How to make a -100dB RF signal?
Can we produce it with VCO+PLL+Attenuators?
Is there any better way?
(50 ohm output)
It is possible to produce -100dBn signal by generating a relatively strong signal and attenuating it afterwards
The problem at these low signal levels is shielding leakage at the higher level source.
Put your source in a cast metal box and use feedthrough filter capacitors where the power connections go through the box wall. Apply metal foil to the junctions between the box body and lid. You may want to put the box far away from your receiver and use a long cable to get the signal to the receiver. Put ferrite beads around the cable at the source end to prevent conduction on the outside of the cable.
And use high quality connector as N or SMA and double screened coax-cable as (least) RG223, RG214 or semi ridge both internaly in generator and external out to DUT - not BNC or PL-connector on RG58 you can see on HAM-radio equipment...
Bad leaking internal RG58 can make well designed internal attenuator to worthless state...
-100 dB RF signal is a meaningless term.