The HFXT has internal variable load capacitors (Cap Array Q1 and Cap Array Q2) in the IC and does not require external capacitors to be mounted. The load capacitance is set in the Sysconfig TI DEVICES section under Device Configuration by enabling Override HFXT Cap Array Trims and setting the appropriate values for Cap Array Q1 and Cap Array Q2. To avoid having a load capacitor imbalance on the two sides of the crystal, the maximum difference between Cap Array Q1 and Cap Array Q2 cannot be more than one. If Override HFXT Cap Array Trims is not enabled, then the fault values for Cap Array Q1 and Cap Array Q2 are used.

Table 6-2 shows the resulting total capacitance measured on an evaluation board versus Cap Array values. Note, that the resulting capacitance value includes parasitic capacitances, which is why the lowest setting is not 0pF. There are two regions where an increase in the Cap Array values do not change the effective capacitance. The two aforementioned regions are when the Cap Array value goes from 15 to 16 and from 47 to 48. If an automated search algorithm is used for finding the optimum Cap Array value, then take caution to remove either 15 or 16 and 47 or 48 from the search algorithm.

The best way to measure the accuracy of the HF crystal oscillator is to output an demodulated carrier wave from the radio and measuring the frequency offset from the wanted frequency using a spectrum analyzer. The relative offset of crystal frequency, typically stated in parts per million (ppm), is the same as the relative offset of the RF carrier.

For testing purposes, Cap Array values can be adjusted in SmartRF™ Studio. This simplifies tuning greatly by allowing on-the-fly updates of the load capacitance. The optimum value found in SmartRF Studio can then be entered into Sysconfig in the applicable software project.