To be able to compare two frequencies we first need to convert a frequency to a voltage.

One might just use an RC filter .... but at 1Hz thats not feasable.

So i came up with a different approach.

I take the positive part of the squarewave output, and charge a capacitor. On the negative half i discharge it.

Higher frequency = less time to charge = lower voltage

Lower frequency = more time to charge = higher voltage

Easy, right?

On the output we get this.

Now we average this output with another I-controller integrator.

No we do the same thing with our reference frequency