[SI-LIST] Re: Crystal Oscillator Overtones.
- From: Parthasarathy Sampath <parthsv@xxxxxxxxx>
- To: "Moeller, Merrick" <mmoeller@xxxxxxxxxxxxxxx>
- Date: Thu, 12 Jun 2003 17:36:31 -0700 (PDT)
Hi,
I did some mathematical analysis on impedance of
the crystal along with its tank and capacitive load.
This circuit offers '0' resistance and inductive
impedance at 33Meg.
I inferered that, at 33Meg, the crystal and load
offering zero resistance, serves as short for the
input and hence Maximum gain is observed in open-loop
AC simulation, while transient analysis shows no
oscillations.
Is my inference correct and if so what is the
remedy.
All suggestions are welcome.
Thanks,
Regards,
Partha!
PS:I tried to add a small resistor in series to
inductor of tank circuit. It did suppres the 33Meg
problem but then, gain at 60Meg(3rd overtone) dropped
too low compared to fundemental.
--- "Moeller, Merrick" <mmoeller@xxxxxxxxxxxxxxx>
wrote:
> In theory the tank should be a capacitive load
> to get the Phase Shift you want around the loop.
> When you are performing your gain measurement
> what bandwidth measurement are you using? It should
> be very narrow around your frequency of interest.
> You may not be displaying a high enough resolution
> of data to pick up the gain spike and phase shift
> your are looking for at your overtone frequency.
>
> Merrick
>
> sorry about the late reply
>
> -----Original Message-----
> From: Parthasarathy Sampath
> [mailto:parthsv@xxxxxxxxx]
> Sent: Tuesday, June 03, 2003 12:58 PM
> To: Moeller, Merrick
> Cc: si-list@xxxxxxxxxxxxx
> Subject: RE: [SI-LIST] Re: Crystal Oscillator
> Overtones.
>
>
> Hi Merrick,
> The phase crosses '0' at three different points
> corresponding to Fundamental, 2nd overtone and 3rd
> overtone.
> The impedance offered by tank at 3rd overtone(the
> case for which I have highest gain at this freq) is
> infact Inductive!!!!
> This is what is more confusing for me. How will
> this circuit oscillate when I have one load as
> capacitive and another as inductive? and I cannot
> get
> any good gain at 3rd overtone if the tank impedance
> is
> capacitive.
>
> Thanks
>
> Regards,
> Partha!
> --- "Moeller, Merrick" <mmoeller@xxxxxxxxxxxxxxx>
> wrote:
> > How does the phase shift look at your
> > peak AC Gain?
> >
> > The initial condidtions on your simulation
> > can provide misleading oscillations. If the
> > signal dies out you probably aren't actually
> > running. Check the phase.
> >
> > Run Neg R. It will represent a combination of
> > phase and gain.
> >
> > The High frequency of your tank circuit may be
> > misleading your gain measurements. What is the
> > tank impedance?
> >
> > M
> >
> > -----Original Message-----
> > From: Parthasarathy Sampath
> > [mailto:parthsv@xxxxxxxxx]
> > Sent: Friday, May 30, 2003 5:06 PM
> > To: Moeller, Merrick
> > Cc: si-list@xxxxxxxxxxxxx
> > Subject: RE: [SI-LIST] Re: Crystal Oscillator
> > Overtones.
> >
> >
> > Yes, it does include 1nf cap in series with tank
> > circuit.
> > First i test for ac gain whihc inlcudes the
> > invertor
> > and crystal oscillator and once i achieve required
> > gain with correct inverter size, I go for
> transient
> > analysis to make other measurements.
> > Another peculiar thing is, with tank ckt freq
> > twice
> > the 3rd overtone, I have good ac gain, while in
> > transient analysis, the ouput dies out.
> >
> > Regards,
> > Partha
> > --- "Moeller, Merrick" <mmoeller@xxxxxxxxxxxxxxx>
> > wrote:
> > > First, are you using a decoupling cap from
> > > your tank inductor to ground? Second what
> > > type of simulation are you running...
> > > A negative R simulation is good in this
> > > case you should see your negative R cross
> > > 0 at your tank resonant frequency.
> > >
> > > Merrick
> > >
> > >
> > > -----Original Message-----
> > > From: Parthasarathy Sampath
> > > [mailto:parthsv@xxxxxxxxx]
> > > Sent: Friday, May 30, 2003 3:01 PM
> > > To: si-list@xxxxxxxxxxxxx
> > > Subject: [SI-LIST] Re: Crystal Oscillator
> > Overtones.
> > >
> > >
> > > > My question is at freq above tank corner
> freq,
> > > the
> > > > tank becomes inductive and whole circuit
> becomes
> > > > imbalanced.
> > > 'The above statement should be "At freq below
> > tank
> > > frequency, the tank should become inductive and
> > the
> > > whole circuit becomes inductive with one load of
> > > crystal being capacitor and other inductor(due
> to
> > > tank)".
> > >
> > >
> > > __________________________________
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