LTC6946-1: using oscillators to generate reference frequency for local oscillator signal
From the data sheet, the LTC6946 needs a \$V_{REF}\$ signal that's between \$0.5\mathrm{V}\$ and \$2.7\mathrm{V}\$ peak-peak, and it presents an impedance that looks like a \$3\mathrm{pF}\$ capacitor in parallel with an \$8\mathrm{k\Omega}\$ resistor.  (This is in the section titled &quot;Reference Inputs&quot;).
From the data sheet, the ECS-TXO-3225 is HCMOS compatible.  Here, you're just supposed to know that means that it can drive a healthy (\$\pm20\mathrm{mA}\$, if they're telling the truth) current into a load, with a low of \$0\mathrm{V}\$ and a high of \$V_{DD}\$ (which is \$3.3\mathrm{V}\$ if you use the recommended voltage).  They do actually state rise and fall times into a \$15\mathrm{pF}\$ load, from which you could compute currents if you were inclined.
So, you have an oscillator that'll produce up to \$3.3\mathrm{V_{P-P}}\$, going into a pin that can stand up to \$2.7\mathrm{V_{P-P}}\$.
I would make a simple voltage divider, followed by the DC blocking capacitor shown in the LTC9646 data sheet.  Something like the enclosed schematic.  The two resistors attenuate the signal down to 1.7V or so, which is comfortably in the range that the LTC9646 wants to see.



simulate this circuit &ndash; Schematic created using CircuitLab