FreeLists / si-list / [SI-LIST] Re: Crystal oscillator simulation in time domain by Pspice

[Parthasarathy Sampath <parthsv@xxxxxxxxx>]

   Agreed, you will probably observe oscillations of
few millivolts or probably few hundred millivolts. But
this method of reducing 'Q' produces rail-rail
oscillations. 
  As i said earlier, try using Spectre and I believe
its a better option for these kind of circuits!
   Ramping up your VDD/DVDD for few ns/us will be
necessary for better results.


Regards,
Partha!

--- Sogo Hsu <sogo.hsu@xxxxxxxxxxx> wrote:
> Hi, Partha and Peter,
> 
> In the simulation, we have put a noise source in the
> circuit. 
> Actually, the circuit oscillated. However, the
> amplitude is too tiny 
> to observe. We took Fourier Transformation for the
> tiny sinusoidal 
> waveform. The 'oscillated' frequency depicted a
> negative feedback of 
> complete close loop. I suspect that  the algorithm
> does merely pick 
> the solution can meet convergence criterion.
>  
> Best Regards,
> 
> Sogo Hsu, Ph. D.
> 
> Simulation center/PCEG/Foxconn
> 
> --- In si-list@xxxxxxxxxxxxxxx, Parthasarathy
> Sampath <parthsv@xxxx> 
> wrote:
> > Hi Sogo,
> >    We faced a similar problem when attempting
> design
> > of crystal oscillators. This problem is acute for
> > lower order frequencies. 
> >    As you have said, pspice/hspice algorithm is
> unable
> > to solve the circuit to develop oscillations. The
> main
> > reason we belive is, crystal osc inductance has
> high
> > 'Q', the energy concentrated at parallel
> resosance.
> > Reducing the 'Q', spreads out this energy over a
> wider
> > range of freq. Use parameters with lower 'Q' and
> you
> > will certainly see oscillations.
> >    Also, for lower order freq Crystals require
> higher
> > resistance across the inverter(basically bias
> > resistor) to enable more current to flow through
> > crystal and you should see higher amplitude. But
> the
> > drawback with higher resistor is, you start-up
> will be
> > low - Reason, inverter takes long time to get
> itself
> > autozeroed to its bias point - Higher RC delay.
> > 
> > Hope this Helps,
> > 
> > Regards,
> > Partha!
> > 
> > PS: I did check out oscillator simulations on
> Spectre
> > and results were pretty good. You may not require
> > lower order 'Q' parameters.
> > 
> > 
> > --- Sogo Hsu <sogo.hsu@xxxx> wrote:
> > > Hi, Gurus,
> > > 
> > > I tried to simulate a typical oscillator and
> > > investigate the start-up 
> > > time by Pspice. The oscillator consists of an
> > > inverter and crystal. 
> > > the crystal was modeled by it's equivalent
> circuit.
> > > However, we were 
> > > unable to start up the oscillation successfully.
> It
> > > seems that is the 
> > > limitation of Newton-Raphson algorithm. The
> > > algorithm ensures the 
> > > circuit simulation can converge to solution.
> > > Accordingly, it gives up 
> > > the oscillation solution to avoid 'diverge'
> issue.
> > > How can I solve 
> > > this issue and bypass the converge requirement
> to
> > > achieve 
> > > oscillation? Thank you for your comments in
> advance.
> > > 
> > > Sogo Hsu Ph. D.
> > > Foxconn
> > > 
> > >
> >
>
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