Answer

For the crystal itself, there are three tolerance considerations which go into determining the full crystal frequency tolerance:

1. Fundamental frequency tolerance at room temperature.
2. Frequency stability across a given range of temperature.
3. Frequency tolerance due to aging.

The reason crystal selection is so important is because for proper operation between the radios in a network, it is very important to have the transmit frequency offset between them as small as possible.  Since the transmit frequency offset is directly dependent on the crystal frequency offset, crystal tolerances become the key parameters to go by.  To properly specify the crystal within the three parameters above, the designer must also determine the temperature range the EM3xx is to operate in and how long the product is to be in use, i.e., the life expectancy of the product must be known.  The total of these three parameters must then be about +/- 35 ppm maximum in order for the crystal being considered to be used.  Additionally, the crystal circuit consists of the EM3xx and load capacitors as well as the crystal itself and these components must be taken into consideration.

The EM3xx has a TX frequency offset range which is +/- 2.4 ppm.  This range of the transmitter frequency must be subtracted from the 802.15.4-2003 +/- 40 ppm specification when determining the allowable crystal tolerance.  Additionally, there is a crystal frequency tolerance resulting from the tolerance of the load capacitors.  For example, a 5 % tolerance capacitor will result in a larger +/- ppm crystal frequency than will a 1 % tolerance capacitor.  For cost reasons, most designers select 5 % tolerance capacitors which generally result in about another +/- 2.5 ppm crystal frequency.  This really depends on both the internal crystal makeup in conjunction with the load capacitors, but +/- 2.5 ppm can generally be assumed during design and then verified in prototype hardware.  The load capacitor tolerance effect must also must be subtracted from the 802.15.4-2003 +/- 40 ppm specification leaving about +/- 35 ppm as the maximum allowable crystal frequency tolerance.

Application note AN700, http://www.silabs.com/Support%20Documents/TechnicalDocs/AN700.pdf, section 2.6, Transmit Frequency Test, provides details regarding the relationship between the EM3xx radio center frequency offset and the crystal frequency offset.  Use the method given in AN700 to adjust the value of the load capacitors by verifying the frequency offset of the radio.  Be aware the frequency offset at room temperature can vary from application to application, depending on the temperature range of the application and / or aging of the crystal.  For this reason, the designer should always check with the crystal manufacturer for the best room temperature frequency offset to use for a specific temperature range.  For example a particular crystal might shift in the positive direction over the life span and therefore require a slightly negative frequency offset at room temperature.

Contact the crystal vendor of choice any time there are special considerations for the end application.  Special considerations include when the parameters of the application the EM3xx is to be used in are extreme, for example, high temperature LED or automobile applications or very long life expectancies such as water and gas metering or solar applications.  In these type of applications, many crystal vendors have additional processes for their crystals to meet the requirements specified, but there are limitations and tradeoffs and generally extra cost.  For this reason it is best to involve a crystal vendor early on in the design cycle.

Finally, the designer must make sure the selected crystal meets the High-Frequency Crystal Oscillator Specification in the EM3xx datasheet, http://www.silabs.com/Support%20Documents/TechnicalDocs/EM35x.pdf.  For a list of crystal part numbers known to be a good fit for the EM3xx and are known to be in use in various product applications, refer to the schematic appendix page in one of the EM3xx reference designs on the Silicon Labs web site, for example the EM358x USB Ceramic Balun Reference Design, http://www.silabs.com/Support%20Documents/Software/EM358x_REF_DES_CER_USB_INV-F.zip

For more details about tuning the 24 MHz crystal, refer to following KBA article: http://community.silabs.com/t5/Wireless-Knowledge-Base/EM3xx-24MHz-Crystal-Tuning/ta-p/157154.