The scope was never intended to have a reference input but it doesn't necessarily stop us.
The schematic got me thinking, what if I added a small board with some PLL/MCU that would take external 10MHz, pick up 25MHz or RF out from ADF3460 and do one of the following:
1. Pull or trim 20MHz crystal (voltage controlled capacitor?) or
2. Offset VTUNE with respect to filtered CP (see schematic / scribble)
to match multiplied reference to either of those signals?
Those will not work.
The unit has an oscillator, not a crystal.
You cannot pull the VCO DC signal, as the loop will fight against that.
The simplest is probably to change the 25MHz Oscillator, to a VCTCXO model.
As a first pass, you can simply trim that to a reference
It looks like the PLL can accept a clipped sine reference, which expands your choices.
Digikey show stocks of
O 25,0-JTP32CSV-F-K-3,3-1510-1015-LF = VCTCXO 25 MHz Clipped Sine Wave 3.3V ±280ppb -40°C ~ 85°C 3mA 3225
and also TXEAADSANF-25.000000 ±500ppb (cheaper)
Well suited to a 'trim and forget' setup.