Estimating values

Before starting to tune, you must estimate an approximate value for the capacitors C_tune1 and C_tune2.

Coming up with an estimation is important especially when using the method described in Varying the frequency, because this method uses the estimated value to find the correct value of C_tune1 and C_tune2.

Note: C_tune1 and C_tune2 must have the same value.

Complete the following steps to estimate the value of the capacitors, C_tune1 and C_tune2:

Find an approximate inductance (L_ant) of the Near Field Communication (NFC) tag antenna using an appropriate method:
- Find the value in the antenna datasheet.
- Calculate the inductance based on the antenna dimensions.
Calculate the value of the parallel capacitors, based on the inductance of the NFC antenna.

C_tune1 and C_tune2 must have the same value, the internal capacitance C_int is approximately 4 pF, and L_ant is the inductance of the NFC antenna.

Use the below formula to calculate the value of the parallel capacitors.

Figure 1. Resonance frequency for an LC-circuit