The LMX2572 data sheet (page 18, paragraph 7.3.7) says that the "output impedance of the buffer is very small, the buffer can be AC-coupled to drive a 50 ohm load."
Can you clarify the "very small" comment? This sounds like they are quite likely emitter follower outputs. In my application I wish to use RFOUTAM and RFOUTAP outputs in a differential configuration.
Does "very small" mean on the order of a few ohms, or <10 ohms? I plan to use a series ~50 ohm resistor (with DC block) at each port, then pass through a 2-to-1 balun to drive a 50 ohm load.
This will be passed through a BPF for spectrum cleanup, and the BPF will wish to see a low VSWR 50 ohm source drive impedance.
In theory, the "very small" output impedance and 2x 50 ohm resistors should generate a reasonable 100 ohm differential source impedance to convert using the balun transformer to 50 ohms unbalanced.
My application generates an Fo of 2.5 GHz, with the LMX2572 VCO at 5 GHz. Thank you.