heres my 2ps worth...
mobil wrote on Dec 14th, 2006, 4:45pm:(1). To simulate divider's phase noise, in my understanding, the divider is a driven component same as mixer or LNA. Hence, the 'sweeptype' should be 'absolute' in pnoise setup and the 'oscillator' option in pss should be inactive. But it actually consists of a VCO in circuit, so I still active the 'oscillator' option in pss and selected the 'oscillator node' of VCO's output node. Is that all right under this configuration?
The pss form should be set to oscillator. The guess at the beat frequency should be the lowest common divisor of the present frequencies. In your case you should set it to 15GHz (as you divide by 2). Also I would choose the divder output as the oscaillation node. As for the pnoise sweep you can chosse absolute or relative as you deem fit. If you wanted look at the phase noise at 1MHz offset from the 15Ghz divider output, you could set it to relative to harmonic 1, at 1MHz, OR absoulte at 15.001GHz.
mobil wrote on Dec 14th, 2006, 4:45pm:(2). The divider is self-oscillating around 15G Hz without any input signal. The phase noise of this self-oscillation is not good about -70 dBc/Hz @ 1M Hz. But when it's driven by VCO, the phase noise of divider is around -100 dBc/Hz. Which one should be considered?
-100dBc/Hz is what is quoted. This is the phasenoise of your vco cascaded with your divider. This will only be the divider noise if you know the VCO noise is lower than -94dBc/Hz (@1MHz offset). So if your VCO noise was say -104dBc/Hz at 1 MHZ offset from 30GHz, and you subsequently measured -100dBc/Hz at 1MHz at the divider output, then you could say the -100 is the divider noise floor.
The noise of a divider self oscillating is always poor.
point 3 is answered above.
cheers
aw