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You have to make a closed loop system. The feedback will be frequency which will be continuously monitored for any changes and a will be corrected accordigly. You can use microcontroller for this purpose.
It will compare its center frequency to an input frequency and create an output accordingly. Similiar to tone decoders, check out a data sheet for a 567 tone decoder, you'll get a pretty good explanation.
Do you just want a fixed frequency source? Are you using a dedicated IC for this function?, because there is usually a voltage feedback which you can just fix. If you do this, you will not get regulation, it will be free-running..
A PLL is a good way of making a frequency ultra-stable, but I don't think it is necessary in an inverter application. PLLs are used at high frequencies where stability is important because of matching transmitters and recievers...
When the feedback for oscillation coming from inverter transformer, the frequency depend from load. Use independent oscillator circuit for output transistors driving.No needed extra-stable circuit e.g. PLL. An 555 or any digital circuit stable enough.