5.1.1.1 System Design with Compensation of Initial XTAL and XTO Tolerances

The initial frequency tolerance of the system is given by the crystal tolerance, by the integrated capacitors on pins XTAL1 and XTAL2 and by the initial transconductance tolerance of the XTO. The tolerance can be lowered to ±1 ppm by measuring the CLK_OUT frequency at the end of the module production and by correcting the frequency programming of the fractional-N PLL in the EEPROM. This initial tolerance depends on the measuring accuracy and the resolution of the fractional-N PLL.

The XTO has a remaining influence of less than ±4 ppm. For more details, refer to the ATA8210/ATA8215 UHF ASK/FSK Receiver Data Sheet (9344E), section 4, parameter number 13.50.

The resulting tolerance is given by:

• ±5 ppm residual XTO tolerance from above
• Crystal frequency temperature tolerance; for example, ±30 ppm in the temperature range of -40°C and +85°C⁽¹⁾
• Crystal aging of ±10 ppm

In this example, ±45 ppm of total tolerance must be compensated. In a 433.92 MHz system, this results in a ±19.52 kHz total frequency tolerance on one side of the communication system. If both sides of the system have the same tolerances, a maximum frequency offset between transmitter and receiver of ±2 x 19.52 kHz = ±39.05 kHz must be taken into account.

Using 10 kBit/s FSK with ±10 kHz frequency deviation results in a required IF bandwidth of 106.38 kHz. In this example, IF bandwidth of either 110 kHz or 123 kHz can be chosen. For more details on the impact of the IF bandwidth on system performance, see Estimation of Maximum Frequency Offset Between Receiver and Transmitter from Related Links. If choosing 110 kHz IF bandwidth, 1.5 dB less sensitivity has to be considered for the link budget under worst case frequency tolerance conditions. On the other hand, no loss in sensitivity due to frequency offsets has to be considered if 123 kHz IF bandwidth is selected.