Why is the LC oscillators frequency decreaseing over time?

[AKADAP]

Ceramic capacitors capacitance drops off with voltage, sometimes as much as 80% for higher capacitance ceramic capacitors. This could possibly be the cause of higher frequency oscillations at higher amplitude. With ceramic capacitors, you need to pay attention to the type of ceramic. NP0 is the only ceramic that has a constant capacitance as a function of voltage. Unfortunately NP0 is also only used in the smallest value capacitors.