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Themostcommonexceptiontotheruleiswhenasmall
Fig.5Frequency/LoadCapacitance
capacitor,avaricapdiodeforexample,isplacedinseries
withthecrystalinthenon-invertingamplifiercircuitinor-
dertoprovideadegreeoffrequencyadjustment.Insucha
casethecrystalmustbecalibratedforresonancewiththe
meanvalueofthatcapacitance.
Pullability
!
Thepullabilityofacrystalisitsfrequencychangefora
givenchangeofloadcapacitance.Thisisoftenexpressed
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inkHzasthedifferencebetweenitsseries-resonancefre-
quencyandthatwhenresonantwithagivenloadcapaci-
tance.Thisoffsetcanbecalculatedinpartspermillion
using:
Fractionalloadresonancefrequencyoffset(DL).The
actualfrequencychangefromFtoFforagivenvalue
rL
!!
ofC
L,
6
C10
1.
Dppm
L
2(CC)
0L
Fig.5showsatypicalcurvefortheeffectoffrequency
Fig.6TypicalCrystalPullingSensitivity
changewithrespecttochangeinloadcapacitance.
Alternatively,itiscommontoexpressacrystal’spullabil-
ityasatrimsensitivityinppmperpFchangeofloadca-
pacitance.Thisisrepresentedapproximatelyby:
6
DC.10
L1
ppm/pF
2
C
2(CC)
L
0L
andisshowngraphicallyinFig.6forvariousvaluesof(C
0
+C).
L
TypicalValues
FrequencyVibrationModeC(fF)C(pF)
10
1.0to1.999MHzFundamental5to83
#$%
2.0to3.999MHz6to12
4.0to6.4999MHz8to205
6.5to30.0MHz16to256
21.0to90.0MHz3rdOvertone1.0to2.5
60.0to150.0MHz5thOvertone<0.70
85.0to210.0MHz7thOvertone<0.40
EuropeTel:+44(0)146074433Fax:+44(0)146072578258AmericasTel:+19199410430Fax:+19199410530
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APPLICATION
NOTES
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