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From: Mike Monett
X-Mailer: Mozilla 2.02 (Win16; I)
Subject: Re: Querry, Kick Starting Crystal Oscilator.
References: <3DCA905A.59FD928B@mmm.com.DELETETHIS> <3DCC9A71.7DF3C3A9@mfi.net>
Date: Sun, 10 Nov 2002 11:03:10 -0800
NNTP-Posting-Date: Sun, 10 Nov 2002 11:03:12 EST
Organization: Bell Sympatico
James Meyer wrote:
> On Sun, 10 Nov 2002 06:03:10 +0000 (UTC), email@example.com (Michael Stein)
> >Picture a parallel LC with a constant current (DC) being forced through
> >the L. The voltage across the L and C is constant. Now suddenly
> >disconnect the current source.
> >Why should there be a delay in the start of ringing and why should
> >this be dependent on the Q of the LC? And if the ring doesn't
> >start at full amplitude, then where does the energy from the L go?
>The equivalent inductance associated with the motional impedance of a
> typical 32KHz crystal is HENRIES. How long would it take to get a significant
> current to flow through that amount of inductance? The series capacitance is
> picofarads. How much voltage would you have to apply to get significant
Actually, this is a very good method to kick-start a crystal oscillator in
SPICE. Just use .IC statements to set the voltage across R1 and L1, which sets
the current through the inductance.
When the transient analysis starts, the voltages are removed and the energy
stored in the inductor is transferred to the series capacitor as in a
conventional LC resonant circuit.
This doesn't really help if the circuit Q is high. SPICE takes too long to show
the resulting decay or buildup, and will probably run out of memory. The
solution is to recalculate the crystal parameters using a much lower Q. However,
the loop gain and phase can be simulated in frequency analysis using the normal
As far as kick-starting an actual crystal, there is a problem on where to
connect the leads to force a current through L1. It doesn't really exist.
"L1: The motional inductance of the crystal is determined by the
mechanical mass of quartz in motion. The lower frequencies
(thicker and larger quartz wafers) tend to run at a few Henries,
while higher frequencies (thinner and smaller quartz wafers) tend
to run at a few milli-Henries."
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