4.1.1.2 System Design without Compensation of Initial XTAL and XTO Tolerances

If tuning at the end of module production is not possible, additional tolerances have to be considered and result in broader system bandwidth. The resulting tolerance is given by:

• The internal load capacitance variation and the XTO variation amounts for a motional capacitance of 4 fF in the crystal to ±10 ppm (see parameter no. 13.40 in Oscillators and CLK_OUT) + ±4 ppm (see parameter no. 13.50 in Oscillators and CLK_OUT) = ±14 ppm
• The initial frequency tolerance of the crystal of, for example, ±40 ppm
• The temperature tolerance of the crystal, for example, ±30 ppm in the temperature range of -40°C to +85°C⁽¹⁾
• Crystal aging, for example, ±10 ppm

The total frequency tolerance is ±94 ppm. For a 433.92 MHz system, the total frequency tolerance on one side of the transceiver system is ±40.78 kHz. If both sides of the system have the same tolerances, a maximum frequency offset between transmitter and receiver of 2 x ±40.78 kHz = ±81.57 kHz has to be taken into account.

Finally, using 10 kBit/s FSK with ±10 kHz frequency deviation results in required IF bandwidth of 191.4 kHz. For the system design, IF bandwidth of 219 kHz IF bandwidth can be selected. No loss in sensitivity due to frequency offsets has to be considered due to the 27.6 kHz bandwidth margin (see Estimation of Maximum Frequency Offset between Receiver and Transmitter).