cc1101 868 mhz ref design 26 mhz crystal

Ram, 

This should be possible to switch, however there is a bigger question. What are your exact frequency of operation (Start frequency and Stop frequency). The reason I ask is, with 26MHz reference clock you will get a spur at every N*13MHz, however using a 27MHz XTAL, you will get a spur at every N*13.5MHz. This can be good or bad depending on what exact frequencies you plan on using in your system.

Regards,
/TA

For compliance with modulation bandwidth requirements under EN 300 220 in the 863 to 870 MHz frequency range it is recommended to use a 26 MHz crystal for frequencies below 869 MHz and a 27 MHz crystal for frequencies above 869 MHz.

As Thomas wrote, there will be spurs at N x reference frequency (= N x crystal frequency / 2) which is also folded around RF. The closer the spur is to the carrier the higher it will be in amplitude. The spurs should be approx 2 MHz or more away from the carrier to be below the ETSI limit of -36 dBm. 

Example: 868 MHz operation and 26 MHz crystal. There will be a spur at 871 MHz (67 x 13 MHz) and also one folded around RF at 865 MHz. In this case the spurs will be 3 MHz away from the carrier and will be below -36 dBm.

There are several crystal manufacturers out there: NDK, Epson, TEW,.... I am sure you will find 26 MHz if you do a search. Make sure the ESR and crystal loading is according to the CC1101 data sheet figures. 

TA12012 & Sverre,

Thanks a lot for the info. I had no idea about these spurs.

I intend to use the device at 866 MHz.  

Can you tell me if the spurs will be far away enough in both cases (26 MHz crystal and 27 MHz crystal).

Sverre,

I am not clear about the spur at 865 MHz.

I will try to look for a 26 MHz crystal then.

Thx

Ram

Previous example: 868 MHz operation and 26 MHz crystal. There will be a spur at 871 MHz (67 x 13 MHz). This spur is 3 MHz away from the carrier, and this spur will also folded around RF; i.e 868 MHz - 3 MHz = 865 MHz.

Using a 27 MHz crystal there will be a spur at 864 MHz. When operating at 866 MHz there will thus be spur at 864 MHz and also at 868 MHz (spur @864 MHz is also folded around RF). The spurs will be 2 MHz away from the carrier and should be sufficiently far away from the carrier (in Hz) to be below the -36 dBm limit when measuring the modulation bandwidth.

Hi Sverv,

  I have ordered a 26 MHz crystal (+/- 10 %ppm tolerance and stability) from digikey.

  For 866 Mhz RF, one spur will be at 858, another one at 871 and one at 861 MHz due to the fold. This should be ok since the spurs are now at least 5 MHz away. Is my assumption right ?

  Another question. We are using the balun specified in the 868 Mhz CC1101 reference design (0868BM15C0001). it's operating frequency range is from 863 MHz to 873 MHz. I am hoping we can use this balun for 866 MHz. If so, do we need to modify the  matching network ?

Thx

Ram

The balun iworks fine also at 866 MHz. No need for any modifications. 

Your assumption regarding the spurs is correct.