[hpsdr] External 10MHz reference input questions

[Jean-Claude JCA. Abauzit]

The input can be grounded via J24: When does it need to be grounded/not grounded?
The input can be terminated into 50 ohms via J22: when does it need to be terminated/not terminated?
When not terminated the input resistance is something like 2.2 kohms.
The voltage at J20 must not be more than 3.3V but Q3 and Q4 are powered by 5V via 100 ohms: what is the maximum voltage allowed on the external input in order to have 3.3V at J20?
If I get this OCXO
http://www.ebay.com/itm/NEW-MTI-Crystal-Oscillator-10MHz-OCXO-Telecom-Stratum3-1ppb-day-atomic-clock-/270998646224?pt=LH_DefaultDomain_0&hash=item3f18c72dd0#shId
which puts a 9dBm sine wave into ??? ohms it would give 0.6V (may be) if the external input is terminated into 50 ohms or 4V (may be) if the external input is not terminated into 50 ohms.
How would you use this OCXO on Hermes?

Thanks and 73 de PJ2BVU Jean-Claude

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