they attenuate less above the center than they do below the center.  So, a 868 chronos will work better at 915 than a 915 Chronos will at 868.

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Actually the Chronos watch itself is identical in the 868MHz and 915MHz versions. So if you're only interested in using the watch you can simply reprogram the firmware to your desired frequency band.

 

The USB dongle is actually not too bad either, though you'll have to do a bit of soldering. The hardware design (schematic & layout) is again identical in both the 868 & 915MHz versions. You can actually check the BOMs here: http://www. ti.com/lit/pdf/slau292 . The only difference is the antenna length, which is done with the 0Ohm (R7) resistor that extends the wavelength when moving from 915MHz band to 868MHz band. So for your case, you'll need to un-solder the resistor to shorten the wavelength. 

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A suggestion before you begin to 'convert' it you may want to grab TI's smart RF tool. You will need that to generate the data to compile in for the firmware update.

The register access and locations are different but the CC430 and the CC1101 are mostly similar in what registers do what.

(mostly)

 

I don't know what the firmware in the watch is as I've haven't had then time to play (frolic even) apart from setting the time (seriously).

 

Anyhow the smart RF tool will help you know what changes when you change the base frequency. Be aware it's not a perfect tool (heh) it can dump out code though for the register set on the CC430 in the watch just look at the settings carefully (their are a few to say the least).

 

Cyb

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