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<IMG src="tech/t_tech.gif" width=376 height=81 border=0 alt="Technical Notes">
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<IMG SRC="tech/defn.gif" WIDTH=99 HEIGHT=17 BORDER=0 ALT="Definitions">
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<IMG SRC="tech/esr.gif" WIDTH=61 HEIGHT=16 BORDER=0 ALT="ESR" VSPACE=5><BR></TD>
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Is the value of the equivalent series resistance that the crystal
exhibits in the resonant circuit. Generally, the lower the ESR value
of a crystal, the more active it is, thus a lower drive level is
required to make the crystal resonate.<BR><BR></FONT>
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<IMG SRC="tech/load.gif" WIDTH=183 HEIGHT=16 BORDER=0 ALT="LOAD CAPACITANCE" VSPACE=5><BR></TD>
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For parallel mode operation, the crystal unit is designed to operate
as an inductive impedance. The load capacitance is the principal external
influence in determining the operating frequency of the oscillator. It
should be noted that a part of the actual load capacitance consists of
circuit and stray capacitances. These factors should be carefully
evaluated in order to ensure that the actual load capacitance presented
to the crystal is identical to the defined value of load capacitance for
which it is designed to operate.<BR><BR></FONT>
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<IMG SRC="tech/drive.gif" WIDTH=121 HEIGHT=15 BORDER=0 ALT="DRIVE LEVEL" VSPACE=5><BR></TD>
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The level of drive imposed on an oscillator crystal is usually specified
in terms of the power dissipated in it. Ideally, the crystal oscillator
should be regarded as a source of stable frequency. Overdriving a quartz
crystal can cause the frequency to shift either permanently or for the duration
the drive is imposed. The frequency shift of an overtone crystal is often
caused by a high drive level, e.g. higher than 200 µW. Designers should
take care that variations due to component tolerances, supply voltages,
etc, do not result in rated drive levels being exceeded.<BR><BR></FONT>
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<IMG SRC="tech/overtones.gif" WIDTH=95 HEIGHT=14 BORDER=0 ALT="OVERTONES" VSPACE=5><BR></TD>
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It is possible to excite vibrations in the quartz plate which are mechanical
overtones of some fundamental vibration. Overtone modes may be excited on
the 3rd, 5th, 7th, 9th harmonic of AT - cut thickness shear plates. The
properties for a given crystal unit and given order of overtones are quite
different from those of its fundamental frequency or other orders of
overtones. Consequently, no reliance can be put on the behaviour of a
crystal unit at any frequency other than that for which it was designed.<BR><BR></FONT>
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<IMG SRC="tech/aging.gif" WIDTH=50 HEIGHT=14 BORDER=0 ALT="AGING" VSPACE=5><BR></TD>
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Is the long-term stability of the frequency of a crystal functioning in a
constant operating condition. Frequency shifts, usually expressed in PPM
(parts per million) move positively or negatively with time. Aging
performance will vary according to the type of encapsulation employed,
e.g. cold weld produces typical rates of 1 PPM/year, resistance weld
typically 2 PPM/year, and solder seal 5 PPM/year.<BR><BR></FONT>
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<IMG SRC="tech/t_basic.gif" WIDTH=319 HEIGHT=18 ALT="Basic Quartz Crystal Characteristics" BORDER="0"></TD>
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<DIV ALIGN="center"><IMG SRC="tech/d1.gif" WIDTH=200 HEIGHT=117 ALT="Diagram 1" BORDER="0" VSPACE=8></DIV>
<IMG SRC="tech/t_circuit.gif" WIDTH=217 HEIGHT=14 ALT="Crystal Equivalent Circuit" BORDER="0" VSPACE=8><BR>
<FONT FACE="Arial, Helvetica" SIZE="-1">
The equivalent circuit<SUB> </SUB>of a crystal, as illustrated, is useful<SUB> </SUB>to explain
the basic concepts governing<SUB> </SUB>the crystal's performance. C<SUB>0</SUB> represents the
