3.2 Calculating Crystal Loading Capacitance
Crystals will resonate at their specified frequency (i.e., be “on-frequency”) if the capacitive loading across the crystal’s terminals is
the same as specified by the crystal manufacturer’s loading capacitance (CL) specification. The total loading capacitance presented
to the crystal must factor in all capacitance sources such as parasitic “stray” capacitance as well as added loading capacitance.
Stray capacitance comes from sources like PCB traces, capacitive coupling to nearby components, as well as any stray capacitance
within the device itself. For “on-frequency” oscillator operation, all capacitance sources must be factored into determining the correct
additional capacitance to be presented to the crystal to match its required CL.
The Si5332 contains variable
internal
loading capacitors (CL
VAR
) to provide the necessary added crystal matching capacitance such
that external matching capacitors are not needed. The figure below shows the Si5332’s internal variable capacitance and the two main
sources of stray loading capacitance, PCB stray (CLS
EXT
) and Si5332 device stray capacitance on XA/XB input pins (CLS
INT
).
Internal Stray
Capacitance CLS
INT
External Stray
Capacitance CLS
EXT
XA
XB
Internal Variable
Capacitance CL
VAR
Figure 3.6. Sources of Crystal Loading Capacitance
Using the Si5332’s internal variable loading capacitors (CL
VAR
), the crystal’s required CL can be matched by adding capacitance to
the external stray (CLS
EXT
) and internal device capacitance (CLS
INT
) to match the crystal’s requirements. A value for CL
VAR
must be
selected such that:
Required
Crystal
CL
=
CL
VAR
+
CLS
INT
+
CLS
EXT
Or rearranged:
CL
VAR
=
Crystal
CL
-
CLS
INT
-
CLS
EXT
Equation 1
Note:
the required Crystal CL must be greater than or equal to the total stray capacitance quantity (
CLS
INT
+ CLS
EXT
) or the crystal
can’t be used as CL
VAR
is always >= 0.
For the following example, a Crystal CL value of 10 pf will be used. The internal stray capacitance (CLS
INT
) of the Si5332 is 2.4
pf. External PCB stray capacitance (CLSEXT) is usually in the order of 2-3 pf given a reasonably compact layout. The Si5332 EVB
external stray capacitance is ~ 2.75 pf. Given these example values, the required CL
VAR
can be calculated as shown below, using
Equation 1.
CL
VAR
= 10
pF
- 2 . 4
pF
- 2 . 75
pF
= 4 . 85
pF
Using Equation 1
As shown in the figure above, the internal variable loading capacitor (CL
VAR
) is implemented by using
two
capacitors
in series
: one
connected to the XA pin (CL
XA
) and one to the XB pin (CL
XB
) of the Si5332. The CL
VAR
value is therefore determined by the values of
CL
XA
and CL
XB
.
For capacitors in series, if we keep CL
XA
= CL
XB
, we can simply double the value of CL
VAR
to arrive at the correct CL
XA
and CL
XB
value. (This is a technique commonly used for calculating crystal loading capacitors when external loading capacitors are used.) Now,
CL
XA
and CL
XB
values are simply:
Si5332-AM1/2/3 Automotive Grade Device Reference Manual • Input Clocks
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com
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Rev. 0.3 • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice • July 26, 2021
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