Dear All,
I am currently involved in LC VCO development. In this activity I am facing some problem understanding the feasibility of the small signal analysis, like PXF or PAC, in the frame of autonomous circuit simulations.
As already suggested in this forum (see
http://www.designers-guide.org/Forum/YaBB.pl?num=1108757064), the PXF analysis can give some quantitative insight on figures like PSRR. I performed the PXF (Relative Harmonic=1, sideband=-1, sweeptype=relative and frequency sweep between 100Hz to 1MHz to simulate the periodic transfer function from the VDD baseband frequency to the carrier frequency range on the output) on my VCO.
I got a similar result reported in
http://www.designers-guide.org/Forum/YaBB.pl?num=1092741082, a pretty huge gain at frequencies close to the carrier (100dB). I did not fully understand the explanation given in the topic of the reported link, being the result of PXF a voltage to voltage transfer function does it mean that if I have 1uV sine on VDD I get 100mV sine on the output?
On the other side, when performing the PXF one calculates the transfer function from any source,to a single output. Now the question is what is the meaning of this transfer function in an oscillator, i.e. what is the SOURCE in oscillator circuit? By definition oscillator is autonomous (not-driven) circuit. If I introduce the periodic small signal sinusoidal excitation that I sweep and according to which I want to get the transfer functions, I would lock the phase. This means that my circuit is not autonomous anymore.
So I would say you cannot obtain periodic-steady state transfer function for an oscillator due to fundamental problem of 'what is the transfer function in case of an oscillator circuit?'
I guess I must be wrong in some of my assumptiom and I would be glad to get Your feedback to clarify my ideas on this topic,
Regards,
Ettore