We noticed the dang SR560 is injecting noise into our beatnote spectrum at high gain.  I found a ZHL-3A low-noise RF amplifier and we've added it in after the transmission beatnote RFPD and in front of the new mixer (ZX05-1MHW-S+) in our PLL loop to amplify our beatnote with low noise. 

Our beatnote is ~100MHz.  Amplitude is 2 dBm.  The noise figure of the RF amp at 100 MHz is ~5.5.  RF amplifies the beatnote to 19 dBm (we attached a 10 dBm attenuator to avoid saturation of our mixer, which is rated to 23 dBm RF input).

Before adding in the RF amp, the PLL OLG UGF is ~25kHz.  The SR560 gain is 20.  The Marconi FM Devn is 10 kHz/V.

After adding the RF amp, I took the same spectra with the same PLL settings as before to get a direct comparison.

The plot shows the direct comparison.  The pictures show the settings of the PLL.

Edit (awade) Thu Dec 7 21:23:50 2017: Put the actual part/model numbers.  Links die after a time, you need to actually list the part otherwise it will be a mystery.