The next graphics plot the phase noise for the IQ+ LO RevB compare with other two oscillators, my Marconi 2024 signal Generator (@288 MHz) and the Crystal Oscillator from a well know commercial transverter (@ 116 MHz), al Mesurements are done with the same Spectrum Analyzer). Markers are at 100Hz, 1KHz and 5KHz.
Marconi 2024 phase noise @ 288MHz
IQ+ LO RevB. phase noise @ 288MHz
Crystal LO phase noise from "well know brand" 2m transverter @ 116MHz
The next table resume the 3 measurements, you can see not a big difference for the IQ+ LO Rev.B phase noise, existing crystal oscillators capables to reach -145dBc/Hz phase noise like the oscillators in the WSE converters But then you lose stability and you add the limitation about changing your central frequency in your band pass. I decide to use the Si570 because I reach a extraordinary compromise between RF level, harmonics, phase noise and "agility" to change the frequency with a resolution of 1Hz depending on the control software.