DA6283.002
11 November, 2010
EXTERNAL COMPONENT SELECTION
Quartz Crystal and VCXO Module Information
To ensure the best system performance, the crystal
parameters should be considered carefully. Pulling
is an important parameter which can be calculated
according to an equation 1. Layout guidelines in the
following section should be followed. The frequency
of the crystal is tuned by load capacitors. There are
integrated variable load capacitors on the MAS6283
and they are controlled by an external voltage at the
VC pin. It is recommended to connect a 1 nF
capacitor between VDD and VSS
.
The external
crystal should be located as close to the chip as
possible. In case of a PCB mounted module, it is
usually advisable to mount a crystal on the same
side with the VCXO IC to minimize stray
capacitance. Often vias between the crystal pins
and the XIN and XOUT pins of the VCXO IC
increase stray capacitance. There should be no
noisy signal traces underneath or close to the
crystal.
Equation 1
Crystal Pulling Sensitivity
S
=−
C
1
ppm
[values are given in the units described below]
2(
C
0
+
C
L
)
2
pF
10
6
Where,
C
L
= Load capacitance in series with the crystal
C
0
= Shunt capacitance of the crystal
C
1
= Motional capacitance of the crystal
Example 1
If we choose a crystal with the following values
C
L
= 8.0 pF,
C
0
= 2.0 pF,
C
1
= 6.7 fF
the equation 1 yields
S
=
−
6.7
×
10
−
15
2 2.0
×
10
−
12
+
8.0
×
10
−
12
10
6
(
)
2
= −
33.5
ppm
pF
If a crystal load differs from 8 pF the oscillator will have frequency offset at V
C
= 1.65 V. Thus if you need to use
1.65 V VC voltage with a crystal which C
L
is other than 8 pF you have to design the crystal for a specific nominal
frequency. The following guidelines show how to define the crystal’s nominal frequency.
Separate crystal C
L
as C
L_XTAL
and MAS IC C
L
as C
L_IC
.
To define specific nominal frequency for the crystal first calculate load difference
∆C
L
[pF] as in an equation 2.
Equation 2
∆
C
L
=
C
L
_
IC
−
C
L
_
XTAL
Calculate frequency difference
∆f
[ppm] as in an equation 3. Pulling S comes from the equation 1.
Equation 3
∆
f
= ∆
C
L
×
S
5/13
MAS [ MICRO ANALOG SYSTEMS ]
