DA6283.002
11 November, 2010
The crystal nominal frequency f
NOM_XTAL
is calculated, as shown in an equation 4.
Equation 4
∆
f
f
NOM
_
XTAL
=
f
NOM
×
1
+
6
10
Where,
f
NOM
= Desired nominal frequency of the VCXO module
f
NOM_XTAL
= Crystal nominal frequency (without MAS IC load capacitance)
Crystal nominal frequency optimization is calculated in an example 2.
Example 2
VCXO module target frequency f
NOM
is 35 MHz. Crystal characteristics are crystal load C
L_XTAL
= 12.5 pF and
pulling S = 30 ppm/pF.
MAS6283 C
L_IC
= 8 pF when V
C
= 1.65 V.
Calculate load difference
∆C
L
according to the equation 2.
∆
C
L
=
C
L
_
IC
−
C
L
_
XTAL
=
8
pF
−
12.5
pF
= −
4.5
pF
Calculate frequency difference
∆f
according to the equation 3.
∆
f
= ∆
C
L
×
S
= −
4.5
pF
×
30
Now f
NOM
= 35 MHz.
According to the equation 4
ppm
= −
135
ppm
pF
∆
f
f
NOM
_
XTAL
=
f
NOM
×
1
+
6
10
−
135
6
=
34995275
Hz
=
35
×
10
×
1
+
6
10
The specified crystal has to have a nominal frequency of 34.995275 MHz without load capacitance. This offset is
compensated with 8 pF load capacitance though a crystal C
L
= 12.5 pF.
6/13
MAS [ MICRO ANALOG SYSTEMS ]
