Thanks to everybody for great discussion.?
Despite initial diversion I have found that U313 Agilent 1GC1-4210 has struck yet again.? It had weird failure mode where it would change its division ratio at the bottom 30% of its input range of 4GHz..8GHz to half the intended value.? This caused its output to double in frequency and become filtered out by downstream LPFs.? So I chucked U313 out and replaced it with good honest organic grown 24GHz Hittite prescalers.? E4435B now works just how it was supposed to.? If there is enough interest I can create a drop-in PCB that will replace 1GC1-4210.
Cheers
Leo
