I'm trying to create what amounts to as accurate a frequency counter as I can possibly build. At the moment, I am considering a 3.3v CMOS output 20 MHz OXCO.
The application notes for the oscillator have all sorts of caveats in order to achieve the specified tolerance. One of them is output loading. They want a 15 pF load. They talk about buffering the output, but given that the output is going straight into the input of the ATMega328P and nowhere else, I am inclined to skip that.
The question at the moment is, if you fuse the atmega for an external oscillator, how much capacitance should I assume for the pin? 0? Should I just use a 15 pF cap in parallel (to ground) between the OXCO and the ATMega? Anybody else have any advice?
My real issue is that I have no other means to calibrate or check the calibration of this system once it's done (none of my other tools have a design tolerance as tight as what I want to achieve here - which is around 10 ppb). I'm going to just sort of have to ride on a wing and a prayer, so I want to get the design as close to ideal as possible. Moreover, this OXCO is super expensive, so I want to get my money's worth out of it. I know for sure one thing is that I need to design a beefy power supply for it, as it needs nearly an amp at 3.3 volts, but that's a whole 'nother story.