Hi Jerry,
Thanks for the comments. On the Si5351, it is not uncommon for the AC source impedance of an RF amplifier or buffer to appear to be 50 ohms. However, the available source current commensurate with the available operating voltage NOT able to drive a resistance load of 50 ohm without introducing distortion. In the op amp arena this is comparable to large signal vs. small signal performance. Not unlike slew rate limits in operational amplifiers. Fortunately, the LO terminations from the SA612 or SA602 are larger than 50 ohms and he further buffers the Si5351 with 390 ohm and a voltage divider.
