Varying the frequency

Measure the voltage swing of a set of capacitors and find out which one is closest to a resonance frequency of 13.56 MHz.

For a given value of C_tune1, C_tune2, and R_{IN_OHM}, vary the frequency of the poller antenna using a signal generator (13-14 MHz) and measure the maximum voltage swing between the pins NFC1 and NFC2 using an oscilloscope in differential mode. Measure the voltage swing of a set of C_tune values. The value of C_tune1 and C_tune2 that results in the highest voltage swing at 13.56 MHz, will be best value among the tested ones for that antenna. The value of C_tune1 and C_tune2 with the highest voltage swing will be the value that results in an LC circuit with resonance frequency closest to 13.56 MHz, among the tested values.

The required equipment for this method is:

Signal generator that can generate signals up to at least 14 MHz.
Near Field Communication (NFC) antenna with resonant frequency of 13.56 MHz (the poller antenna in Setup for the method of varying the frequency).
Oscilloscope
Active probes
Active probes are required to minimize the capacitance between the NFC antenna and the oscilloscope. The reason that active probes are required is that they have much less capacitance than passive probes.

Complete the following steps to tune the antenna with the method of varying the frequency:

Calculate an approximate value for C_tune1 and C_tune2, like described in Estimating values.
Create a good setup for the poller and tag antennas that will make the distance and relative placement between the antennas constant for all measurements. Any change in distance and placement will lead to inaccurate measurements.

Figure 1. Setup for the method of varying the frequency

Note: In order to obtain accurate and useful results, the distance and relative placement between the poller antenna and NFC tag antenna must be exactly the same for all measurements. A slight change of distance or relative placement may change the maximum voltage swing considerably, and hence change the validity and accuracy of the results. In order to avoid overflow, the maximum voltage swing should be maximum 80% of VDD.
Set the internal resistor R_{IN_OHM} in nRF52832 System on Chip (SoC) to a fixed value using nrfjprog to change R_{IN_REGVALUE}.

See Configuring the chip to set R_{IN_REGVALUE} to a fixed value.
Measure the maximum voltage swing in the frequency region 13–14 MHz for a set of C_tune values. Ensure that the voltage never exceeds VDD as it will affect the results. If the VDD is exceeded, do one of the following actions to reduce the voltage swing:
- Increase the distance between the two antennas.
- Increase the value of R_{IN_REGVALUE}.
- Reduce the power of the signal generator connected to the poller antenna.
Choose the value of C_tune1 and C_tune2 that results in the largest voltage swing at a signal frequency of 13.56 MHz.

Figure 2. Maximum voltage swing for various C_tune values