Network analyzer

The network analyzer (S11, dB magnitude) measures the return loss in the coil antenna over a frequency span to find the optimal value of the parallel capacitors.

The frequency that results in a dip is the resonance frequency of the Near Field Communication (NFC) tag antenna. When using this method, the nRF52832 chip does not need to be powered. However, if the nRF52832 System on Chip (SoC) is powered, R_IN has to be set to a fixed value. See Configuring the chip for more details.

The required equipment for this method is:

Network analyzer with minimum one port. The frequency range needs to cover 13.56 MHz.
Coil antenna, for example, a metal wire in a coil.
NFC tag antenna connected to the board with the nRF52832 SoC.

Complete the following steps to tune the antenna with the network analyzer method:

Connect the coil antenna to a network analyzer and the NFC tag antenna to the nRF52832 device.

The network analyzer should power the coil antenna slightly, with for example -10 dBm.

Figure 1. Setup for the network analyzer method
The distance between the NFC tag antenna and the coil antenna should be within the following limits:
- Lower limit: A too short distance between the two antennas will pull the resonance frequency down. The distance should therefore not be shorter than approximately 1 cm.
- Upper limit: The longer the distance between the antennas is, the weaker the loss dip in the network analyzer will be. To measure the resonance frequency, the loss dip must be visible.
Measure the return loss in the coil antenna over a frequency span, for example 10–15 MHz. The resonance frequency of the NFC tag antenna is the frequency that results in a dip. Change the value of C_tune1 and C_tune2 to make it resonate on 13.56 MHz:
- If the resonant frequency is lower than 13.56 MHz, the value of C_tune1 and C_tune2 should be decreased.
- If the resonant frequency is higher than 13.56 MHz, the value of C_tune1 and C_tune2 should be increased.

Figure 2. Example reading from network analyzer

Note: The dip at approximately 13.56 MHz shows that the resonance frequency of the NFC tag is almost at 13.56 MHz.