I bought an HP E1740a time interval analyzer for about 80€.
There is no free software, what you can find on the internet requires a dongle. I have to do it myself.
My hardware is an old laptop with a PCICIA slot for the Firewire controller to talk to the VXI rack via VISA running on windows XP.
I measure 500,000 times the period of the OCXO on input 1 an save it to internal RAM with a resolution of 50ps. That takes about 1s.
Each measurement is a double value and needs 8 bytes to store, so we have to read 4 Mbytes and save it as ascii to disk. That takes about 14min.
A lot of time to measure and save 20 ms in the life of a 10Mhz oscillator to a 11 Mb file
I imported the data into Timelab and this is what it looks like.
Sample time is 1E-7 - Timelab can only do 1E-6 so the measurement time is 20ms and not 200ms
* measured against the internal reference TCXO