Please check the first message. Resolution of 100MHz is needed. Otherwise I would already done this using just CPU. The fastest CPU I can use is not even close to that.
I think you mean that you need a resolution of 10ns in the time domain, not quite the same thing. A
resolution of 100 MHz is in the frequency domain, suggesting that you need to be able to
resolve frequency to within 100 MHz (i.e. be able to
just tell the difference between 100 MHz and 200 MHz,). It might seem pedantic (you can blame a morning spent looking at Z transforms), but there is a world of difference.
The STM32F4 series will run their timers with at least 10ns resolution (with the one exception of the STM32F401) and have input capture modes that are designed to do exactly this kind of timing of an external signal. I can personally attest to that because the breadboard I have in front of me is using an STM32F411 to do
exactly that. The range starts at around the £2 level in quantity, £3 one off pricing. An official, fully populated, ST Nucleo64 development board for the STMF410 is under £12.
Now, whether one can find anybody with stock of any STM32 microcontrollers at the moment is another question, but that's down to the current Chipagedon situation.