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From: Tony Williams
Subject: Re: Querry, Kick Starting Crystal Oscilator.
Date: Mon, 11 Nov 2002 13:46:48 +0000 (GMT)
References: <3DCA905A.59FD928B@mmm.com.DELETETHIS> <3DCC9A71.7DF3C3A9@mfi.net>
NNTP-Posting-Date: Mon, 11 Nov 2002 13:48:35 +0000 (UTC)
User-Agent: Pluto/1.14i (RISC-OS/3.60)
In article ,
Michael Stein wrote:
> Taking the specs for the C-TYPE/C-002RX (32768 Hz) at the bottom:
> maximum drive level: 1 uW max
> load cap: 6 pf
> series resistance: 45 Kohm max
> motional cap: 2.0 fF max
> shunt cap: 0.8 pF max
> insulation resistance: 500 Mohm min
> With the motional capacitance of 2.0 fFk then L is:
> 2.3591e-11/2.0e-15 = 1.1796e+04
> or 11796 H. Quite a bit larger than just "Henries".
> ............. The model for the inside of the
> crystal is supposed to look like this:
> | |
I think you may be missing a series-resistance in there.
My old telecomms textbook has an interesting sum
in it. That defines the ratio of the amplitudes
of sequential cycles of an RLC circuit.
It says; Ratio= exp(pi*R/w*L)
Actually, I think there is a typo in the book,
that term in brackets should have a minus sign.
If R=1, then oscillation is at constant amplitude,
if R= negative oscillations increase, and if R=
positive oscillations decrease.
So I plugged in your numbers of 45k/32KHz/11796H.
It said that on unpowering that crystal, the ratio
of sequential amplitudes should be 0.99994. Your
circuit probably loads it further, so the practical
ratio would be less than that.
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