So currently I am trying to build a custom microbalance based on the Arduino MEGA 2560 chip, some At-cut quartz crystals with a 6MHz resonance frequency and a crystal oscillator driver (SN74LVC1GX04). Firstly, I want to try and make a prototype that prints the frequency value through the serial port. If this works properly I will try to add a display, an SD card and maybe some Wi-Fi if it's not too complicated. However, before trying to connect the crystal I wanted to see how accurate and stable can the frequency measurement of the ATMega 2560 can be since for this application they should be as good as possible.
Main problem that I am encountering is that arduino Mega 2560 is running on a 16MHz ceramic resonator and frequency mesurements are not accurate or stable. My idea is to make a custom PCB and replace it with a crystal oscillator (TCXO). My objective is to get a 1Hz accuracy and resolution since this is for accurate microbalance mesurements. However, after looking into it the best TCXO the best that I have been able to find with the same 16MHz resonace frequency have only 2.5ppm frequency stability. I have also seen the option of using a OCXO wich is in the range of ppb wich would be much better but I was unable to find a 16MHz option to fit directly (after changing fuse bits ) in the ATmega2560.
I would need someone experienced to solve doubts and provide guidance on how to follow this project to obtain the desired results.