Driving high impedance load with Crystek Sine Wave Oscillator- CCSS-945X-25-100.000
Is there any way to know the value of load capacitance, this sine wave oscillator can drive ,from the available oscillator specification in datasheet like 5dBm into 50 Ohms i.e, how to convert 5dBm into 50 Ohms to 5dBm into ... pF?


Not really, no. These 50 Ω are a specific impedance for maximum power transfer, i.e. impedance matching.

So, it'd be expected that you match your capacitive load (using some sort of matching network) to the real 50 Ω impedance to achieve maximum power sinking.

If you don't do that matching, you'll get an impedance mismatch, which means not all power will be brought to your sinking device. Look for the reflection and transmission coefficient formula for impedance mismatches. It's easy to deduct how much power you'll deliver to that load.

But, really: it's not clear why you'd even want to know whether your sine oscillator can sink 5 dBm into a 15 pF load – that's not a problem you're facing! Remember, the sine is a harmonic oscillation – so a capacitive load will do nothing but shift the phase.

When you look at the LMK datasheet p. 92, you'll notice that you'll need to terminate your differential transmission line with the nominal transmission line impedance.



Your datasheet tells you that yes, you can drive a 50 Ω load with 5 dBm; calculate whether those 5 dBm mean enough voltage across the 50 Ω termination resistor.

If it's not solidly enough, you will need to add a buffer on your clock distribution board (sounds like a good idea, anyway, since you're calling it distribution board, you probably need some fanout!). 

Make that buffer a clock buffer that converts the single-ended to a differential signal (since you seem to be concerned with cleanliness of the oscillation rather than exact frequency, this is desirable), and go for fig 23 from p. 91