This nomenclature (direct vs. indirect) sound "old".
almost every syntetized generators are "indirect".
Direct:
^^^^^
An array of N fixed frequecncy oscillators (i.e. 1,2,4,8,10,20,40,80,100,200,400,800MHz)
An array of N mixers.
With appropriate switch bank you may get an output freq. from 1 to 1665 MHz with 1 MHz step. Lowest phase noise but expensive and large.
Indirect:
^^^^^^
The PLL are indirect
The minimum step is smaller as the dividing factor is large. Fmax/Fmin depend on VCO and typically is 1 octave. Phase noise is moderately low, in most cases is low enough.
DDS:
^^^^
The waveform of the out signal is "syntetised" point by point.
Extremally low phase noise, but few spuriuos signal near the carrier.
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A short note about stability:
Phase noise (or FM noise) far from the carrier depend on synt. technology. But phase noise close to carrier depend to the frequency standard only (i.e. the Quartz).