PLL has a VCO.
I designed an individual cylinder knock controller in ‘91. Mostly in software, running on a processor with a security bit. Decided that was better than a patent. Hasn’t been ripped off yet.
New version for multi coil Fords called Vampire:
http://www.jandssafeguard.com/VampirePage/J%26S_Vampire.html
“PLL has a VCO”
Yes.
But the technique to detect a varying inductance (a sensor) was different, enough different to get around the patent.
And yes, bury your idea so it is hard to discover.
I design high performance satellite modems. I use a cheap PIC controller on the board as a hardware key. The hardware key connects to a large FPGA which makes it pretty difficult to reverse engineer.
It is possible to take the top off the chip and probe the PIC code but it is beyond normal engineers and companies to perform. It would be cheaper for them to just buy the company ;-)