Attila, > Why do have OCXO a Vref output in the first place?
The purpose of Vref is to optionally allow the oscillator to be fine-tuned using 3 wires to an external user-supplied multi-turn pot, often hiding behind an access hole at the rear of the instrument.
Note that the internal Vref needs to be good enough not to degrade the performance of the crystal oscillator, but not much better.
For example, if your OCXO is stable to 5e-10/day and has a tuning range of 1e-7 with an EFC range of -5 to +5 volts then 1 mV stability on Vref translates to 1e-7 * 1e-4 = 1e-11 in frequency, far more than adequate (50x) for the OCXO. Check my math.
By contrast you would not want a cheap Vref that drifted by, say, 50 mV/day because then half the drift of the oscillator might be due to Vref and half due to the crystal; a waste of a good crystal.
I'll let volt nuts run the actual numbers for Vref that are found inside various OCXO. But it sounds to me like a Zener would be fine for almost any OCXO that you own, especially since it's inside an ovenized box. Using a metrology-grade voltage reference would be a waste of a good reference.
Here's how to get actual data: disconnect Vref, ground EFC, and then measure both Vref voltage and Fout frequency once an hour for a few weeks and compare their normalized daily drift rates.
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