Ok, In the end I have prepared an offset for both frequencies, just in case.
Today I am working with the calculation of phase diferences between two signals.
The preliminary results are quite good. The microcontroller can calculate phase shifts with a resolution close to nanosecond.
When comparing 2 signals with close frequency, it is difficult to know exactly what the difference in frequency is. From the eighth decimal place the noise is very high. However with the phase difference measurement, the frequency differences can be measured very accurately.
Here attached a preliminar result from an excel datasheet.
The frequency of both measured signals is very close with 10000.2554 Hz versus 10000.2560 Hz, with a very small difference in the last decimal place.
With the phase difference, you can easily see the frequency difference much more precisely. The average difference is 67.46 ns per second.
The phase difference has an offset of at least 250us. This is the time it takes for the microcontroller to take the two measurements.
Regards.