static (shunt) capacitance<SUB> </SUB>and is the sum of the capacitance between<SUB> </SUB>the
electrodes and the capacitance added<SUB> </SUB>by the wire leads and holder. The R<SUB>1</SUB>,
L<SUB>1</SUB>, C<SUB>1</SUB> branch is known as the motional arm. C<SUB>1</SUB>
represents the motional capacitance of the quartz, L<SUB>1</SUB> is a function of the mass
and R<SUB>1</SUB> is the sum of the bulk crystal losses. <BR><BR>
<IMG SRC="tech/t_res.gif" WIDTH=253 HEIGHT=14 ALT="Series and Parallel Resonance" BORDER="0"><BR>
<IMG SRC="tech/t_sres.gif" WIDTH=116 HEIGHT=14 ALT="Series Resonance" BORDER="0" VSPACE=8><BR>
When a crystal<SUB> </SUB>is operating at series resonance (f<SUB>s </SUB>), it looks resistive
in the circuit. Thus its impedance at f<SUB>s</SUB> is near zero. In a well designed
series<SUB> </SUB>resonance circuit, correlation is not<SUB> </SUB>a problem and the load
capacitance does not have to be specified. <BR>
<IMG SRC="tech/t_pres.gif" WIDTH=229 HEIGHT=15 ALT="Parallel Resonance" BORDER="0" VSPACE=8><BR>
When a crystal<SUB> </SUB>is operating at parallel resonance (f<SUB>a </SUB>), it looks inductive
in the circuit. The crystal's<SUB> </SUB>impedance reaches its peak at f<SUB>a</SUB>. A change
in circuit reactance values will have<SUB> </SUB>the effect of pulling the frequency
of the<SUB> </SUB>crystal. If the crystal is to be used at<SUB> </SUB>parallel resonance, the load
capacitance should<SUB> </SUB>always be specified. Load capacitance<SUB> </SUB>is the dynamic
capacitance of the total<SUB> </SUB>circuit measured across the<SUB> </SUB>crystal terminals. In
parallel circuit design<SUB> </SUB>the load capacitance should be selected to<SUB> </SUB>operate
the crystal at a stable point on the f<SUB>s</SUB>-f<SUB>a</SUB> reactance curve (as close to f<SUB>s</SUB>
as possible).
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<IMG SRC="tech/t_quality.gif" WIDTH=149 HEIGHT=15 ALT="Quality Factor" BORDER=0 VSPACE=8><BR>
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The Q of a crystal unit<SUB> </SUB>is the Quality Factor of the motional<SUB> </SUB>arm at
resonance. The maximum stability<SUB> </SUB>that can be attained by the crystal
is directly<SUB> </SUB>related to Q. The higher the Q the smaller the band<SUB> </SUB>width (<IMG SRC="delta.gif" WIDTH=11 HEIGHT=10 BORDER=0 ALT=""> f )
and the steeper the reactance slope (f<SUB>s</SUB>-f<SUB>a</SUB>). External circuit reactance<SUB> </SUB>value
changes have a smaller effect<SUB> </SUB>on a high Q crystal (lower pullability) than<SUB> </SUB>
on lower Q devices.<BR>
<IMG SRC="tech/d2b.gif" WIDTH=246 HEIGHT=247 ALT="Diagram 2" BORDER=0 VSPACE=8 HSPACE="10">
<IMG SRC="tech/t_form.gif" WIDTH=78 HEIGHT=14 ALT="Formulas" BORDER="0" VSPACE=4>
<IMG SRC="tech/formulas2.gif" WIDTH=258 HEIGHT=343 BORDER=0 VSPACE=8 HSPACE="10"></FONT></TD>
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<TD width="566"><FONT FACE="Arial, Helvetica" SIZE="-1"><BR>
Established in 1980, West Crystal specializes in the manufacture of precision quartz
crystals, custom made to your specifications. Our strength is small quantities and
fast turn around times, but we tackle larger quantities as well.<BR><BR>
Since our
origins as a supplier of replacement crystals to the 2-way radio market, we have
evolved to satisfy the needs of a wide range of OEM customers, including Avionics, Medical
Systems, GPS, Guidance Systems and Cable TV Equipment Manufacturers.<BR><BR>
We also supply tubular crystals, filters, oscillators and other frequency
management products through our offshore affiliates.<BR><BR>
Our initial work with 2-way radio dealers led to the creation of a separate division
supplying channel elements, extension speakers, mounting brackets, horn speakers, etc.<BR><BR>We are located in Kelowna in the beautiful Okanagan Valley in South Central British
Columbia, and market our products across North America. We also have a smaller presence
in several other countries, including China.<BR><BR>As of September 3, 1997, West Crystal Company Ltd is approved under the ISO 9002 Quality
Standard for the MANUFACTURE OF QUARTZ CRYSTALS.</FONT>
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